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The transmission of PVS by the cutting knife in potato cvs Norland, Kennebec and Russet Burbank was studied in the field and greenhouse. Field studies showed transmission to Norland via tuber infection was significantly less than for Kennebec and Russet Burbank (P = 0.01). However, significant cultivar differences were not found in the greenhouse (P = 0.05). Contamination of the cutting knife by cutting through sprouts resulted in significantly greater numbers of plants infected for cv Kennebec than when sprout contact was avoided (P = 0.05). Although the same trends were observed for cv Norland and Russet Burbank the differences were not significant (P = 0.05). Infectious PVS particles were readily recovered from materials commonly contacted during potato cultivation, storage and processing. Retention time of infectious PVS was usually greater when materials were held at 4°C and 100% relative humidity than at ambient greenhouse conditions. Retention times ranged from 0 hr (PVS not recovered) to 180 hr on unpainted wood. Results demonstrated the need for strict indexing and sanitation procedures during the production of PVS-free potato seed.

The effect of three planting dates on stem, stolon, tuber number and size were compared throughout the 1979 and 1980 growing seasons. A relatively warm April and early May and a relatively cool late May and June in 1980 resulted in significantly higher stem and stolon numbers and a tuber population over double that of 1979. The highest average commercial yields ever recorded in the Columbia Basin were obtained with yields in some fields of 100 tons per ha on 48 hectare irrigation circles. On experimental plots tuber numbers increased from the latter part of May with successive sampling dates until late June and early July when a peak in number was reached after which a significant decline in number occurred. The decrease in tuber number was attributed to the phenomenon of reabsorption which was much greater with later planted potatoes. It occurred only in tubers of approximately 10 g size or smaller. Average tuber size in 1980 was one-half that of 1979 although total yield was 25 tons per ha higher because of significantly larger tuber numbers.

Potato early dying, incited byVerticillium dahliae, was found to be the major factor in reducing the yield potential of potato grown under our conditions. Immediate attempts at control involved the screening of commercial cultivars and lines bred in both the USA and Europe, and various soil fumigation treatments. This work led to the selection of cultivars with reasonably high tolerance and the elucidation of the role that the nematodePratylenchus thornei plays in the disease syndrome. Elimination of the nematode by sprinkler applied metham-sodium in conjunction with some of the above cultivars now produces yields of 40–60 metric T/ha, depending on the cultivar and season. More recently, our research has been directed towards the use of serological methods to detect the pathogen in locally produced seed tubers and the utilization of an extracellular toxin produced by the pathogen in determining resistance to the fungus. With regard to the former, we have been able to produce an antibody that reacts solely toV. dahliae and can be used in several serological tests to determine the presence of the pathogen. Our work has also shown that utilization of the toxin offers a highly reliable technique in determining at least certain forms of resistance.

Nine clones from a heat tolerant PHU-STN hybrid population were evaluated for flower production and percent normal, 2n and non-viable pollen under three temperature regimes at the Southeastern Plant Environmental Laboratories: 30/26, 26/22 and 22/18 C. No flowering was observed at 30/26 C. Flowering was irregular among seven of the clones at 26/22 C and 22/18 C. One clone produced numerous flowers at 26/22 C and 22/18 C. The plot of percent 2n pollen against time for this one clone under 22/18 C revealed a slope which was statistically different from zero but too small to be of practical value. The same plot of this clone under 26/22 C revealed a slope which was not statistically different from zero. Percent 2n pollen was statistically greater (at the 10% level of significance) in the 26/22 C chamber than the 22/18 C chamber.

Pollen from several tuber-bearingSolarium species was exposed to liquid nitrogen (LN2) and tested forin vitro germination and seed set capabilities. Pollen placed in LN2 or in the vapor phase above LN2 retained high levels ofin vitro germination after 11 to 24 months of storage. This pollen also set significant numbers of seed. Brief drying of the pollen over anhydrous CaCl2 prior to freezing increased survival in most samples. Rehydration of dried, LN2 treated pollen prior toin vitro germination testing was necessary to obtain maximum germination percentages. Rehydration, however, was not necessary for seed set, and in several samples decreased seed set compared to pollen that was not rehydrated. Storage of pollen from tuber-bearingSolanum species in LN2 appears to be practical for maintaining pollen for crossing and for germplasm preservation.

Field tests were conducted to determine the effectiveness of an experimental preparation ofBacillus thuringiensis var.thuringiensis against Colorado potato beetle infestations on potatoes and tomatoes. Applications were made weekly with a tractor-towed commercial sprayer at the rates of either 473, 946, or 2,838 ml. ofB.t.t. in 378.41 of water per 0.4 hectare. Significant reductions were observed in treated plots of all stages of the beetle. All untreated plants were completely defoliated by the end of the test due to beetle feeding. No significant differences were observed among the different treatment levels.

A survey of potato (Solanum tuberosum L.) plant stands in upstate New York over a three-year period showed that the average distance between plants was 4 cm wider than intended. Plant stands averaged 83%. The failure to plant seed pieces was the major reason for missing hills. Seed rot was not generally a problem, although one field in the survey was severely affected. Speed of planting up to 6.7 km/h had no consistent effect on average spacing or uniformity of seed piece distribution. Seed type (whole or cut) and mechanical condition of planter influenced spacing. Experiments conducted over four years indicated that the non-uniform seed piece distribution delivered by commercial picker-arm planters does not adversely affect crop yield or tuber size. Non-uniform seed distribution with extremely high coefficients of variation (greater than 100%), however, affected yield, tuber size and tuber number. Tuber numbers and yields were decreased by reducing the number of seed pieces planted per unit area whether or not the pieces were uniformly spaced.

Using the potato seedling B7906-1, stem thinning was carried out on plants having three, four, or five stems per hill. Treatments ranged from intact plants to those thinned by one, two, or three stems each. Data on the effect of stem thinning on yield and other vegetative parameters were analysed using an analysis of variance. Unlike leaf parameters, stem height and number of stolons per plant generally decreased with an increase in stem thinning. Within a control group, represented by the intact plants having the three stem densities used for treatments, the degree of association among different vegetative parameters and the measure of their contribution towards yield were determined by correlation and path coefficient analyses, respectively. Given the effect of number on weight of tubers per plant, the assessment of yield as affected by stem density treatments was confined to the former parameter. Among all parameters, number of stolons per plant was the most effective contributor to number of tubers harvested. The decrease in that parameter as a result of stem thinning, however, was not accompanied by a corresponding decrease in tuber number. The following relationship could be discerned: as number of stolons per plant decreased several stem parameters increased. Based on correlation and path analyses, it is suggested that the induced increase in number of compound leaves per stem could have compensated, directly or indirectly, for the adverse effect of stolon number on yield. Through stem density manipulation, the yield output of a variety in response to quantitatively induced, more diverse, vegetative characteristics could be evaluated.

Plant growth from Russet Burbank seed-tubers of different ages was compared as affected by source of meristematic tissue (apical or basal) and NAA treatment. In controlled environment studies, single-eye seedcores from 17-mo-old seed-tubers produced twice as many shoots compared to those from 5-and 29-mo-old seed-tubers. Seed-tuber age and shoot dry weight/core were inversely related. Dry weight/shoot decreased as age increased to 17 mo and then remained unchanged from 17 to 29 mo. Apical meristems lost growth potential faster than basal meristems as age advanced. Meristem physiological age apparently depends on relative position of the buds as well as on the chronological age of the seed-tuber. NAA (10–40 mg/l) decreased the number of shoots/core, had no effect on shoot dry weight/core but increased the dry weight/shoot when applied to seedcores from 20-mo-old seed-tubers. On seedcores from 8-mo-old seed-tubers. NAA decreased the shoot dry weight (g/core, g/shoot). In field studies, NAA (100 mg/l) increased the total tuber yield of plants grown from seedpieces cut from 8-and 20-mo-old seed-tubers. These increases were manifested in age-specific effects on tuber grade. The effects of seedtuber age on plant growth and tuber yield were partially overcome by seedpiece treatment with NAA.

In four years of field trials approximately75% of hollow heart was initiated in tubers between 5.1 and 7.6 cm in diameter. Only rarely was hollow heart initiated in tubers less than 3.8 cm. The frequency of hollow heart was approximately nine times greater in medium sized tubers (6.4 to 7.6 cm) and 34 times greater in large tubers (over 7.6 cm) than in small tubers (5.1 to 6.4 cm). The number of tubers with hollow heart increased linearly through the last five weeks of the growing season. Hollow heart during the four years ranged from 4200 to 31900 tubers/ha or 2.9 to 13.0% of all tubers over 2.5 cm. There were consistent increases in tuber growth between harvests in each season, inferring an absence of severe plant stress and supporting the hypothesis that hollow heart was due to tissue tension associated with tuber enlargement.

Field studies were conducted over a three-year period to investigate effects of potato virus X (PVX) on the Russet Burbank potato cultivar. Seed (PVX-free and PVX-infected) used for this three-year study originated from the same PVX-free source and was grown and stored under similar conditions. PVX-free seed was found to increase yield over PVX-infected plots by 9 to 32%. With these yield benefits, effects of PVX on either the incidence of verticillium wilt or plant nutrition were not significant. There was a trend (P = 0.10) for a reduction of mean tuber weight when the PVX infection level exceeded 19%. Because of increases of undersized potatoes, the yields of U.S. #1 potatoes were reduced as levels of PVX infection were increased. With a PVX infection level of 36%, the yield of U.S. #1 tubers was reduced by 21% when compared with plots having 0% PVX. At an 88% infection level, the yield of U.S. #l’s was reduced still further (36% lower than plots with 0% PVX). Results demonstrate the importance of the level of PVX infection to potato production.

The frequency of 2n egg formation in 15 haploid Tuberosum-wild species F1 hybrids was determined by seed/fruit (S/F) following 2x × 4x crosses and cytological analyses; the mechanisms of 2n megaspore formation were detected by microscopic observations of megasporogenesis using a stainclearing technique for whole ovules. The variability among clones was from 0–102 in S/F, from 1.3 to 56.9 in percent 2n eggs, and from 2.7 to 38.3 in percent aborted ovules. There was a statistically significant correlation between S/F and cytologically determined frequency of 2n eggs. Both S/F and percent 2n eggs in particular clones varied significantly, and appeared to be affected by environment. The modifications of the meiotic process that resulted in 2n egg formation were: 1) omission of the 2nd meiotic division after a normal 1st division, 2) failure of cytokinesis at Telo II followed by nuclear restitution, and 3) synaptic abnormalities followed by nuclear restitution and a normal 2nd division. Modifications 1 and 2 are genetically equivalent to second division restitution and 3 to first division restitution. Some clones producing 2n eggs also formed a small percentage of 4n megaspores from aberrations during both meiotic divisions. The diverse mechanisms of 2n egg formation suggest the presence of independent genetic control.

Abscisic acid (ABA), sucrose, reducing sugars and proline contents were monitored over an eleven-month period in tubers of three potato cultivars that varied in tuber dormancy. ABA and proline levels increased after top pulling the haulms and were not affected by harvest date. Proline did not change in either a storage temperature or cultivar-specific manner. The highest concentrations of ABA were found in tubers stored at 2°C while the lowest concentrations occurred in tubers stored at 20°C. At 10°C, the end of tuber dormancy in the cultivars Kennebec and Nooksack (but not Sebago) coincided with the decline in ABA content. There was no evidence of a threshold concentration of ABA below which sprouting would occur. Tuber samples of 10 different cultivars were removed from 10°C and placed in 20°C storage (in darkness). Initial ABA concentrations (i.e., at the time of removal from 10°C storage) were positively correlated with duration of dormancy and negatively correlated with subsequent rates of sprout elongation at 20°C. Sucrose content was negatively correlated with duration of dormancy. Reducing sugars responded primarily to storage temperature and did not appear to be related to dormancy or sprouting.

A study into the ecology ofErwinia carotovora in North Dakota was performed to determine possible sources ofErwinia spp. which recontaminate stem cut derived seed stocks (SCDS).Erwinia carotovora pv.carotovora (Ecc) was found to overwinter in naturally infested agricultural soils that had been planted to potato the previous cropping season. However, this bacterium was only present in the 30–71 cm sampling depth.E. carotovora pv.atroseptica (Eca) and Ecc were also found in the rhizospheres of a few weed and crop species but the frequency of recovery was low. Ecc and other strains not identifiable to pathovar, but notE. chrysanthemi, were readily recovered from a variety of water sources and from sunflowers with stem and head soft-rot symptoms. AllErwinia strains were characterized, but not all could be identified to pathovar. WateringErwinia-free potato plants with water naturally infested withErwinia spp. resulted in progeny tubers contaminated withErwinia. The importance of this information with regard to the recontamination of SCDS is discussed.

Farmers in Rwanda do not apply P fertilizer on their crops. Bray and Kurtz #1 extractable P levels are generally less than 2 ppm on these soils. Therefore, a long-term experiment was established to determine the yield response of potatoes to P fertilizer. A maximum accumulated yield of 130 t/ha in 5 crops was obtained with 902 kg/ha of P. The no P treatments produced 72% of the average maximum yield. The external P requirements, determined with P sorption isotherms, averaged 0.04 μg/ml if equilibrated at 15 C and 0.02 μg/ml if equilibrated at 25 C for the 5 crops. The residual efficiency of the applied P fertilizer was less than 20%, suggesting that large applications of P are not advisable on Andepts, which have amorphous materials because the applied P is not available for subsequent cropping seasons.

Potato (Solanum tuberosum L. cv. Russet Burbank) tuber periderm resistance to water loss was monitored during the harvest season. Periderm resistance of tubers from water-stressed plants was compared with tubers from unstressed plants throughout a six-week harvest period, in order to determine the effect of plant condition on tuber resistance to water loss. Tubers stored for ten days at 4°C immediately following harvest were also monitored to determine if storage enhanced periderm resistance and if any difference occurred between tubers from stressed vines compared with those from unstressed vines. During the harvest season, tuber resistance to water loss was positively correlated with plant water stress, as measured by leaf water potential and leaf diffusive resistance. Storage of tubers after harvest also caused an increase in periderm resistance. The commercial practice of vine killing, therefore, may have a positive effect on reducing weight loss in storage by enhancing the resistance of tuber periderm to water loss.

Epidemics of potato late blight progressed more slowly in plots of intermediate-aged plants than in plots of older plants. The suppression of epidemics achieved by intermediate-aged plants relative to old ones was equivalent to the suppression achieved by weekly applications of chlorothalonil of as much as 0.30 kg a.i./ha. The age effect on epidemic development was most noticeable for late-season cultivars and least noticeable for early-season cultivars.

Tuber and stem rots induced by Erwinia carotovora pv. carotovora (Ecc) and E. carotovora pv. atroseptica (Eca) are serious diseases of potato (Solanum tuberosum L.) in the southern regions of Chile (2). Resistance in potato to soft rot and black leg has been investigated extensively in Europe and North America (7). However, the native Chilean potato collection has received little attention as a possible source of resistance to soft rot bacteria. This collection consists of 496 introductions that have been gathered mostly from southern Chile. It is maintained by the Instituto de Produccion y Sanidad Vegetal of the Universidad Austral de Chile under an agreement with the International Board for Plant Genetic Resources (4). To detect material that might be resistant to Ecc and Eca, tubers of S. tuberosum were harvested and stored at room temperature (15°C). About 20 days after harvest 14 tubers from each of 15 different clones were selected and weighed. The selection included tubers representative of the many different shapes and pigmentations characteristic of the native clones. Suspensions of Chilean strains of Ecc and Eca isolated from potato tubers and plants with soft rot and black leg symptoms were prepared (3). Bacterial cells were grown 48 h at 25°C on nutrient agar, suspended in sterile distilled water, and adjusted to an OD = 0.5 at 600 nm with a spectrophotometer (Spectronic 20 TM, Bausch and Lomb, Rochester, NY 14602), equivalent to 2.5 × 108 colony forming units (CFU)/ml. For each strain, seven surface-sterilized (1) tubers were inoculated by injecting 10/zl of the suspension at a depth of 2 cm with a Hamilton syringe (#705-N, The Hamilton Co., Inc., Whittier, CA 90608). Three to five inoculations per tuber were made depending on tuber size. After inoculation the tubers were wrapped with a moist paper towel and one layer of polyvinyl chloride (Saran Wrap®; The Dow Chemical Co., Indianapolis, IN 46268) and then were incubated at 25°C under anaerobic conditions in a BBL gas Pack anaerobic Jar system (BBL-Becton Division, Dickinson and Co., Cockeysville, MD 21030). After 2 days, tubers were cut and the rotted area was removed with running tap water. After drying at room temperature each tuber was again weighed to estimate loss of tissue due to bacterial action.

A diagnostic test for the potato spindle tuber viroid (PSTV) that is based on hybridization of highly radioactive, recombinant DNA complementary to PSTV with PSTV bound to a nitrocellulose membrane and autoradiographic detection of the resulting DNA-RNA hybrids has been evaluated with tubers from 20 potato clones maintained at the International Potato Center (CIP) and with true seed obtained from healthy or PSTV-infected potato plants. The nucleic acid spot hybridization test detected PSTV not only in tuber sprouts from 10 clones that had previously tested positive in polyacrylamide gel electrophoretic (PAGE) analysis, but also in tuber sprouts from 9 clones that had previously tested negative in PAGE analysis. Further tests confirmed the presence of PSTV in these clones. The spot hybridization test detected PSTV in mixtures of seed extracts equivalent to as few as one seed from a PSTV-infected plant and 80 seeds from healthy plants. The spot hybridization test was shown to be more sensitive and reliable than PAGE analysis; it is suitable for the testing of large numbers of samples with a minimum expenditure of labor and materials.

Micropropagation is a tissue culture technique adapted for the rapid multiplication of disease-free seed stocks. Procedures for propagating potatoes in the laboratory and acclimating plantlets in the greenhouse are available, but information on cultural practices for maximizing tuber yield of plantlets when transplanted to the field is lacking. Centennial Russet and Russet Burbank plantlets were transferred from culture jars to three sizes of transplant containers for establishment under greenhouse conditions before transplanting to the field. Length of field growing season was varied by using two transplant dates and two vine kill dates. Survival of field transplanted plantlets was above 95 percent in both 1983 and 1984. Cultural practices significantly affected the tuber yield of plantlets of both cultivars. Total yield and yield of tubers larger than 35 mm in diameter increased with increasing transplant container volume. Transplant container volume had no effect on the yield of tubers less than 35 mm in diameter. Highest total yields and tuber production per plantlet for both Centennial Russet and Russet Burbank were obtained from the longest growing season (early transplant date with late vine kill). Yield of Russet Burbank plantlets increased more in response to a longer growing season than did Centennial Russet.

Wounding of potato stems by the application of clean sand at a 45° angle at 550 kPa pressure followed by a spray-mist ofPseudomonas solanacearum proved to be the most reliable method of rapidly inoculating a large quantity of potato plants to incite bacterial wilt in the field. The technique was 100% efficient (320/320) and it was estimated that 10,000 plants/hr could be inoculated by using this method. Because of its hygroscopic nature, sand coated withP. solanacearum created too many mechanical problems to be of practical value for field use even though the amount of wilt after 21 days was not significantly different from that of the best treatment. In addition, viability of inoculum as a bacterial suspension in water was greater than as coated sand.

The Sunrise potato variety is an early maturing variety with round to oblong, moderately smooth, white skinned tubers with medium-shallow eyes. Its characteristics are closely parallel to those of Superior, except Sunrise is resistant to race A of the golden nematode (Globodera rostochiensis). Sunrise is primarily a table potato; however, in southern areas it can be used for chips when processed without prolonged storage at low temperatures. Sunrise is resistant to virus X and does not show the net necrosis symptoms caused by leafroll virus. Its parents are Wauseon and B 6563-2, a selection from the USDA breeding program.

PVX infected plantlets from two potato cultivars grownin vitro with 0.3, 3, 30 or 300 ppm kinetin were exposed to temperatures of 28 or 35 C. After 3 wk, axillary buds were isolated and grown aseptically in organogenic media, followed by PVX testing by ELISA. The serological test was also run on whole plantlets at the end of the kinetin-temperature exposure. No donor plants exposed to 28 C nor the plantlets derived from their buds gave an ELISA (-) reaction, regardless of the kinetin content of the media or that of the cultivar. At 35 C the virus was suppressed to undetectable levels in several whole plantlets. In the cultivar Alpha, 2 out of 6 resulting plantlets after isolation of buds were virus-free in the presence of 3.0 mg/1 kinetin during the treatments. From Atzimba, about 15–40% of the regenerated plants were ELISA (-), without any relationship to the cytokinin content in the media. Heat had a stronger influence on virus elimination than kinetin and the results varied with the cultivar.

The ability of abscisic acid (ABA) to inhibit synthesis of the phytoalexin, rishitin, in aging arachidonic acid-treated potato tuber discs was studied in a time-restricted manner. The extent of inhibition was decreased to near zero when the ABA was applied up to 24 h after cutting the discs, regardless of whether the arachidonic acid treatment was at 0 or 24 h after cutting. Application of ABA to potato discs more than 24 h after cutting the discs caused high levels of inhibition in discs treated with arachidonic acid at 24 h, but not in discs treated with arachidonic acid at 0 h. Apparently ABA interferes with at least two separate aging events that are required for phytoalexin synthesis. One event is initiated by cutting the potato tissue, and the other by the arachidonic acid treatment.

Development ofHelminthosporium solani Dur. & Mont. on artificially inoculated, excised potato tuber periderm, incubated in darkness and high humidity at 20 to 24°C, was studied using light and scanning electron microscopy. Spore germination occurred within 16 hours, and appressoria were observed 2 days after inoculation. Penetration of periderm was evident 4 days after inoculation. Conidiophores with young conidia developed on the periderm surface within 7 days of inoculation. Well-developed conidia were visible often by 9 days after inoculation. Rudimentary stromata were observed beneath conidiophore groupings at later stages of development. In histological studies,H. solani hyphae were found in the phellem, phelloderm, and cortex of infected tuber sections. Basal cells of conidiophores or stromata were observed on the surface and in the outer tangential cork cell layers. Several layers of suberized and sometimes collapsed cortical cells were observed beneath disrupted and collapsed, infected periderm. Cavities sometimes formed between periderm and cortex, and callosities were observed on cell walls of some cortical cells beneath infected periderm.

Disease development was observed in field plots to which two different fungicides, each at two rates, were applied regularly with either a conventional or a controlled droplet sprayer. The type of fungicide did not affect disease severity, but more disease damage occurred in the controlled droplet sprayer plots than in conventional sprayer plots. Reduced rates of fungicide resulted in higher disease incidence ratings than recommended rates of fungicides by end of the spraying season. Lower marketable tuber yields and greater incidences of late blight tuber rot were related to foliar disease levels, the different sprayer types, and rates of applied fungicide. No significant differences in terms of potato top desiccation efficacy between the two sprayer types were obtained. In addition, total tuber yield and level of vascular discoloration were similar for both the controlled droplet and conventional sprayer.

‘Arma’ is a high-yielding cultivar of potato (Solanum tuberosum) with medium specific gravity, bred in Northern Ireland and selected in the Negev region in Israel as being suited there for the spring crop, between February and July. In the Negev, during these months, periods of dry weather occur with maximum temperatures up to 40 C. ‘Arma’ is a late-maturing tablestock cultivar, resistant to heat stress, Verticillium wilt, and early blight. Tuber shape is long-oval, distinctly regular, with shallow eyes; the skin is yellow and the flesh is creamy-white. Its long rest period makes it suitable for storage with relatively few problems. The heat stress resistance and the long rest period make Arma suitable for regions with hot climates.

The practice of transplanting microplants from tissue-culture to the field was compared to normal tuber propagation with respect to the transmission and translocation of potato virus S (PVS) and potato virus Y (PVY) to the daughter tubers of Red Pontiac, Shepody, Kennebec, and Russet Burbank cultivars in Prince Edward Island in 1984 and 1985. In general, the use of microplants to produce seed stocks appeared to increase the risk of infection with both viruses, although the results for tuber infection with PVY were only significant in one of the years.

The effect of early blight on tuber yield of two potato (Solanum tuberosum L.) cultivars (Russet Burbank and Norland) was evaluated using number and timing of applications of three chemicals to generate different disease epidemics in 1981 and 1982. Under the non-irrigated farming conditions in Minnesota, there were no significant differences among captafol, triphenyltin hydroxide and maneb-Zn in blight control to increase yield. Of the five spray schedules tested, a 7–10 day spray schedule initiated when blight severity exceeded 0.01%, and a 7–10 day schedule initiated when blight was found above the lower third of the plant, generally gave significant, positive yield responses with all three chemicals. At Rosemount in 1982, the greatest yield increases (relative to unsprayed) obtained were 34.9% and 48.6% in total tuber yields for Norland and Russet Burbank, respectively. At Rosemount in 1982, the highest yield increases (relative to unsprayed) obtained were 92.2% and 56.6% in total tuber yields for Norland and Russet Burbank respectively. At Grand Forks in 1981, the highest yield increases were 126.8% and 34.6% in total tuber yields for Norland and Russet Burbank respectively. Across all three experiments, maximum yield losses (percentage of [[yield of ‘healthy’—yield of unsprayed check]/yield of ‘healthy’’]) were 58.4% (US #1 tubers) and 55.9% (total tubers) for cv. Norland and 34.4% (US #1 tubers) and 36.2% (total tubers) for cv. Russet Burbank.

Seventeen clonal selections of Sangre were made from a tuber-unit seed lot planted at the San Luis Valley Research Center in 1982. Seven selections had typical vine size and 10 had larger vine size than the standard cultivar. Progeny of each selection were increased and observed in 1983. During the three-year interval from 1984–1986 comparative performance trials were conducted. All clones selected for larger vines were significantly taller and later maturing than the standard. Clones 10 and 11 also had significantly greater total yields and clones 10, 11, and 14 had significantly greater U.S. No. 1 yields. Tuber shape of these clones was comparable to the standard. Results indicate that improvement may be made in the Sangre cultivar through clonal selection.

Feeding by psyllid nymphs causes “Psyllid yellows,” a characteristic yellowing of shoots which results in a dramatic loss of tuber yield. If psyllid infestation is not controlled, the onset of shoot yellowing and growth reductions can occur within two weeks. When insecticides are applied after yellowing, recovery of plants often is not complete. In this study comparisons were made of the physiology and growth of tops and tubers of recovered and permanently injured potatoes of the cultivars ‘Denali,’ ‘Kennebec,’ and ‘Norgold Russet.’

We identified clones inSolanum tuberosum L. gp. Tuberosum breeding populations that, although susceptible to infestation by potato leafhopper,Empoasca fabae (Harris), exhibited a tolerance which was manifested by the ability to produce higher than expected tuber yields when heavily infested with leafhoppers. Such tolerance was noted in a number of clones in varying degrees, but one, B6930-1, was exceptional. It sustained only a 19% loss in tuber yield compared to ‘Katahdin,’ a susceptible cultivar, that averaged 33% lower tuber yields under similar infestation levels. We studied the seasonal growth patterns of this clone and compared them to those of Katahdin, and B6705-10, a breeding clone with known resistance to leafhopper infestation. We found that much of B6930-l’s tolerance results from its early, vigorous vine growth and its early and rapid rate of tuber bulking, enabling this clone to escape some of the effects of leafhopper feeding, whereas Katahdin was usually just beginning to tuberize at the height of leafhopper infestation.

Enrichment techniques and serological strain typing methods were used to determine the source of stem and progeny tuber populations ofErwinia carotovora (Ec) in 1980 and 1981. Plots were established in a field that had been planted with potatoes since 1972. Seed tubers of cultivars Russet Burbank and Norgold Russet were assayed forErwinia carotovora pv.carotovora (Ecc) andErwinia carotovora pv.atroseptica (Eca) by tuber peeling and direct plating and plating following enrichment in a pectate-based broth. Levels of seed tuber infestation ranged from undetectable to 2.4 × 104 colony-forming units per g peel tissue. Prior to planting Ecc was detected in soil samples from field plots by enrichment in a pectate-based broth. Ec was isolated from symptomless plants with increasing frequencies from 8 weeks following planting regardless of the level of Ec populations in seed tubers. At harvest progeny tuber recontamination was low and not related to levels of seed tuber Ec populations. In 1980, serological typing of representative Ecc isolates indicated serogroups III, XXIX, and unknown strains isolated from the soil but not the seed tubers prior to planting were isolated from stems and progeny tubers. Serogroup XXIX was isolated from soil, seed, stems and progeny tubers in 1981. Throughout both years, Ecc was isolated more frequently than Eca from seed, soil, stems, and progeny tubers. These data emphasize the potential for reinfestation of seed stocks and infection of plants by populations of Ecc that overwinter in soil under Wisconsin conditions.

Ohio field studies in microplots in the past 6 years have demonstrated a synergistic interaction of the lesion nematodePratylenchus penetrans withVerticillium dahliae in potato early dying. Tests with various populations ofP. penetrans andV. dahliae, alone and in combinations, have shown that disease occurs when both pathogens are present at populations which individually have little or no effect. Yield loss is not necessarily associated with foliar symptom development, but occurs with the addition of high temperature stress during tuberization. Comparative studies with two other species ofPratylenchus commonly found in Ohio potato soils have shown thatP. crenatus does not interact withV. dahliae and thatP. scribneri interacts only slightly. Since all three species feed and reproduce well on potato,Verticillium interactions probably are not due to wounding. The involvement ofPratylenchus withV. dahliae in potato early dying is an important factor that must be considered in the development of future crop-loss predictive systems.

The cultivars Atlantic, Belchip, Denali, Monona, Norchip and Russet Burbank were harvested at weekly intervals from early August to early October in 1980–81 and analyzed for the following attributes: 1) sucrose rating (SR) (mg sucrose/g fresh tuber) 2) specific gravity (Sp.G.); 3) total yield and 4) chip color. At the final harvest, samples of each cultivar were stored and analyzed for color to determine chipping ability after storage.

Bacterial ring rot-infected tuber and stem tissue reacted withCorynebacterium sepedonicum antiserum in agar immunodiffusion (AID). The AID test was as effective as immunofluorescence (IF) in confirming ring rot in potato tubers submitted for diagnosis. Precipitin bands developed in AID with tuber preparations that contained, in a 1:10 dilution of the preparation, at least 100 IF positive cells/microscope field with a 100 X oil objective. Only a low percentage of preparations with less than 100 IF positive cells were positive with AID. Samples inoculated withC. sepedonicum strains which produced non-mucoid colony types on YGM medium did not consistently develop precipitin bands in AID which suggests that the usefulness of the test may be limited where non-mucoid strains occur naturally.

Apical stem cuttings removed from potato plants (cv. Russet Burbank) infected withCorynebacterium sepedonicum and expressing mild symptoms of bacterial ring rot were demonstrated to be free from the pathogen. This material remained free of ring rot through two additional stem cutting generations and when grown in the field for four successive tuber generations.

The effect of irregular in-row spacing on tuber production and yield of Russet Burbank potatoes was tested at 2 locations in 1979 and 1980. At a similar mean spacing, no yield reductions were observed when irregular seedpiece spacings (CV levels 25, 50 and 75%) were compared with evenly spaced seedpieces (CV level of 0%). No significant interaction was found between irregular spacing and seedtypes with mass ranging from 59.0 to 67.9 g. No significant interaction was found between three accurate spacings (30.4, 45.7 and 60.9 cm) and the same range of seedpiece mass; however, the crop grown from large seedpieces had a higher marketable yield at the widest spacing tested. Mainstem numbers per plant were influenced by seedpiece mass while axillary branches increased with decreases in intra-plant and inter-plant competition. The incidence of hollow heart was generally unaffected by irregular inrow spacing. Incidence of this disorder was highest in 1979 and appeared to increase with increased plant stresses.

Adequate soil water is needed for satisfactory yield and quality of potato tubers. With sprinkler irrigation systems it is common practice to apply more water than the crop uses in order to maintain high soil water levels. A study was initiated to evaluate the response of Russet Burbank potatoes to a wide range of daily sprinkler irrigation rates when grown on two soilsa loam and a sand—differing in water holding capacity. The results from the two soils were very different. On the loam soil, yields generally increased with increased applications of water, up to the equivalent of 40 to 50% estimated Et. Irrigation treatment effect on percent No. 1 tubers was inconclusive. In 1978, percent No. 1 tubers increased with water applied up to about 70% estimated Et. In 1980, irrigation rates between the equivalent of about 20 and 80% estimated Et had little effect on tuber grade. Yields and percent No. 1 tubers were depressed at irrigation rates greater than about 80% estimated Et. On the sand, yields and percent No. 1 tubers increased with increased irrigation rates up to about 100% and 80% estimated Et, respectively. Tuber specific gravity was not affected to an important degree by irrigation treatment on either soil. These results indicate that a good crop of potatoes can be grown on a loam soil at daily irrigation rates considerably less than estimated Et rates, while such reductions will decrease yields and grade on a sandy soil.

Neo-Tuberosum clones from the Cornell breeding population were evaluated as female parents in combination with Tuberosum for production from true seed. Field trials of hybrid seedling families were carried out in 1982 and 1983. Families were also grown in a hot greenhouse to measure combining abilities for tolerance to heat stress. General combining ability was highly significant for yield, vine and tuber characteristics, and heat tolerance. Several clones were identified which produced outstanding seedling families for yield, uniformity, and tuber appearance. Some individual crosses were equal to cultivars in yields, uniformity, and tuber appearance. Hybrids of Neo-Tuberosum and Tuberosum appear to be an excellent choice for potato production from true seed.

In both 1981 and 1982, 51 potato fields were surveyed every 7–10 days for early blight severity and fungicide use. Early blight severity data from each field were used as input to a regression model to estimate yield loss in that field. The model for early maturing potato cultivars was Percent Yield Loss=0.8183+0.6441* (% blight increment between days 56 and 66)+0.6102* (% blight increment between days 66 and 76)+1.3480* (% blight increment between days 76 and 86). The model for late maturing cultivars was Percent Yield Loss=2.1846-4.7734* (% blight on day 56)+0.7440* (% blight on day 76)+ 0.5676* (% blight on day 96). In both 1981 and 1982, early blight was found to be present in all fields of the survey. In 1981, early blight was estimated to have caused mean losses of 6.5% (Red River Valley), 3.7% (Central sands) and 2.3% (Southern muck). In 1982, losses from the same areas were 5.4%, 3.7% and 2.6% respectively. In 1981, individual county losses ranged from 2.3% for Freeborn to 12.9% for Marshall while in 1982 losses were from 2.0% for Freeborn to 6.7% for Clay. Although fungicide use data were incomplete, the county with lowest yield loss had the most fungicide applications.

Phenolic content and discoloration were determined for 13 commercial potato cultivars and 6 potato clones grown at 2 locations over a period of 5 successive years. Tubers from 6 potato cultivars and 5 potato clones resistant to the golden nematode were compared with tubers from 7 potato cultivars and 1 clone known to be susceptible to the golden nematode. Tubers from resistant plants were lower in phenols and discolored less than tubers from susceptible plants.

The effect on true potato seed (TPS) weight of supplemental nitrogen (N) applied during seed development was investigated using crosses DTO-33 × R128.6 (“A” produced in the field) and Atzimba × R128.6 (“B” produced in the field and in a screenhouse). Dry weights of tops and tubers of the mother plants were also measured in the screenhouse.

A simple potato growth model was validated in 1985 and 1986 at two dryland sites, Grand Forks, ND and Rosemount, MN, and one irrigated site, Hancock, WI. At each site, Russet Burbank potatoes were grown to maximize yield. Leaf area index and leaf, stem, root, and tuber biomass data were collected from replicated experiments every two weeks. Soil matric potentials, minimum and maximum temperatures, and solar radiation also were measured. Among sites, crop development rates and solar radiation receipts were similar, but final tuber yields differed by as much as 35 Mg/ha. Model predictions of leaf, stem, and tuber dry weights were similar to observed data both in terms of maximum values attained and seasonal development except for the experiment in Grand Forks in 1986 where the model over-estimated final tuber yield by 23 Mg/ha (115%). Soil aeration or salinity, factors not currently modeled, may account for this discrepancy. Modeled yield responses to defoliation were compared with field data from one of the experimental locations. At levels of defoliation below fifty percent, proportional yield reductions were similar between observed and modeled crops.

The Islander potato variety is a medium maturing variety with oblong, shallow-eyed, white-skinned tubers. High dry matter and good chip color characterize this variety which is better adapted to Long Island, New York and New Jersey than to other northeastern areas. It is resistant to golden nematode and net necrosis; moderately resistant to common scab and early blight; and less susceptible than most varieties to acid scab and Rhizoctonia. The Islander is a product of the cross Chipbelle × Norchip.

Ribavirin (virazole) treatment of cultured axillary bud tips (3-4 mm) was tested as a method of eradicating potato virus Y (PVY) and potato virus S (PVS) from two potato cultivars, Norchip and Desiree.

In a genetic study with cultivated diploid potatoes the distribution of anthocyanin to the tuber flesh was found to be controlled by a single dominant allele. The gene symbolPf (pigmented flesh) is proposed. The geneI, which controls the distribution of pigment to the tuber skin, is necessary for the expression ofPf and is closely linked to thePf locus. ThePf gene is a potentially useful genetic marker which does not appear to be associated with any deleterious traits and can be screened for without laboratory equipment.

Field observations of potato plants showed various types of roots. There were roots originating at the base of the main stem (basal roots); roots arising from the mainstem at the junction of the stolon to the mainstem (junction roots); roots on the stolons (stolon roots) and roots growing directly from the base of the buds on the tuber (tuber roots). The path of water uptake and movement was studied by examining the transport of a water soluble dye from each type of roots. The results showed xylem connections from the different types of roots to the tuber as well as to the aerial part of the plant. However, under field conditions the basal and the junction roots do not appear to transport water to the tuber. The stolon and the tuber roots, on the other hand, transported water to the tuber under similar moisture conditions. To our knowledge this is the first report documenting the presence of potato tuber roots. Our study clearly demonstrates the role of tuber and stolon roots in tuber water uptake. These roots may play a role in tuber calcium uptake which is thought to move primarily with the water in the xylem.