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Bipolaris sorokiniana (Sacc.) Shoemaker is a major foliar and root-infecting pathogen of cool-season forage and turf grasses and small grains in the southeastern United States (2). In North America, B. sorokiniana has been reported from bermudagrass (Cynodon dactylon [L.] Pers.) once in California in 1961 (1), and rarely from other warm-season grasses in the southeastern United States. In May, July, September, and October 2002, B. sorokiniana sporulation was observed on leaves of common bermudagrass exhibiting necrotic lesions and dieback in waste application fields on three commercial swine farms in Chickasaw, Lowndes, and Webster counties, MS. Leaves were collected (50 per farm per month), surface-disinfested, plated on water agar, and observed for fungal sporulation on leaf surfaces after 7 to 10 days (3,4). The pathogen was detected on 1 to 3 farms each month in leaves that were infected with numerous other dematiaceous hyphomycetes (3,4). Three randomly selected single-spore isolates of B. sorokiniana from each of bermudagrass and annual ryegrass (Lolium multiflorum Lam.), collected at the Webster County farm, were compared for select features of morphology and pathogenicity on bermudagrass. Isolates differed significantly in growth rates, amount of sporulation, and spore sizes on cornmeal agar, but differences were not consistently related to hosts of origin. In plants inoculated by atomizing equal quantities of spores (2.8 × 104/ml) onto foliage, isolates of B. sorokiniana from bermudagrass and ryegrass both caused significantly (P = 0.05) more severe foliar necrosis after 10 days than B. cynodontis (5 pots of seeded plants per treatment in each of two experiments). B. sorokiniana was reisolated from disinfested, symptomatic bermudagrass leaf tissue following inoculations. To our knowledge, this is the first report of B. sorokiniana on bermudagrass in North America outside of California (1) and indicates that this pathogen is highly virulent on bermudagrass in the southeastern United States (3,4). Of potentially greater importance is the fact that one of the most common and widespread forage and turf grass species in the southeastern United States can serve as an alternate host for maintenance and increase of inoculum of B. sorokiniana during summer months. References: (1) R. M. Endo. Plant Dis. Rep. 45:869, 1961. (2) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory, On-line publication. ARS, USDA, 2003. (3) R. G. Pratt. Agron. J. 92:512, 2000. (4) R. G. Pratt. Plant Dis. 85:1206, 2001.

Johnsongrass (Sorghum halepense (L.) Pers.), broadleaf signalgrass (Brachiaria platyphylla (L.) Beauv.), and yellow foxtail (Setaria glauca L.) are common volunteer grasses in bermudagrass (Cynodon dactylon (L.) Pers.) pastures in the southeastern United States. Johnsongrass and broadleaf signalgrass are potential forages whereas yellow foxtail is a noxious weed. In 1999 and subsequent years, necrosis and dieback of leaves, stems, and roots, stunting, and plant death were observed on all three species in bermudagrass pastures in north Mississippi (3). Symptoms on johnsongrass and yellow foxtail were most severe where bermudagrass exhibited severe symptoms of infection caused by dematiaceous hyphomycetes (2,3); symptoms on broadleaf signalgrass often occurred independently. Symptomatic leaf tissues from 15 to 33 plants of each species and stem and root tissues from 4 to 14 plants of johnsongrass and yellow foxtail were surface disinfested, plated on water agar, and examined for sporulation after 5 to 10 days (2,3). Pathogens were identified by specific morphological features of spores and sporulation as on bermudagrass (3), and axenic cultures were established by spore transfers to cornmeal agar. Bipolaris cynodontis (Marig.) Shoemaker, Curvularia lunata (Wakk.) Boedijn, C. geniculata (Tracy & Earle) Boedijn, and Exserohilum rostratum (Drechs.) Leonard & Suggs were isolated from symptomatic leaves of all three grasses and frequently also observed on stems and roots. B. stenospila (Drechs.) Shoemaker was observed only on broadleaf signalgrass (19 of 33 plants) and B. spicifera (Banier) Subr. on johnsongrass and yellow foxtail. Species most frequent on leaves (58 to 100%) were B. spicifera, C. lunata, and E. rostratum on johnsongrass and yellow foxtail and B. cynodontis, B. stenospila, and E. rostratum on broadleaf signalgrass. The three grasses were grown from seed in potting mix in the greenhouse (one plant per 375-cm3 container), and five replicates 31 to 60 days old were inoculated with a mixture of three isolates of each pathogen observed on them in two experiments. Conidia produced from infested wheat and oat grain were atomized onto foliage (1.2 to 4 × 104 conidia per ml, 20 ml per plant) as described (2). All pathogens incited similar necrotic lesions and streaks on the three grasses after 12 to 15 days, and B. stenospila also caused extensive golden yellow chlorosis on broadleaf signalgrass. All pathogens caused significant (P = 0.05) necrosis (means = 5 to 35% of foliage necrotic based on visual estimates, controls = 1 to 3%), and all were reisolated and grown in pure culture by spore transfers to cornmeal agar from surface-disinfested, symptomatic leaf tissue of each grass. When bermudagrass grown from seed was inoculated at similar spore concentrations, isolates of E. rostratum, B. cynodontis, and B. spicifera from two or all three grasses caused symptoms as severe as did isolates from bermudagrass. Results document new North American or worldwide records of occurrence and pathogenicity for B. cynodontis, C. geniculata, and C. lunata on all three grasses, B. stenospila and E. rostratum on broadleaf signalgrass, and B. spicifera on johnsongrass and yellow foxtail (1). These volunteer grasses, bermudagrass, and the six fungi all appear to represent large, interacting complexes of multiple hosts and potentially cross-infecting pathogens. Reference: (1) D. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory. Online publication. USDA, ARS, 2005. (2) R. Pratt, Agron. J. 92:512, 2000. (3) R. Pratt. Phytopathology 95:1183, 2005.

Fungal diseases of annual ryegrass (Lolium multiflorum Lam.), a cool-season species grown for forage and turf in the southeastern United States, and bermudagrass (Cynodon dactylon (L.) Pers.), a warm-season perennial, usually are considered distinct. In May 2002 and 2004, symptoms of leaf and stem necrosis were observed simultaneously in forage bermudagrass and overseeded annual ryegrass on a swine waste application site in Mississippi. Sporulation by nine species of Bipolaris, Curvularia, Drechslera, and Exserohilum was observed on symptomatic leaves of ryegrass that were surface disinfested and plated on agar each year, and axenic cultures were established by spore transfers. These isolations represent four new worldwide and two new North American records of occurrence of fungal pathogens on ryegrass, and three new records for the southeastern United States and Mississippi. Bipolaris cynodontis, a common bermudagrass pathogen, and Drechslera dictyoides were observed most frequently on ryegrass during both years. In excised leaves of ryegrass inoculated with infested agar discs, most isolates of B. cynodontis from ryegrass and bermudagrass were equally virulent and caused necrosis equal to or greater than that caused by D. dictyoides, an established ryegrass pathogen. Isolates of B. cynodontis from both hosts also caused similar symptoms in foliage of ryegrass and bermudagrass following spore inoculations. Pathogenicity to ryegrass of five other species of dematiaceous hyphomycetes that represented new worldwide or North American records of occurrence also was demonstrated by foliar inoculations. All pathogens were reisolated from symptomatic tissues and grown in axenic culture for fulfillment of Koch's postulates. Results demonstrate that most of the same species of dematiaceous hyphomycetes infect both forage ryegrass and bermudagrass in Mississippi and indicate that B. cynodontis, in particular, may be a common and virulent pathogen of ryegrass in the southeastern United States.

Sclerotinia trifoliorum and Sclerotium rolfsii are pathogens for which similar mechanisms of parasitism have been proposed. This suggested that resistance to these pathogens may be related in a common host plant. This study was undertaken to determine whether selection for quantitative resistance to Sclerotinia trifoliorum in alfalfa also increases resistance to Sclerotium rolfsii as expressed in excised leaf tissues and whole plants. Resistance in excised leaf tissues was evaluated according to the rate of necrosis induced by Sclerotium rolfsii following inoculation with mycelium. Resistance to Sclerotium rolfsii in whole plants was evaluated according to their survival following crown inoculations. Three alfalfa populations previously selected from cv. Delta for quantitative resistance to Sclerotinia trifoliorum exhibited enhanced resistance to Sclerotium rolfsii, in comparison with Delta or with susceptible populations, in excised leaf tissues. When whole plants of Delta and two of these populations, Sclerotinia trifoliorum resistant (STR) and Mississippi Sclerotinia resistant (MSR), were inoculated with Sclerotium rolfsii at 3 to 8 weeks of age, significant (P = 0.01) differences in survival were attributed to plant age at inoculation and alfalfa populations. Survival of both MSR and STR was significantly (P = 0.05) greater than for Delta; the best differential results were obtained by inoculating plants 5 to 7 weeks old. To evaluate relationships of resistance to Sclerotinia trifoliorum and Sclerotium rolfsii over a broader genetic background, additional populations were selected for resistance to Sclerotinia trifoliorum from four other alfalfa cultivars by leaf-inoculation techniques, and this resistance was confirmed by whole-plant inoculations. In excised leaf tissues, all four of these populations also expressed enhanced resistance to Sclerotium rolfsii in comparison with either parent cultivars or populations of comparable size selected at random. These results establish that selection for quantitative resistance to Sclerotinia trifoliorum in alfalfa also confers enhanced resistance to Sclerotium rolfsii as expressed in excised leaf tissues and whole plants. Results suggest, therefore, that genes for quantitative resistance to Sclerotinia trifoliorum and Sclerotium rolfsii in alfalfa are likely to be synonymous, overlapping, or closely linked.

USA, environmental stresses include saturated soils in the winter and hot, drought conditions in the summer. Summers in the southeastern USA produce a harsh environment Though environmental and other factors can reduce for survival of white clover (Trifolium repens L.) stolons. Long periods of drought and hot temperatures are interspersed with rain showers white clover growth and stands at any time of the year, that create ideal conditions for fungal pathogenesis. Previous studies complete death of the stand usually occurs in the sumindicated that Macrophomina phaseolina (Tassi) Goidanich may be mer (Gibson and Hollowell, 1966). an important pathogen that limits survival of white clover stolons in Fungal disease pressure plays an integral role in the the summer. The objective of this study was to determine the range survival of white clover stolons in the summer. The in response of 20 white clover cultivars, germplasms, and breeding normal summer environment of the southeastern USA and naturalized populations for resistance to M. phaseolina using a is ideal for fungal pathogenesis. Fungicides have been leaf tissue assay. Discs were cut from leaves excised from 50 plants shown to improve white clover yield and persistence of each entry and inoculated with an agar plug cut from the margin (James et al., 1980; Pederson et al., 1991). Fungicide of a M. phaseolina colony. Leaf discs were scored according to the rate of necrosis induced by the pathogen. The experiment was conducted as treatment during the summer months improves white a randomized complete block with four replicates and was repeated clover stolon density, growing point density, and stolon with 50 additional plants from each entry. Differences in responses length (Pederson and Pratt, 1995). Germplasm sources of entries to inoculation with M. phaseolina were observed in each differ in their response to fungicide treatment; Brown run of the experiment. Brown Loam Syn. 2 germplasm and North Loam Syn. 2 germplasm showed less response to fungiGA population had the least disease and the greatest number of plants cide treatment than ‘Regal’ and ‘Louisiana S-1’ white selected as resistant to M. phaseolina. Large-leaf plants selected for clover (Pederson and Pratt, 1995). These data suggest resistance gave highly consistent responses when retested, with 35% that this germplasm may have some resistance or tolerof the plants having no leaf necrosis following inoculation with M. phaseolina. The leaf tissue assay was not as reliable for selecting ance to summer fungal diseases. Naturalized populaconsistent resistant phenotypes among small-leaf white clover entries, tions that have survived in pastures for a number of as 37% of the plants selected as resistant were rated as susceptible years have also been suggested as sources of resistance upon retesting. Resistance to M. phaseolina was observed in adapted or tolerance to summer stolon diseases (Cope, 1978). white clover germplasm, and development of new cultivars with this Numerous fungi are frequently isolated from white resistance should improve white clover summer survival. clover stolons including Colletotrichum spp., Curvularia trifolii (Kauff.) Boedijn, Fusarium spp., Mycoleptodiscus terrestris (Gerdemann) Ostazeski, M. phaseolina, tolonsare the means by which individual white clo- Rhizoctonia spp., Sclerotium rolfsii Sacc., and others ver genotypes persist in the field. The seedling tap- (Halpin et al., 1963; Latch and Skipp, 1987; McGlohon, root system only survives for the first year or two of 1959; Pederson and Pratt, 1995). The relative imporgrowth. When the taproot, primary stem, and basal por- tance of individual fungal species for stolon survival has tion of the stolons die, plant survival is entirely depen- rarely been addressed. Some fungi appear to play no dent on new stolon growth from the terminal ends. Indi- active role in stolon necrosis, arriving only as secondary vidual stolons from the same plant are genetically pathogens when death and decay have already begun. identical, but may no longer be connected. New growth Other fungi may be quite pathogenic. Following a previand branching continues at the stolon tips while the ous study of white clover stolon growth with and without older basal portion of the stolon dies. The genotype will fungicide treatment (Pederson and Pratt, 1995), Pratt survive as long as new stolon growth at the tip exceeds (1998, unpublished data) evaluated the pathogenicity of the progression of necrosis from the basal end.

Previous studies established that excised leaf tissues of alfalfa can be inoculated with Sclerotinia trifoliorum to select for heritable resistance, but the original procedures were not practical for use in large-scale screening programs. In this study, simplified leaf inoculation procedures for more rapid screening for resistance, based on direct application of leaf tissues to colonies of the pathogen on agar media, were evaluated. Cotyledons, unifoliate leaves, and leaflets of trifoliolate leaves of plants 7 to 21 days old from three relatively susceptible cultivars and one resistant germ plasm were applied, with and without wounding, directly to colony margins of S. trifoliorum on cornmeal agar, V8 juice agar, and water agar. Leaves were scored according to the rate and extent of development of necrosis. Significant differences between alfalfa populations were expressed in unifoliate leaves and trifoliolate leaflets but not in cotyledons. Disease severity in the resistant germ plasm (Mississippi Sclerotinia-Resistant [MSR]) was less than in the three cultivars on all agar media. Wounding of leaf tissues increased disease severity and greatly reduced the incidence of symptomless leaves, which are indicated to be escapes, but wounding generally did not prevent expression of resistance in MSR. Results indicate that initial screening for resistance to S. trifoliorum in alfalfa seedlings may be accomplished by applying wounded unifoliate leaves and leaflets of trifoliolate leaves directly to colonies on cornmeal or V8 juice agars.

Effective breeding for economically important resistance to Sclerotinia trifoliorum Eriks in alfalfa (Medicago sativa L.) requires that the plant breeder have an understanding of any genetic basis for variability in measurements of resistance and the likelihood that selection for resistance to one isolate confers resistance to other isolates. Fifty alfalfa plants were repeatedly evaluated for their resistance to five isolates of S. trifoliorum. Tips of excised stems were inoculated with a S. trifoliorum isolate and stem necrosis was measured after 14 d. The inoculation of five stems from each of 50 plants with a single isolate was repeated five times. For the length of stem necrosis, the interaction of pathogen isolate and alfalfa plant was not significant but differences among alfalfa plants and among isolates were significant. The plants that, on average, developed larger necrotic regions did not have more or less variation in their replicated measurements of necrosis than those plants which averaged much smaller necrotic regions. The genotype of the alfalfa plant had a greater effect on the variability in replicate measurements of disease than the genotype of pathogen isolate. Resistance to different isolates were significantly correlated (r = 0.48-0.60) and selection of some plants for resistance to one isolate appeared to improve resistance to other isolates.

White clover (Trifolium repens L.) stands in the southeastern USA survive during summer drought predominantly through persistence of stolons. The decay and death of stolons during the hot, humid summer may be partially due to fungal diseases. This study determined the effect of summer fungicide applications on stolon survival of 'Regal' and 'Louisiana S-1' white clover compared with drought-tolerant Brown Loam Syn. No. 2 germplasm (BLSyn). The three white clover entries were grown in the field on a Catalpa silty clay (fine, montmorillonitic, thermic Fluvaquentic Hapludoll) at Mississippi State, MS, in three separate 1-yr studies. During summer stolon dormancy, plots of each entry were either untreated or sprayed biweekly with benomyl [methyl 1-(butylcarbamoyl)-2-benzimidazole-carbamate]. In 2 of 3 yr, BLSyn had 23 to 63% greater ground cover, 33 to 55% greater stolon density, and 30 to 47% greater relative live stolon length following the summer drought than Louisiana S-1. In all 3 yr, BLSyn had greater relative live stolon length (7-18%) following the summer drought than Regal. Plots treated with benomyl had 14 to 52% greater stolon density (in 2 of 3 yr), 14 to 40% greater stolon growing point density, and 9 to 27% greater relative live stolon length than untreated plots. Benomyl treatment gave less of an increase in relative live stolon length of BLSyn than in the two cultivars. These results suggest that fungal pathogenesis may reduce white clover stolon survival during summer dormancy, and that greater fungal disease resistance could be part of the mechanism of improved summer survival in BLSyn white clover.

White clover (Trifolium repens L.) stands commonly decline within 2 to 3 yr after establishment, due to insects, fungi, nematodes, viruses, and other factors. This study determined the effect of application of insecticide and fungicide, and infectation of southern root-knot nematode, Meloidogyne incognita (Kofoid & White) Chitwood, alone and in combinations, on white clover persistence, yield, and crude protein (CP) concentration

Bipolaris hawaiiensis has been reported on bermudagrass (Cynodon dactylon) and other Cynodon spp. from subtropical areas around the world (2). This pathogen has not previously been reported on bermudagrass in North America (1) nor has its virulence been compared with that of other Bipolaris spp. on this host. In July and October 1999, frequencies of dematiaceous hyphomycetous pathogens in live but symptomatic leaves of bermudagrass were determined on two poultry waste application sites in Smith and Covington counties, MS, where foliar disease symptoms were widespread. Common bermudagrass was being grazed in Covington County, and cv. Alicia was being grown for hay in Smith County. At each date and site, 100 stems with leaves exhibiting symptoms of chlorosis and necrosis were collected, and a single leaf with well-developed symptoms from each stem was assayed for pathogens by surface-disinfesting, plating on water agar, and observing fungal sporulation. Multiple species of pathogens were detected on most leaves. Identities and mean frequencies of observed pathogen species across both sites and sampling dates were Exserohilum rostratum (62%), Bipolaris cynodontis (98%), Curvularia lunata (28%), C. geniculata (20%), B. spicifera (3%), and B. hawaiiensis (3%). B. hawaiiensis was detected at both sites and on both sampling dates. It was distinguished from B. cynodontis by smaller conidia (14 to 28 μm long) and from B. spicifera by more than three pseudosepta per conidium. Virulence of B. hawaiiensis on bermudagrass, compared with B. cynodontis and B. spicifera, was assessed in two identical inoculation experiments using three pathogen-inoculated treatments plus an uninoculated control. In each experiment, foliage of 12-week-old plants in five replicate pots per treatment was sprayed with 4 × 104 conidia per ml of water of each pathogen. The pots were incubated under 12-h plant-growth lights at 25°C for 3 days in a water-saturated atmosphere to initiate infection and then grown for seven additional days in ambient air under plant-growth lights at 25°C. All three pathogens induced symptoms of chlorosis and necrotic lesions. Symptoms induced by B. hawaiiensis were similar in severity to those produced by B. spicifera and less severe than those produced by B. cynodontis. To our knowledge, this is the first report of B. hawaiiensis on bermudagrass in North America. The site in Smith County also apparently represents its northernmost known point of occurrence on this continent (2). References: (1) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. The American Phytopathological Society, St. Paul, MN, 1989. (2) A. Sivanesan. Graminicolous Species of Bipolaris, Curvularia, Drechslera, Exserohilum and their Teleomorphs. Mycol. Pap. No. 158, CAB International Mycological Institute, Wallingford, U.K., 1987.

Excised leaf tissues of alfalfa were inoculated with Sclerotinia trifoliorum to determine whether leaf-inoculation techniques may be used to screen for heritable and effective resistance to the pathogen. Leaf tissues were placed on water agar in petri plates, inoculated with mycelia of S. trifoliorum, and incubated at 17°C. Host responses were evaluated according to the rate and extent of necrosis that developed during 1 week. One thousand plants of cultivar Delta were screened for resistance by successive inoculation of excised unifoliate leaves, first trifoliate leaflets, and disks of tissue from later leaves. The five plants considered most resistant were intercrossed, and progeny were compared with those of five plants selected for susceptibility, five selected at random, and the parent cultivar. Progeny of resistant plants had significantly (P = 0.05) less severe disease than did progenies of susceptible and random plants and the parent cultivar when evaluated by inoculation of unifoliate leaves, first trifoliate leaflets, leaf disks, excised stems, and whole plants. In field experiments with natural infection during two growing seasons, progeny of resistant plants had less severe disease and produced significantly (P = 0.05) higher yields of forage through the first or second harvests than did progenies of susceptible and random plants and the parent cultivar. These results establish that inoculation of excised leaf tissue is an effective and efficient means to screen for resistance to S. trifoliorum in alfalfa.

Eight cultivars and one experimental population of alfalfa were artificially inoculated with five isolates each of Sclerotinia trifoliorum and S. sclerotiorum. Isolates of both species originated from different forage legume hosts and geographic areas in the United States. Inoculations were performed by dusting dried and comminuted mixtures of infested wheat and oat grain over foliage of 4-wk-old plants. Plants were maintained at 17-20 C with intermittent atmospheric saturation for 24 days after which plant survival was evaluated. Isolates of both Sclerotinia species differed significantly (P < 0.01) in virulence. Alfalfa cultivars differed significantly (P = 0.02) in susceptibility, and responses of cultivars to the two species were generally similar. Morida 77 was the most susceptible of the eight cultivars to both Sclerotinia species, and 5472 was the least susceptible. An experimental population (STR), previously selected from cultivar Delta for resistance to S. trifoliorum, expressed the least susceptibility to both Sclerotinia spp. Cultivar × isolate interactions were not significant for S. sclerotiorum but were significant (P < 0.01) for S. trifoliorum. These interactions appeared to be caused by differences in virulence of isolates and did not suggest the occurrence of pathogenic races. Significant (P < 0.01) experiment × cultivar and experiment × isolate interactions also occurred for both species; possible causes are discussed. Results indicate that responses of these alfalfa cultivars to S. trifoliorum and S. sclerotiorum are generally similar, that selection for resistance to S. trifoliorum in alfalfa may also confer resistance to S. sclerotiorum, that no evidence for different pathogenic races was detected among the isolates, and that host of origin is not an important determinant for the virulence of Sclerotinia isolates on alfalfa

The variation in lengths of necrotic tissue following replicated stem inoculations with Sclerotinia trifoliorum was characterized on 25 plants in each of four alfalfa populations. The plants were ranked for resistance based on measurements in 12 replications, and the five most resistant and the five most susceptible (selection pressure of 20%) plants in each population were identified. Subsets consisting of 11, 9, 7, and 5 replicates were systematically or randomly selected by a computer program from the original 12 replicates, and the plants were ranked again and selected for resistance. The probability of reselecting plants previously identified as either one of the five most susceptible or resistant plants in each population was determined for each size subset

Reproduction and colonizing behavior of pea aphids, Acyrthosiphon pisum (Harris), were studied on reproductive arrowleaf clover, Trifolium vesiculosum Savi, using four treatments: healthy, infected with bean yellow mosaic virus (BYMV), infected with a Phytophthora sp. that caused root rot, or infected with both BYMV and the Phytophthora sp. Reproduction of aphids in no-choice experiments was significantly lower on plants in the two treatments with the root rot disease than on healthy or BYMV-infected plants. The highest level of reproduction was observed on healthy plants. Aphid colonization also was lower on fungus-infected plants whether BYMV was present or not. Results of these studies suggest that a complex relationship exists between aphid vectors, virus disease, and fungal root disease in arrow leaf clover. Presence of fungal root rot disease may influence reproduction and movement of alate aphids and epidemiology of aphid-transmitted viruses during both fall and spring periods of alate aphid activity, when plants are in vegetative or reproductive stages, respectively.

Hyphae of male and female isolates of Pythium sylvaticum grown from inoculum blocks with and without 0.01 g/liter cholesterol met and produced sexual unions in water agar. Fewest oospores formed when sterol was omitted from both male and female sides. Sterol added to the male side only did not cause a significant increase in percentage of mature oospores formed in sexual unions, but addition of sterol to the female side resulted in greatly increased numbers of oospores. When two pairs of isolates were mated and cholesterol was added to both sides, a further increase in oospore formation occurred that was significant at the 5% level with one pair and at the 1% level with another pair. These results are viewed as a dosage effect due to the relative amounts of sterol carried in the male and female game-tangia and not to a specific effect on either sex. Rare mature oospores were found in single cultures of the female type isolate of P. sylvaticum. Oospore formation in crosses of isolates N14 and N58 of Phytophthora capsici increased significantly when either isolate was grown from a medium containing 0.03 g/liter cholesterol. These results are viewed as effects of bisexual mating by both Phytophthora isolates.

Pythium sulcatum n. sp. was isolated from carrots with browned and forked roots grown in muck soils from Wisconsin and Florida. It is primarily distinguished by small oogonia and oospores and by antheridia which are variable in shape and frequently adorned with single or multiple transverse furrows and folds, for which the species is named. Zoospores are rarely produced; vesicles always arise from filamentous sporangia indistinguishable from vegetative hyphae. Vegetative hyphal bodies vary in shape and are outnumbered by sexual bodies on most media. Radial growth rates of P. sulcatum on potato dextrose agar are slower than those of P. irregulare, P. paroecandrum, and P. sylvaticum over a wide temperature range, and P. sulcatum may easily be distinguished from these species on that basis.

(...) Cultivar Woogenellup was grown in plots from September to May of each year. Infection of foliage by C. zebrina was initiated by applying infested debris to plots in late winter. Control plots received autoclaved debris. Burrs (seed heads) were harvested each spring and counted. Seed were threshed, counted, weighed, measured volumetrically, and evaluated by a standard germination test. (...)

Deep tillage and roguing of diseased plants were evaluated for effects on oospore populations of Peronosclerospora sorghi in soil and for control of sorghum downy mildew in grain sorghum. Roguing was conducted to simulate effects of 1- to 3-yr rotations with resistant crops by preventing completion of the disease cycle and addition of new inoculum to soil. Deep tillage (moldboard plowing to 30- 35 cm) was most effective when plowing was conducted in a downwind direction and not preceded by deep disking. Most oospores occurred at 0- 20 cm in soil with conventional tillage, whereas they were redistributed to 20- 40 cm with optimal deep tillage. Sorghum downy mildew incidence following optimal deep tillage was 14% of that obtained following conventional tillage. When deep tillage was conducted in an upwind direction or preceded by deep disking, oospore densities in upper soil and the incidence of sorghum downy mildew were less effectively reduced. Annual roguing of diseased plants from plots of susceptible cultivars reduced oospore densities to 22- 27%, and systemic downy mildew incidence to 11- 20% of levels observed in nonrogued plots after 3 yr. Results indicate that deep tillage, when properly conducted, and crop rotations or other treatments that prevent addition of new oospores to soil for 3 yr may provide economically beneficial control of sorghum downy mildew. However, some infection still occurred after almost 4 yr with little or no replenishment of oospore populations in soil

Etude des relations existant entre la densite de plantation, la competition, les caracteristiques de croissance, la pourriture interne de la couronne chez Trifolium vesiculosum. Identification et pouvoir pathogene des champignons associes a la maladie