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Weed competition severely constrains cassava root yield in sub-Saharan Africa; thus, good weed control measures, including the use of herbicides, are increasingly important. Herbicide trials were conducted at five locations across eastern, western, and north-central Nigeria over two cropping seasons (2014 and 2015). Nineteen premixed PRE herbicides applied at different rates were evaluated for efficacy on weeds and selectivity on cassava. Manual hoe-weeding at 4, 8, and 12 wk after planting (WAP) and two S-metolachlor + atrazine treatments commonly used by cassava growers were included for comparison. Six of the 19 PRE herbicide treatments (indaziflam + isoxaflutole, indaziflam + metribuzin, flumioxazin + pyroxasulfone, isoxaflutole, acetochlor + atrazine + terbuthylazine, and terbuthylazine + S-metolachlor) consistently provided 80% to 98% broadleaf and grass weed control up to 8 wk after treatment. Overall, PRE herbicide treatments and cassava yield were significantly positively correlated. Herbicide treatments terbuthylazine + S-metolachlor, flumioxazin + pyroxasulfone, diflufenican + flufenacet + flurtamone (respectively, 60 + 60 + 60, 120 + 120 + 120, 90 + 360 + 120, and 135 + 360 + 180 g ha−1), acetochlor + atrazine + terbuthylazine (875 + 875 + 875 g ha−1), S-metolachlor + atrazine (870 + 1,110 g ha−1), oxyfluorfen (240 g ha−1), indaziflam + isoxaflutole (75 + 225 g ha−1), indaziflam + metribuzin (75 + 960 g ha−1), and aclonifen + isoxaflutole (500 + 75 g ha−1) contributed to yields exceeding twice the Nigerian national average of 8.76 tonnes ha−1. These treatments had root yields of 1.4 to 2 times higher than plots that had been hoe-weeded three times. There were some adverse herbicide treatment effects such as delayed cassava sprouting and temporary leaf bleaching observed in indaziflam and diflufenican + flufenacet + flurtamone treatments, whereas sulfentrazone caused prolonged leaf crinkling. The PRE applications alone at rates safe for cassava did not provide adequate season-long weed control; supplemental POST weed control is needed about 10 WAP for satisfactory season-long control.

Timely results from whole-plant, herbicide-resistant weed screenings are crucial to heighten grower awareness. However, the high degree of physiological dormancy of giant ragweed (Ambrosia trifidaL.) seed exacerbates this process. The most effective methods for alleviating dormancy, to date, are either labor-intensive (embryo excision) or require several weeks (soil stratification). This research describes a conditioning process involving clipping and aeration of seed in water that is highly effective at alleviating dormancy and requires less skill and time compared with previous techniques.Ambrosia trifidaseeds were collected over 2 yr at two different collection timings (September 25, “early”; October 25, “late”), subjected to various treatments intended to release dormancy, and evaluated for emergence over 18 d in the greenhouse. The use of germination-promoting chemicals (ethephon, gibberellic acid, and thiourea) generally provided no increase in emergence compared with water and occasionally produced seedlings with abnormal growth unsuitable for further experimentation. Conditioning yielded between 30% and 33% emergence for both early and late collections of seeds with no afterripening period compared with 0% emergence for seeds imbibed in water. Following an 8-wk period of dry storage at 4 C, conditioning yielded nearly 80% emergence for both collection timings, while emergence of seeds imbibed in water was 10% and 27% for early and late collections, respectively. Soil stratification in moist soil for 8 wk at 4 C was the second most effective treatment, yielding 46% to 49% emergence across both collections. Parameters of the Weibull function further indicated the conditioning treatment had the fastest rate of emergence and shortest lag phase between planting and first emergence. Methods to germinateA. trifidawithout an afterripening period have previously been unsuccessful. Therefore, the seed-conditioning method outlined in this work will be useful in expediting the confirmation of herbicide-resistantA. trifidaincidences.

Giant ragweed is a highly competitive weed that continually threatens crop production systems due to evolved resistance to acetolactate synthase–inhibiting herbicides (ALS-R) and glyphosate (GR). Two biotypes of GR giant ragweed exist and are differentiated by their response to glyphosate, termed here as rapid response (RR) and non–rapid response (NRR). A comparison of data from surveys of Indiana crop fields done in 2006 and 2014 showed that GR giant ragweed has spread from 15% to 39% of Indiana counties and the NRR biotype is the most prevalent. A TaqMan®single-nucleotide polymorphism genotyping assay was developed to identify ALS-R populations and revealed 47% of GR populations to be ALS-R as well. The magnitude of glyphosate resistance for NRR populations was 4.6 and 5.9 based on GR50and LD50estimates, respectively. For RR populations, these values were 7.8 to 9.2 for GR50estimates and 19.3 to 22.3 for LD50estimates. A novel use of the Imaging-PAM fluorometer was developed to discriminate RR plants by assessing photosystem II quantum yield across the entire leaf surface. H2O2generation in leaves of glyphosate-treated plants was also measured by 3,3′-diaminobenzidine staining and quantified using imagery analysis software. Results show photo-oxidative stress of mature leaves is far greater and occurs more rapidly following glyphosate treatment in RR plants compared with NRR and glyphosate-susceptible plants and is positively associated with glyphosate dose. These results suggest that under continued glyphosate selection pressure, the RR biotype may surpass the NRR biotype as the predominant form of GR giant ragweed in Indiana due to a higher level of glyphosate resistance. Moreover, the differential photo-oxidative stress patterns in response to glyphosate provide evidence of different mechanisms of resistance present in RR and NRR biotypes.

BACKGROUND The glyphosate-resistant rapid response (GR RR) resistance mechanism in Ambrosia trifida is not due to target-site resistance (TSR) mechanisms. This study explores the physiology of the rapid response and the possibility of reduced translocation and vacuolar sequestration as non-target-site resistance (NTSR) mechanisms. RESULTS GR RR leaf discs accumulated hydrogen peroxide within minutes of glyphosate exposure, but only in mature leaf tissue. The rapid response required energy either as light or exogenous sucrose. The combination of phenylalanine and tyrosine inhibited the rapid response in a dose-dependent manner. Reduced glyphosate translocation was observed in GR RR, but only when associated with tissue death caused by the rapid response. Nuclear magnetic resonance studies indicated that glyphosate enters the cytoplasm and reaches chloroplasts, and it is not moved into the vacuole of GR RR, GR non-rapid response or glyphosate-susceptible A. trifida. CONCLUSION The GR RR mechanism of resistance is not associated with vacuole sequestration of glyphosate, and the observed reduced translocation is likely a consequence of rapid tissue death. Rapid cell death was inhibited by exogenous application of aromatic amino acids phenylalanine and tyrosine. The mechanism by which these amino acids inhibit rapid cell death in the GR RR phenotype remains unknown, and it could involve glyphosate phytotoxicity or other agents generating reactive oxygen species. Implications of these findings are discussed. The GR RR mechanism is distinct from the currently described glyphosate TSR or NTSR mechanisms in other species. © 2017 Society of Chemical Industry

Agricultural environments allow study of evolutionary change in plants. An example of evolution within agroecological systems is the selection for resistance to the herbicide glyphosate within the weed, Conyza canadensis. Changes in survivorship and reproduction associated with the development of glyphosate resistance (GR) may impact fitness and influence the frequency of occurrence of the GR trait. We hypothesized that site characteristics and history would affect the occurrence of GR C. canadensis in field margins. We surveyed GR occurrence in field margins and asked whether there were correlations between GR occurrence and location, crop rotation, GR crop trait rotation, crop type, use of tillage, and the diversity of herbicides used. In a field experiment, we hypothesized that there would be no difference in fitness between GR and glyphosate‐susceptible (GS) plants. We asked whether there were differences in survivorship, phenology, reproduction, and herbivory between 2 GR and 2 GS populations of C. canadensis in agrestal and ruderal habitats. We found that geographic location was an important factor in the occurrence of GR C. canadensis in field margins. Although not consistently associated with either glyphosate resistance or glyphosate susceptibility, there were differences in phenology, survivorship, and herbivory among biotypes of C. canadensis. We found equal or greater fitness in GR biotypes, compared to GS biotypes, and GR plants were present in field margins. Field margins or ruderal habitats may provide refugia for GR C. canadensis, allowing reproduction and further selection to occur as seeds recolonize the agrestal habitat. Agricultural practices may select for ecological changes that feed back into the evolution of plants in ruderal habitats.

A segment of the debate surrounding the commercialization and use of glyphosate-resistant (GR) crops focuses on the theory that the implementation of these traits is an extension of the intensification of agriculture that will further erode the biodiversity of agricultural landscapes. A large field-scale study was initiated in 2006 in the United States on 156 different field sites with a minimum 3-yr history of GR-corn, -cotton or -soybean in the cropping system. The impact of cropping system, crop rotation, frequency of using the GR crop trait, and several categorical variables on seedbank weed population density and diversity was analyzed. The parameters of total weed population density of all species in the seedbank, species richness, Shannon's H′ and evenness were not affected by any management treatment. The similarity between the seedbank and aboveground weed community was more strongly related to location than management; previous year's crops and cropping systems were also important while GR trait rotation was not. The composition of the weed flora was more strongly related to location (geography) than any other parameter. The diversity of weed flora in agricultural sites with a history of GR crop production can be influenced by several factors relating to the specific method in which the GR trait is integrated (cropping system, crop rotation, GR trait rotation), the specific weed species, and the geographical location. Continuous GR crop, compared to fields with other cropping systems, only had greater species diversity (species richness) of some life forms, i.e., biennials, winter annuals, and prostrate weeds. Overall diversity was related to geography and not cropping system. These results justify further research to clarify the complexities of crops grown with herbicide-resistance traits to provide a more complete characterization of their culture and local adaptation to the weed seedbank.

Question How do different weed management systems based on glyphosate-resistant (GR) crops affect weed community structure and composition? Location A total of 156 agricultural fields across six US states. Methods Fields were selected and categorized into three weed management systems based on a 3-yr history: (1) a single continuous GR crop; (2) a rotation of two GR crops; and (3) a GR crop rotated with a non-GR crop. Weed species population density was surveyed in 20 0.5-m2 plots per field four times each season from 2006 to 2010. Weed population density, species richness and diversity were analysed using repeated measures mixed models to test the effect of year, geographic location (based on plant hardiness zone) and weed management system (as described above). Weed community composition was analysed using NMDS ordination. The importance of geographic location, planted crop and weed management system were analysed using analysis of similarity and permutational ANOVA based on the Bray–Curtis similarity among sites calculated from the maximum weed species population density per site per year. Results A total of 329 weed species were identified across the six states and all time periods. Weed communities were most strongly correlated with geographic location and secondarily to the crop planted. Weed management system affected similarity among weed communities through an interaction with site location but not every year. The weed management systems of crop rotation and GR trait rotation generally reduced weed population density and species diversity, but the effect of crop rotation varied by geographic location. Conclusions The overall effects of weed management systems in fields including the GR trait on arable weed communities reinforce the need for locally adapted weed management programmes to steward the GR crop trait to manage for high weed diversity while reducing crop–weed competition and maximizing yield.

A segment of the debate surrounding the commercialization of genetically engineered (GE) crops, such as glyphosate-resistant (GR) crops, focuses on the theory that implementation of these traits is an extension of the intensification of agriculture that will further erode the biodiversity of agricultural landscapes. A large field-scale study was conducted in 2006 in the United States on 156 different field sites with a minimum 3-yr history of GR corn, cotton, or soybean in the cropping system. The impact of cropping system, crop rotation, frequency of using the GR crop trait, and several categorical variables on emerged weed density and diversity was analyzed. Species richness, evenness, Shannon's H′, proportion of forbs, erect growth habit, and C3species diversity were all greater in agricultural sites that lacked crop rotation or were in a continuous GR crop system. Rotating between two GR crops (e.g., corn and soybean) or rotating to a non-GR crop resulted in less weed diversity than a continuous GR crop. The composition of the weed flora was more strongly related to location (geography) than any other parameter. The diversity of weed flora in agricultural sites with a history of GR crop production can be influenced by several factors relating to the specific method in which the GR trait is integrated (cropping system, crop rotation, GR trait rotation), the specific weed species, and the geographical location. The finding that fields with continuous GR crops demonstrated greater weed diversity is contrary to arguments opposing the use of GE crops. These results justify further research to clarify the complexities of crops grown with herbicide-resistance traits, or more broadly, GE crops, to provide a more complete characterization of their culture and local adaptation.

Approximately 1,300 growers from 22 states were surveyed during 2010 to determine herbicide use. Cropping systems included continuous glyphosate-resistant corn, cotton, and soybean, and various combinations of these crops and rotations with non–glyphosate-resistant crops. The most commonly used herbicide for both fall and spring applications was glyphosate followed by synthetic auxin herbicides. Herbicide application in spring was favored over application in the fall. The percentage of growers in a glyphosate-only system was as high as 69% for some cropping systems. Excluding glyphosate, the most frequently used herbicides included photosystem II, mitotic, and protoporphyrinogen oxidase inhibitors. A higher percentage of growers integrated herbicides other than glyphosate during 2010 compared with 2005. Extensive educational efforts have promoted resistance management by increasing the diversity of herbicides in glyphosate-resistant cropping systems. However, a considerable percentage of growers continued use of only glyphosate from the period of 2005 to 2010, and this practice most likely will continue to exert a high level of selection for evolved glyphosate-resistant weed species.

Almost 1,650 corn, cotton, and soybean growers in 22 states participated in a 2010 telephone survey to determine their attitudes with regard to which weed species were most problematic in glyphosate-resistant (GR) crop production systems for corn, cotton, and soybean. The survey is a follow-up to a previous 2005 to 2006 survey that utilized a smaller set of growers from fewer states. In general, growers continued to estimate weed populations as low and few challenges have been created following adoption of GR cropping systems. Pigweed and foxtail species were dominant overall, whereas other species were more commodity and state specific. Corn, cotton, and soybean growers cited velvetleaf, annual morningglory, and waterhemp, respectively, as predominant weeds. Growers in the South region were more likely to report pigweed and waterhemp (Amaranthus spp.), whereas growers in the East and West reported horseweed. When growers were asked with which GR weeds they had experienced personally, horseweed was reported in all regions, but growers in the South more frequently reported pigweed, whereas growers in the East and West regions more frequently reported waterhemp. Comparisons with the previous 2005 survey indicated that more growers believed they were experiencing GR weeds and were more aware of specific examples in their state. In particular, the Amaranthus complex was of greatest concern in continuously cropped soybean and cotton.

In 2010, a grower survey was administered to 1,299 growers in 22 states to determine changes in weed management in the United States from 2006 to 2009. The majority of growers had not changed weed management practices in the previous 3 yr; however, 75% reported using weed management practices targeted at glyphosate-resistant (GR) weeds. Growers were asked to rate their efforts at controlling GR weeds and rate the effectiveness of various practices for controlling/preventing GR weeds regardless of whether they were personally using them. Using the herbicide labeled rate, scouting fields, and rotating crops were among the practices considered by growers as most effective in managing GR weeds. Sixty-seven percent of growers reported effective management of GR weeds. Between the 2005 and 2010 Benchmark surveys, the frequency of growers using specific actions to manage GR weeds increased markedly. Although the relative effectiveness of practices, as perceived by growers, remained the same, the effectiveness rating of tillage and the use of residual and POST herbicides increased.

A 2010 survey of 1,299 corn, cotton, and soybean growers was conducted to determine their attitudes and awareness regarding glyphosate-resistant (GR) weeds and resultant implications on weed management practices. An additional 350 growers included in the current study participated in a 2005 survey, and these answers were compared across time so that cross-sectional and longitudinal comparisons of responses could be made. Most growers surveyed in 2010 were aware of the potential for weeds to evolve resistance to glyphosate; however, many growers were not aware of glyphosate resistance in specific weeds in their county or state. Growers in the South were different from growers in other geographic regions and were significantly more aware of local cases of GR weeds. Awareness of GR weeds did not increase appreciably from 2005 to 2010, but the percentage who reported GR weeds as problematic was significantly higher. Grower reports of GR weeds on-farm in 2010 were up considerably from 2005, with growers in the South reporting significantly more instances than growers in other regions. Growers in the South were also more likely to consider glyphosate resistance a serious problem. Overall, 30% of growers did not consider GR weeds to be a problem. It appears that most growers received information about glyphosate resistance from farm publications, although in the South this percentage was less than for other geographic regions. Growers in the South received more information from universities and extension sources.

Field studies were conducted to determine the response of sublethal glyphosate and dicamba doses to processing tomato flowering loss and marketable yield. Dose–response studies for both herbicides were conducted on four commercial processing tomato lines (two different lines within each study) and plants were sprayed at either the vegetative stage or the early bloom stage. Both glyphosate and dicamba caused higher yield losses when sprayed at the early bloom stage. A 25% yield loss was observed with 8.5 and 7.5 g ae ha−1 for glyphosate and dicamba, respectively, at the early bloom stage and 43.9 and 11.9 g ae ha−1 for glyphosate and dicamba, respectively, at the early vegetative stage. Overall, these tomato cultivars were more sensitive to dicamba than to glyphosate. We conclude that glyphosate and dicamba drift could have serious implications on tomato yields especially if the drift occurs during flowering. Nomenclature: Glyphosate; dicamba; tomato, Lycopersicon esculentum Mill.

Weeds that emerge between rows in fresh market tomatoes after the critical period of competition are not suppressed by the crop and can produce large quantities of seed. A living mulch planted between rows might limit weed seed production. Buckwheat was seeded between tomato rows after the critical period in 2007 and 2008 in field studies near Lafayette, IN. Weeds were allowed to emerge after the critical period (CP), controlled throughout the growing season (no seed threshold [NST]), or mowed to limit seed production (MOW). Buckwheat and MOW plots were mowed twice after the critical period in 2007 and once in 2008. Seed banks were sampled after the critical period and in the following spring. Tomato yields were not reduced by growing buckwheat between rows. Seed bank densities for common purslane and carpetweed, which escaped mowing due to their prostrate habits, increased in all treatments. Germinable seed bank densities were 306 seeds m−2or less in the NST and buckwheat treatments but 755 seeds m−2or more in the CP treatments for species with erect habits in both years. Seed bank densities were lower in the MOW treatment than in the CP treatments in 2007 but not in 2008. In a parallel experiment conducted in adjacent plots, buckwheat was seeded at five rates (0, 56, 112, 168, and 224 kg seed ha−1). Plots were mowed and emergent weeds sampled as described for the intercrop experiment. Weed densities before mowing decreased linearly with buckwheat seed rate. After mowing, no relationship was detected between seed rate and weed densities. This study supports the hypothesis that a living mulch planted after the critical period can be used to limit seed bank growth without reducing tomato yields, but additional research is needed to better understand the effect of mowing on living mulch growth and weed suppression.

BACKGROUND: A survey was conducted with nearly 1200 growers in US states (Illinois, Indiana, Iowa, Mississippi, Nebraska and North Carolina) in 2005 with the objective in part of determining the awareness of the potential for development of glyphosate resistance, the experience with glyphosate-resistant (GR) weeds and the sources of information that growers had utilized for information on glyphosate resistance. Growers were asked a series of questions to determine the level of glyphosate resistance awareness and to list the sources of information used to learn about glyphosate resistance issues. RESULTS: The majority of the growers (88%) were aware of a weed's potential to evolve resistance to herbicide, while 44% were aware of state-specific documented cases of GR weeds, and 15% reported having had personal experience with GR weeds. Among sources of information concerning glyphosate resistance issues, farm publications, dealers/retailers and university/extension were the most frequent responses (41, 17 and 14% respectively). Based on a 1-10 effectiveness scale, growers ranked tillage the least effective practice (5.5) and using the correct label rates of herbicides at the proper timing for the size and type of weeds present the most effective practice (8.6) with respect to how effectively the practices mitigated the evolution of GR weeds. CONCLUSION: Results from this survey can be used by researchers, extension specialists and crop advisors further to bridge the information gap between growers and themselves and better to disseminate information concerning glyphosate resistance and glyphosate resistance management practices through more targeted information and information delivery methods.

BACKGROUND:Glyphosate-resistant(GR)cropshavechangedthewaygrowersmanageweedsandimplementcontrolstrategies. Since the introduction of GR crops, growers in many instances have relied on glyphosate almost exclusively to control a broad spectrum of weeds. This overreliance on glyphosate has resulted in the evolution of glyphosate resistance in some weed species. Growers and scientists are concerned about the sustainability of GR crops and glyphosate. When a grower is making decisions about weed control strategies, economic costs and benefits of the program are primary criteria for selection and implementation. Studies across six states were initiated in 2006 to compare the economics of using a weed resistance best managementpractice (BMP) system with a grower’s standard production system. RESULTS: Resistance BMP systems recommended by university scientists were more costly but provided similar yields and economic returns. Rotationof GR crops resulted ina highernet return(maize and soybean) compared with continuous GR crop (cotton or soybean) or rotating a GR crop with a non-GR crop (maize). CONCLUSION:GrowerscanimplementweedresistanceBMPsystemswiththeconfidencethattheirnetreturnswillbeequivalent in the short run, and, in the long term, resistance BMP systems will prevent or delay the evolution of GR weeds in their fields, resulting in substantial savings. c � 2011 Society of Chemical Industry

The growth regulator herbicides 2,4-D and dicamba are used to control glyphosate-resistant horseweed before crops are planted. With the impending release of 2,4-D–resistant and dicamba-resistant crops, use of these growth regulator herbicides postemergence will likely increase. The objective of this study was to determine the effectiveness of various growth regulators on Indiana horseweed populations. A greenhouse dose–response study was conducted to evaluate the effectiveness of 2,4-D ester, diglycolamine salt of dicamba, and dimethylamine salt of dicamba on control of four populations of horseweed in the greenhouse. Population 66 expressed twofold levels of tolerance to 2,4-D ester and diglycolamine salt of dicamba. Population 43 expressed an enhanced level of tolerance to diglycolamine salt of dicamba but not to the other herbicides. Diglycolamine salt of dicamba provided the best overall control of populations 3 and 34. Additionally, a field study was conducted to evaluate standard use rates of 2,4-D amine, 2,4-D ester, diglycolamine salt of dicamba, and dimethylamine salt of dicamba on control of various sized glyphosate-resistant horseweed plants. Control of plants 30 cm or less in height was 90% or greater for all four herbicides. On plants greater than 30 cm tall, diglycolamine salt of dicamba provided 97% control while 2,4-D amine provided 81% control. Diglycolamine salt of dicamba provided the highest level of control of glyphosate-resistant horseweed, followed by dimethylamine salt of dicamba, 2,4-D ester and 2,4-D amine, respectively. This research demonstrates that horseweed populations respond differently to the various salts of 2,4-D and dicamba, and it will be important to determine the appropriate use rates of each salt to control glyphosate-resistant horseweed.

Horseweed can be a problematic weed in no-till soybean fields and populations can vary in their response to 2,4-D. The objective of this study was to evaluate the growth and seed production of four horseweed populations after exposure to 2,4-D. 2,4-D amine was applied at 0, 140, 280, and 560 g ae ha−1 to 5- to 10-cm-tall horseweed plants. An additional treatment of 280 g ha−1 of 2,4-D + 840 g ae ha−1 of glyphosate was included in the study. At 2 wk after treatment (WAT), injury ranged from 47 to 98%, but by 6 WAT the injury ranged from 89 to 100% for all four populations. Between 6 and 12 WAT some individual horseweed plants started to recover. No differences in dry weights were observed between the four populations in the untreated checks at 0, 2, 6, and 12 WAT. At 280 g ha−1 of 2,4-D amine, seed production was reduced by greater than 95%. However, three of the four horseweed populations had plants that survived and produced seed after exposure to 840 g ha−1 of glyphosate + 280 g ha−1 of 2,4-D. One plant produced seed after exposure to 560 g ha−1 of 2,4-D. These results suggest that horseweed can evolve resistance to 2,4-D and no fitness penalities were observed in populations that had higher levels of tolerance to 2,4-D.

Greenhouse studies were conducted to determine the prevalence of resistance to acetolactate synthase (ALS)-inhibiting herbicides in 266 Indiana horseweed populations, both glyphosate-susceptible and glyphosate-resistant, and to characterize the response of selected biotypes to combinations of glyphosate and cloransulam. Populations with individuals resistant to ALS inhibitors were more frequent in the northern half (38% of the populations in the NW and 50% of the populations in the NE) of Indiana than in the southern half (26% of the populations in the SW and 5% of the populations in the SE). Only 2% of the populations appeared to be resistant to both glyphosate and ALS inhibitors in an initial greenhouse study. Horseweed populations with resistance to ALS inhibitors exhibited herbicide doses required for 50% reduction in plant growth (GR50) values ranging from 14 to 255 g ai ha−1of cloransulam. The resistant : susceptible (R : S) ratio for four horseweed populations with suspected resistance to glyphosate and ALS inhibitors ranged from 0.3 to 50 and from 2.5 to 8.1 for cloransulam and glyphosate, respectively. The tank mixtures exhibited an antagonistic response to 3 of the 16 combinations of cloransulam and glyphosate on the susceptible population. The tank mixtures exhibited primarily an additive response to those same combinations in the multiple-resistant populations, but the response was occasionally synergistic for two of the four populations. The additive response between glyphosate and cloransulam indicates that, where the level of resistance is fairly low, combinations of these herbicides should be more effective for control of multiple-resistant populations compared with application of a single herbicide.

Glyphosate-resistant (GR) crops have facilitated increases in conservation tillage production practices and simplified weed control in GR corn, soybean, canola and cotton. Increased reliance on glyphosate, many times as the only active ingredient used, has resulted in weed species shifts and the evolution of weed populations resistant to glyphosate. However, weed shifts and the evolution of herbicide resistance are not new in regard to glyphosate use. Similar effects have been documented to many other historically important weed control advancements for agricultural crop production. GR crop technology was devel- oped to utilize glyphosate for postemergence weed control and industry scientists suggested that there was little fear of weed shifts and resistance evolution due to the broad spectrum of weeds controlled by glyphosate. However, over the last decade, the most problematic weeds in agronomic cropping sys- tems have shifted away from perennial grass and perennial broadleaf weeds to primarily annual broadleaf weeds. The evolution of several GR annual broadleaf weeds in GR cropping systems has been documented, and glyphosate resistance mechanisms in weeds are currently poorly understood.

Horseweed is an increasingly common and problematic weed in no-till soybean production in the eastern cornbelt due to the frequent occurrence of biotypes resistant to glyphosate. The objective of this study was to determine the influence of crop rotation, winter wheat cover crops (WWCC), residual non-glyphosate herbicides, and preplant application timing on the population dynamics of glyphosate-resistant (GR) horseweed and crop yield. A field study was conducted from 2003 to 2007 in a no-till field located at a site that contained a moderate infestation of GR horseweed (approximately 1 plant m−2). The experiment was a split-plot design with crop rotation (soybean–corn or soybean–soybean) as main plots and management systems as subplots. Management systems were evaluated by quantifying in-field horseweed plant density, seedbank density, and crop yield. Horseweed densities were collected at the time of postemergence applications, 1 mo after postemergence (MAP) applications, and at the time of crop harvest or 4 MAP. Viable seedbank densities were also evaluated from soil samples collected in the fall following seed rain. Soybean–corn crop rotation reduced in-field and seedbank horseweed densities vs. continuous soybean in the third and fourth yr of this experiment. Preplant herbicides applied in the spring were more effective at reducing horseweed plant densities than when applied in the previous fall. Spring-applied, residual herbicide systems were the most effective at reducing season-long in-field horseweed densities and protecting crop yields since the growth habit of horseweed in this region is primarily as a summer annual. Management systems also influenced the GR and glyphosate-susceptible (GS) biotype population structure after 4 yr of management. The most dramatic shift was from the initial GR : GS ratio of 3 : 1 to a ratio of 1 : 6 after 4 yr of residual preplant herbicide use followed by non-glyphosate postemergence herbicides.

A survey of farmers from six U.S. states (Indiana, Illinois, Iowa, Nebraska, Mississippi, and North Carolina) was conducted to assess the farmers’ views on glyphosate-resistant (GR) weeds and tactics used to prevent or manage GR weed populations in genetically engineered (GE) GR crops. Only 30% of farmers thought GR weeds were a serious issue. Few farmers thought field tillage and/or using a non-GR crop in rotation with GR crops would be an effective strategy. Most farmers did not recognize the role that the recurrent use of an herbicide plays in evolution of resistance. A substantial number of farmers underestimated the potential for GR weed populations to evolve in an agroecosystem dominated by glyphosate as the weed control tactic. These results indicate there are major challenges that the agriculture and weed science communities must face to implement long-term sustainable GE GR-based cropping systems within the agroecosystem. Nomenclature: Glyphosate.

Over 175 growers in each of six states (Illinois, Indiana, Iowa, Mississippi, Nebraska, and North Carolina) were surveyed by telephone to assess their perceptions of the benefits of utilizing the glyphosate-resistant (GR) crop trait in corn, cotton, and soybean. The survey was also used to determine the weed management challenges growers were facing after using this trait for a minimum of 4 yr. This survey allowed the development of baseline information on how weed management and crop production practices have changed since the introduction of the trait. It provided useful information on common weed management issues that should be addressed through applied research and extension efforts. The survey also allowed an assessment of the perceived levels of concern among growers about glyphosate resistance in weeds and whether they believed they had experienced glyphosate resistance on their farms. Across the six states surveyed, producers reported 38, 97, and 96% of their corn, cotton, and soybean hectarage planted in a GR cultivar. The most widely adopted GR cropping system was a GR soybean/non-GR crop rotation system; second most common was a GR soybean/GR corn crop rotation system. The non-GR crop component varied widely, with the most common crops being non-GR corn or rice. A large range in farm size for the respondents was observed, with North Carolina having the smallest farms in all three crops. A large majority of corn and soybean growers reported using some type of crop rotation system, whereas very few cotton growers rotated out of cotton. Overall, rotations were much more common in Midwestern states than in Southern states. This is important information as weed scientists assist growers in developing and using best management practices to minimize the development of glyphosate resistance.

Corn and soybean growers in Illinois, Indiana, Iowa, Mississippi, Nebraska, and North Carolina, as well as cotton growers in Mississippi and North Carolina, were surveyed about their views on changes in problematic weeds and weed pressure in cropping systems based on a glyphosate-resistant (GR) crop. No growers using a GR cropping system for more than 5 yr reported heavy weed pressure. Over all cropping systems investigated (continuous GR soybean, continuous GR cotton, GR corn/GR soybean, GR soybean/non-GR crop, and GR corn/non-GR crop), 0 to 7% of survey respondents reported greater weed pressure after implementing rotations using GR crops, whereas 31 to 57% felt weed pressure was similar and 36 to 70% indicated that weed pressure was less. Pigweed, morningglory, johnsongrass, ragweed, foxtail, and velvetleaf were mentioned as their most problematic weeds, depending on the state and cropping system. Systems using GR crops improved weed management compared with the technologies used before the adoption of GR crops. However, the long-term success of managing problematic weeds in GR cropping systems will require the development of multifaceted integrated weed management programs that include glyphosate as well as other weed management tactics.

A phone survey was administered to 1,195 growers in six states (Illinois, Indiana, Iowa, Mississippi, Nebraska, and North Carolina). The survey measured producers' crop history, perception of glyphosate-resistant (GR) weeds, past and present weed pressure, tillage practices, and herbicide use as affected by the adoption of GR crops. This article describes the changes in tillage practice reported in the survey. The adoption of a GR cropping system resulted in a large increase in the percentage of growers using no-till and reduced-till systems. Tillage intensity declined more in continuous GR cotton and GR soybean (45 and 23%, respectively) than in rotations that included GR corn or non-GR crops. Tillage intensity declined more in the states of Mississippi and North Carolina than in the other states, with 33% of the growers in these states shifting to more conservative tillage practices after the adoption of a GR crop. This was primarily due to the lower amount of conservation tillage adoption in these states before GR crop availability. Adoption rates of no-till and reduced-till systems increased as farm size decreased. Overall, producers in a crop rotation that included a GR crop shifted from a relatively more tillage-intense system to reduced-till or no-till systems after implementing a GR crop into their production system.

A telephone survey was conducted with growers in Iowa, Illinois, Indiana, Nebraska, Mississippi, and North Carolina to discern the utilization of the glyphosate-resistant (GR) trait in crop rotations, weed pressure, tillage practices, herbicide use, and perception of GR weeds. This paper focuses on survey results regarding herbicide decisions made during the 2005 cropping season. Less than 20% of the respondents made fall herbicide applications. The most frequently used herbicides for fall applications were 2,4-D and glyphosate, and these herbicides were also the most frequently used for preplant burndown weed control in the spring. Atrazine and acetochlor were frequently used in rotations containing GR corn. As expected, crop rotations using a GR crop had a high percentage of respondents that made one to three POST applications of glyphosate per year. GR corn, GR cotton, and non-GR crops had the highest percentage of growers applying nonglyphosate herbicides during the 2005 growing season. A crop rotation containing GR soybean had the greatest negative impact on non-glyphosate use. Overall, glyphosate use has continued to increase, with concomitant decreases in utilization of other herbicides. Nomenclature: 2,4-D; acetochlor; atrazine; glyphosate; corn, Zea mays L.; cotton, Gossipium hirsutum L.; soybean, Glycine max (L.) Merr.

Weed control strategies based on conserving crop yields rather than preventing weed seed production may result in increased weed densities and management costs over the long-term, particularly in less competitive crops such as tomatoes. The effect of crop, tillage, and duration of weed control on weed seed bank size and composition was examined from spring 2001 to spring 2003 near Lafayette, IN. Main plots in 2001 and 2002 contained soybean or tomato planted in rotation (soybean-tomato, tomato-soybean). Subplots were managed with either conventional or no-till practices. Sub-subplots contrasted threshold strategies in which weeds were either controlled for four to six weeks (period threshold, PT) or throughout the growing season (no-seed-threshold, NST). Seed banks were sampled annually in the spring. Emergent weeds were counted` at four and twelve weeks after planting (WAP) in 2001 and 2002. Weed seed banks did not significantly change in the NST plots in any year. However, seed bank densities increased substantially following tomatoes in PT plots. In contrast, weed seed bank densities decreased following soybeans in PT plots. The difference in seed banks and emergent weeds between soybean and tomatoes could be attributed primarily to greater suppression of giant foxtail by the soybean canopy. Giant foxtail control was greater in PT soybeans than in PT tomatoes in both years and giant foxtail comprised most of the PT tomato seed bank in 2002 and 2003. Tillage did not affect weed seed banks in any year. This study highlights the need to control later emerging weeds in tomatoes to prevent large increases in the weed seed bank.

2,4-D is often used as a preplant burndown herbicide to help control horseweed and other broadleaf weeds before planting in no-till corn and soybean production. Isolated instances of poor horseweed control have occurred in production fields. The objective of this research was to evaluate the response of various horseweed populations to 2,4-D. In the first study, 478 horseweed populations from Indiana were subjected to 280 g ae ha−1of 2,4-D amine in the greenhouse. This rate of 2,4-D caused visible injury and prevented all biotypes from forming new leaves for 28 days. There were specific populations where all plants sprayed were alive at 28 days after treatment (DAT), and approximately 10% of all populations had a least one plant that survived 280 g ae ha−12,4-D, resumed growth, and produced seed. In a dose-response study, we observed populations with three-fold more tolerance to 2,4-D. The most tolerant population had a GR90of 513 g ae ha−1and the most susceptible population had a GR90of 121 g ae ha−1based on dry weights. Growth suppression with 2,4-D was not affected by rosette size for rosettes between 0.5 and 10 cm in width.

Late-season field surveys conducted in Indiana from 2003 to 2005 showed that common lambsquarters and giant ragweed plants were present in 11 and 22%, respectively, of randomly sampled soybean fields that also contained glyphosate-resistant horseweed. In the fall of 2005 and 2006, seed from 13 common lambsquarters and 22 giant ragweed populations were collected from previously surveyed fields that had confirmed glyphosate-sensitive or -resistant horseweed. The objective of this study was to determine whether the presence of glyphosate-resistant horseweed was correlated with the presence of common lambsquarters and giant ragweed biotypes with elevated tolerance to glyphosate. Through a series of greenhouse screens, 57% of common lambsquarters and 31% of giant ragweed populations collected from fields that had glyphosate-resistant horseweed expressed elevated levels of glyphosate tolerance. However, elevated tolerance to glyphosate was expressed by 33% of giant ragweed and 100% of common lambsquarters populations collected in fields that had glyphosate-sensitive horseweed. Therefore, under the parameters of this experiment and through different types of analyses, we concluded there was not a strong correlation between the late-season presence of glyphosate-resistant horseweed and common lambsquarters and giant ragweed populations with elevated glyphosate tolerance in the same field. A number of the weed populations expressed significant stunting from exposure to glyphosate, but were able to resume growth. Thus, researchers should evaluate plant regrowth in addition to biomass suppression when making assessments of glyphosate resistance in weed populations through greenhouse and field screening.

Summary Glasshouse and laboratory experiments were conducted on acetolactate synthase (ALS) homozygous resistant Solanum ptycanthum biotypes from Illinois (IL-R) and Indiana (IN-R), and homozygous susceptible biotypes from Illinois (IL-S) and Indiana (IN-S). Genetic similarity of biotypes was assessed by random amplified polymorphic DNA (RAPD) markers, which determined that the Illinois biotypes are more similar to each other than to the IN-R biotype. ALS enzyme activity from the IL-R and IN-R biotypes had I50 values of 362 and 352 μM imazamox respectively. Dose–response experiments using three- to four-leaf-stage plants of the IL-R and IN-R biotypes had GR50 values of 242 and 69 g ae ha−1 imazamox respectively. Whole-plant and ALS enzyme results are different than previously reported values in the literature, which was attributed in the current study to the original IN-R population having individuals that were segregating for ALS resistance. Metabolism studies showed no difference in percentage [14C]imazamox remaining between the IL-R and IN-R biotypes up to 72 h after treatment. The IL-S biotype metabolised [14C]imazamox approximately two times faster than the IL-R and IN-R biotypes and this trait was heritable. Response of F3 plants containing homozygous ALS-resistant alleles from the IL-R biotype in a genetic background of 50% Illinois and 50% Indiana biotypes suggests that genetic factors other than an altered target site or metabolism may also contribute to the magnitude of resistance at the whole-plant level in resistant biotypes.

The effect of different cytokinins on in vitro adventitious shoot regeneration from internodal explants of Menthaxgracilis Sole (scoth spearmint) was investigated. Murashige and Skoog (MS) medium containing 100 mg l−1 myo-inositol, 0.4 mg l−1 thiamine-HCl, 2.0% (w/v) sucrose, 10% (v/v) coconut water and supplemented with 4.5 μM thidiazuron (TDZ) was effective in inducing adventitious shoot formation from callus. The greatest percentage of explants with shoots (85%) with the highest mean number of shoots per explant (29) was obtained with explants from the 1st and the 2nd internodes from 2-wk-old stock plants growing on a medium containing MS basal salts, 2% sucrose, 100 mg l−1 myo-inositol, 0.4 mg l−1 thiamine-HCl, at TDZ 4.5 μM and 10% (v/v) coconut water and solidified with 0.2% (w/v) phytagel. The regenerated shoots rooted on a medium containing MS basal salts, 100 mg l−1 myo-inositol, 0.4 mg l−1 thiamine-HCl, 2.0% sucrose, and 0.054 μM naphthalene acetic acid (NAA). Micropropagated plantlets were transplanted into soil and acclimated to greenhouse conditions. This is the first report describing adventitious shoot regeneration of scotch spearmint.

An in vitro shoot regeneration procedure was developed for native spearmint (Mentha spicata L.) using internodal explants. Shoot regeneration from internodes was evaluated on Murashige and Skoog (MS) media supplemented with individual cytokinins thidiazuron (TDZ), benzylaminopurine (BA), kinetin (KT), or zeatin (ZT) or various pair wise combinations of these. The highest regeneration was achieved by the second internode on a medium containing MS basal salts, B5 vitamins, 10% coconut water, 1.0 mg·L -1 TDZ, 2.5 mg·L -1 ZT, and solidifi ed with 0.2% phytagel. Unlike previous protocols this medium does not need sub culturing and produces elongated shoots in 4 weeks, rather than 6 weeks. Maximum number of shoots (36 per explant after 4 weeks) was observed when internodes from 2-week-old stock plants were used as explant source. The shoots were removed and roots were initiated on medium containing MS basal salts, 0.4 mg·L -1 thiamine-HCL, 100 mg·L -1 myo-inositol, 7.5 g·L -1 agar and 0.01 mg·L -1 ∝-napthaleneacetic acid (NAA) and then plants were transferred to the greenhouse 2 weeks after root initiation, where 100% of the plantlets developed into healthy plants. The genus Mentha (Lamiaceae) is grown throughout the world and contains numer- ous economically important essential-oil producing plants, with Mentha ×piperita L. (peppermint) and Mentha spicata L. (native spearmint) being the most important species. The essential oils are synthesized and produced in the glandular trichomes on leaves and stems. Oil from spearmint contains mostly carvone (Croteau et al., 1994a), which is responsible for the characteristic spearmint fl avor and is widely used in dental care and food products, including candies and gums. By comparison, the oil from peppermint is high in menthol, which is responsible for the characteristic peppermint fl avor. Peppermint production is limited by a soil fungus that causes verticil- lium wilt (Verticillium dahliae Kleb.), which can persist in the soil for up to fi fteen years. In contrast, native spearmint is highly resistant to verticillium wilt and can be grown in fi elds infected with the verticillium wilt fungus. Spearmints do not express the limonene-3- hydroxylase enzyme, which is required for menthol synthesis from geranylpyrophosphate (Croteau, 1991; Croteau et al., 1994b). How- ever scientists have speculated that the activity of limonene-3-hydroxylase might be increased in spearmint using genetic approaches to make it produce oil with high menthol content (Goddijin and Pren, 1995; Lange and Croteau, 1999). Potentially then, a high menthol oil could be produced by native spearmint, even in soils infected with verticillium. Genetic improvement of spearmint to modify oil quality through conventional breeding methods how- ever is constrained by near complete sterility of the species (Constabel, 1990; Croteau et al., 1991; Larkin, 1981). To overcome these constraints, biotechnological approaches may provide a means to insert the genes required for converting limonene to menthol. However, the lack of an effi cient regeneration system has limited our ability to genetically modify native spearmint using gene transfer or mutagenesis and there is a need for a highly effi cient in vitro regeneration protocol for native spearmint if genetic improvements using these new tech- nologies are to be applied. Regeneration of spearmint using cotyledons, hypocotyls (Van Eck and Kitto, 1990), and protoplast (Sato et al., 1993) have had limited success. The best spearmint regeneration system published to date employed leaf explants, however, these produced low shoot regeneration (60%), and in the process, required excessive culture periods (Li et al., 1999). Internodes had high regenera- tion capacity in other species of Mentha (Bhat et al., 2002; Shasany et al., 1998), leading us to speculate that native spearmint might also respond well to internode culture. In this pa- per, we report signifi cant improvement in the regeneration effi ciency of native spearmint from internodal explants using specifi c pair wise combinations of cytokinins as media supplements, and the optimization of explant size, position, and stock plant age. Material and Methods

Trichoderma virens (Gliocladium virens) (Miller et al.) von Arx is a soilborne fungus with a high degree of rhizosphere competence that produces a potent herbicidal compound, viridiol, and therefore has potential for development as a bioherbicide. We investigated the possibility of using composted chicken manure (CCM) as a medium for the production and deployment of T. virens. We chose CCM since the safe disposal of chicken manure presents significant logistic problems, and composted manures, as well as serving as an organic source of nitrogen, have been shown to support the activity of other biological control agents. Composted chicken manure supported the growth of T. virens and the rapid production of high concentrations of viridiol, but only when it was supplemented with large quantities of nutrients, including sucrose (16% w/w). Viridiol was not stable when stored in CCM, with a rapid decline in viridiol concentrations observed in T. virens-inoculated CCM cultures. Clearly, a cheaper alternative to sucrose is required as a carbon source for T. virens in CCM or similar media, and effective storage methods would need to be found for a T. virens-based bioherbicide product. Importantly, CCM did not need to be sterilized to support the growth of T. virens and its concomitant production of viridiol, suggesting that on-farm production systems may be feasible. Trichoderma virens-colonized CCM reduced the emergence and seedling growth of redroot pigweed (Amaranthus retroflexus L.) in a greenhouse experiment and dramatically reduced the emergence of a mixed community of broadleaf weeds in the field.

A number of techniques exist that have potential to be integrated into low-external-input weed management systems but it is not the norm for these different techniques to be investigated in combination. Here, we evaluate the weed management potential of two cultural weed management techniques, cover cropping and fertility management, with two allelochemical-releasing organisms compared to herbicides. Trichoderma virens (=Gliocladium virens), which releases the herbicidal molecule viridiol, is applied incorporated in composted chicken manure (CCM), and rye, which is known to release the herbicidal molecules (3H)-benzoxazolinone (BOA) and 2,4-dihydroxy-1,4-(2H)benzoxazine-3-one (DIBOA), is used as the cover crop. Both T. virens-inoculated CCM and rye cover crops provided significant weed control in some experiments without a significant loss of crop vigor and yield. Yield was comparable to those obtained with chemical herbicides, although the results were not consistent. Clear weed suppressive activity exists with these nonherbicidal treatments, and effective weed management in transplanted vegetables seems feasible with these approaches. However, further refinements in these treatments is needed to ensure reliable performance prior to grower acceptance.

Verticillium dahliae Klebahn is a soil-borne fungal pathogen causing vascular diseases. The pathogen penetrates the host through the roots, spreads through the xylem, and systemically colonizes both resistant and susceptible genotypes. To elucidate the genetic and molecular bases of plant-Verticillium interactions, we have developed a pathosystem utilizing Arabidopsis thaliana and an isolate of V. dahliae pathogenic to both cruciferous and non-cruciferous crops. Relative tolerance (based on symptom severity) but no immunity was found in a survey of Arabidopsis ecotypes. Anthocyanin accumulation, stunting, and chlorosis were common symptoms. Specific responses of the more susceptible ecotype Columbia were induction of early flowering and dying. The more tolerant ecotype C-24 was characterized by pathogen-induced delay of transition to flowering and mild chlorosis symptoms. Genetic analysis indicated that a single dominant locus, Verticillium dahliae-tolerance (VET1), likely functioning also as a negative regulator of the transition to flowering, was able to convey increased tolerance. VET1 was mapped on chromosome IV. The differential symptom responses observed between ecotypes were not correlated with different rates of fungal tissue colonization or with differential transcript accumulation of PR-1 and PDF1.2 defense genes whose activation was not detected during the Arabidopsis-V. dahliae interaction. Impairment in salicylic acid (SA)- or jasmonic acid (JA)-dependent signaling did not cause hypersensitivity to the fungal infection, whereas ethylene insensitivity led to reduced chlorosis and ABA deficiency to reduced anthocyanin accumulation. The results of this study clearly indicated that the ability of V. dahliae to induce disease symptoms is also connected to the genetic control of development and life span in Arabidopsis.

Protocols and media constituents for efficient in vitro plant regeneration of Native Spearmint (Mentha spicata L. cultivar ‘Native Spearmint’) have been defined. Adventitious shoots were initiated either directly from morphogenetically competent cells of explants or primary callus. Leaf explants from at least 2-mo.-old in vitro-maintained shoots exhibited the greatest morphogenetic capacity. Explants derived from basal portions of leaves at the bottom of the shoot were most responsive, with up to a 100% regeneration frequency and greater than nine shoots per explant. Highest frequency of meristemoids and morphogenetic callus were initiated from explants cultured onto a basal medium containing Murashige and Skoog (MS) salts, supplemented with 4 mg thidiazuron (TDZ) per L and 25% (vol/vol) coconut water (CW) for 10 to 14 d in darkness. Bud and shoot development required removal of both TDZ and CW from the medium. Shoot propagules were transferred to basal medium supplemented with 0.01 mg α-naphthaleneacetic acid (NAA) per L and grown under low light for about 2 wk to facilitate shoot elongation. Individual shoots about 1 cm tall were dissected and retransferred onto the same medium. Root initiation began within 4 to 6 d and a functional root system developed within 2 to 3 wk. These plantlets were transferred to soil and acclimated successfully for growth and development in a greenhouse. This is the first report of an efficient regeneration system for Native Spearmint based on adventitious organogenesis.

Field bindweed biotypes differ in glyphosate tolerance. Our previous studies have indicated quantitative genetic inheritance of this trait. In order to study the genetics of these differences we must be able to make sexual crosses, however, self-crossing and backcrossing are difficult due to self-incompatibility (SI) in field bindweed flowers. Therefore research was conducted to characterize and overcome the SI system in field bindweed. We determined SI to be of the multiallelic, sporophytic type based on observations of pollen-stigma interaction, and in the pattern of compatibility of F1 backcrosses to the parent biotypes. Pollen in incompatible crosses produced short pollen tubes that failed to penetrate the stigma surface. Backcrossing showed most F1 plants to be compatible with one parent, indicating dominance interactions between alleles. SI in field bindweed was successfully overcome by both bud pollination and heat treatment which exposed plants to 38 degrees C for 2 days before pollination, however, no seeds were produced. Treatment of stigma tips with a hot soldering iron for 2-3 s allowed abundant pollen tube growth from selfed pollen lying in a zone adjacent to the heat-injured area. This method resulted in production of viable selfed seed at rates equal to that observed in outcrosses.

Two biotypes of field bindweed differing in their susceptibility to glyphosate were used to determine if diluent or carrier volume and additional surfactant could overcome differences in intraspecific response to glyphosate. In greenhouse studies, glyphosate (formulated product) was applied at 1.68 kg/ha in three diluent volumes (142, 189, and 237 L/ha), with and without 1 % (v/v) additional amphoteric surfactant. Nonparametric and ordinal categorical analyses indicated that field bindweed biotype, diluent volume, and surfactant significantly increased glyphosate phytotoxicity 7 DAT. Only biotype and volume were significant 21 DAT. The tolerant biotype was less injured at the 189 and 237 L/ha volumes than the susceptible biotype. Field bindweed injury was similar at a diluent volume of 142 L/ha for both biotypes. These greenhouse studies suggest that control of field bindweed may be improved with glyphosate by using low spray volume in concert with additional surfactant.

The stability and expression of amplified 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) genes was examined in glyphosate resistant tobacco cells grown in glyphosate-free medium, and in plantlets regenerated from resistant cells. Amplified DNA was maintained in resistant cells grown in the absence of glyphosate for three years. Amplified EPSPS genes were retained in regenerated plantlets at levels comparable to those observed in the resistant cells, and EPSPS mRNA was overexpressed (compared to unselected plantlets). However, glyphosate resistance in cell lines grown in glyphosate-free medium declined 7-fold, and in regenerated plantlets approximately 20-fold, compared to resistant cells maintained under glyphosate selection. In plantlets, reduced resistance correlated with lower levels of EPSPS mRNA. Plantlets regenerated from resistant cells exhibited morphological variation, and had an approximate doubling of their nuclear genome size.

A cell suspension culture was established from a transgenic petunia (Petunia hybrida L.) plant which carried genes encoding neomycin phosphotransferase II (nptII) and beta-glucuronidase (uidA, GUS). Two selection experiments were performed to obtain cell lines with increased resistance to kanamycin. In the first, two independently selected cell lines grown in the presence of 350 micrograms/ml kanamycin were eight to ten-fold more resistant to kanamycin than unselected cells. Increased resistance was correlated with amplification of the nptII gene and an increase in nptII mRNA levels. Selection for kanamycin resistance also produced amplification of the linked GUS gene, resulting in increased GUS mRNA levels and enzyme activity. Selected cells grown in the absence of kanamycin for twelve growth cycles maintained increased copy numbers of both genes, and GUS enzyme activity was also stably overexpressed. In a second selection experiment, a cell line grown continuously in medium containing 100 micrograms/ml kanamycin exhibited higher nptII and GUS gene copy numbers and an increase in GUS enzyme activity after eleven growth cycles. In this cell line, amplification of the two genes was accompanied by DNA rearrangement.

Verticillium dahliae Klebahn is a soil-borne fungal pathogen causing vascular diseases. The pathogen penetrates the host through the roots, spreads through the xylem, and systemically colonizes both resistant and susceptible genotypes. To elucidate the genetic and molecular bases of plant-Verticillium interactions, we have developed a pathosystem utilizing Arabidopsis thaliana and an isolate of V. dahliae pathogenic to both cruciferous and non-cruciferous crops. Relative tolerance (based on symptom severity) but no immunity was found in a survey of Arabidopsis ecotypes. Anthocyanin accumulation, stunting, and chlorosis were common symptoms. Specific responses of the more susceptible ecotype Columbia were induction of early flowering and dying. The more tolerant ecotype C-24 was characterized by pathogen-induced delay of transition to flowering and mild chlorosis symptoms. Genetic analysis indicated that a single dominant locus, Verticillium dahliae-tolerance (VET1), likely functioning also as a negative regulator of the transition to flowering, was able to convey increased tolerance. VET1 was mapped on chromosome IV. The differential symptom responses observed between ecotypes were not correlated with different rates of fungal tissue colonization or with differential transcript accumulation of PR-1 and PDF1.2 defense genes whose activation was not detected during the Arabidopsis-V. dahliae interaction. Impairment in salicylic acid (SA)- or jasmonic acid (JA)-dependent signaling did not cause hypersensitivity to the fungal infection, whereas ethylene insensitivity led to reduced chlorosis and ABA deficiency to reduced anthocyanin accumulation. The results of this study clearly indicated that the ability of V. dahliae to induce disease symptoms is also connected to the genetic control of development and life span in Arabidopsis.