Differential resistance to tuber late blight in potato cultivars without R-genes

Summary In 1983 and 1984 there were reports in Norway of more severe late blight infection than usual on tubers of cv. Pimpernel, a cultivar without R-genes known to be somewhat resistant to both haulm and tuber infection. Results from routine tests confirmed earlier experience that cv. Pimpernel shows stable resistance to tuber late blight, although the ranking of cultivars may vary between years. In experiments during three harvest seasons tubers of different cultivars were inoculated with isolates ofP. infestans obtained from stocks of cv. Pimpernel with severe tuber late blight and with other isolates. In these experiments the isolates from cv. Pimpernel caused more rot on tubers of Pimpernel than did other isolates.     

Screening potato cultivars for common scab of potato in a naturally infested field

Summary Twenty seven cultivars of potato were screened for common scab grown in a commercial field in two successive years (1996–97 and 1997–98). Eight cultivars were least susceptible and the others ranged from medium susceptible to very highly susceptible. None of the cultivars was resistant. Most showed a stable resistance reaction in both years.     

Characterization of pectic substances from two potato cultivars with different sensitivities to prewarming

Summary Potato cultivars Chieftain and Kennebec were prewarmed at 70°C for 15 min. prior to heating at 100°C for 18 min. in a water bath. Chieftain exhibited a large increase in fracturability upon treatment while Kennebec showed a smaller increase. Pectic substances of the two cultivars were sequentially extracted in water, sodium hexametaphosphate (0.5%), hydrochloric acid (0.05M, 50°C), and sodium hydroxide (0.05M, 5°C). The largest fraction isolated from both cultivars consisted of hydroxide-soluble pectic substances, and was followed by the acid-soluble fraction. Chieftain contained significantly greater amounts of the hydroxide-soluble fraction than Kennebec. Ion exchange chromatography revealed that the extracted pectic substances from the two major fractions consisted of a charged pectin chain with attached neutral sugars. Gel filtration chromatography revealed wide molecular size distributions for the isolated pectic substances. Pectic neutral sugar content for the hydroxide-soluble fraction (50% of total pectic substances) was greater for Chieftain than for Kennebec.     

The development of mature plant resistance in four potato cultivars against aphid-inoculated potato virus YO and YN in four potato cultivars

Summary Field-grown potato plants of cvs King Edward, Record, Maris Piper and Désirée were inoculated on seven different dates during the growing season of 1987 and 1988 with either potato virus Y^O (PVY^O) or PVY^N, using three viruliferous peach-potato aphids (Myzus persicae) per plant. In each cultivar, the proportion of progeny tubers infected with PVY^O or PVY^N was high in plants inoculated during the four weeks following emergence, the proportion declining to zero or close to zero in the subsequent 4–6 wks.     

Assessing potato cultivars in Tunisia for susceptibility to leak caused byPythium aphanidermatum

Summary The susceptibility of 21 potato cultivars to leak (watery wound rot) caused byPythium aphanidermatum was compared. Whole tubers were inoculated after wounding by dipping into a water suspension of 10³ oospores/ml and incubated for 3 days at 25°C. The mean penetration of tissue calculated from the recording of lesion width and depth and the qualitative assessment of rot extension were highly correlated. For 10 of 12 cultivars tested at least twice, consistent reactions were obtained: cvs Korrigane. Superstar, Safrane and Yesmina were moderately susceptible, whereas Atlas, Diamant, Mondial, Obélix, Spunta and Timate were susceptible to highly susceptible. Tubers originated from two locations and/or two cropping seasons: thus, the performance of these cultivars under different production conditions reveals a satisfactory level of agreement with the test. Moreover, ratings assigned are supported by field or store observations.     

Response of tubers of five potato cultivars to co-inoculation with US-1 and US-8 genotypes ofPhytophthora infestans

Summary The severity of late blight tuber rot in five potato cultivars (Green Mountain, Sebago, Dorita, AC Brador and Island Sunshine) resulting from inoculation with US-1/US-8 combinations of Canadian isolates ofPhytophthora infestans was evaluated. In addition, the potential for mixed (A1∶A2) inoculations to result in oospore formation was examined. In studies conducted in 1996 and 1997, cv. Dorita (and Island Sunshine in 1997) developed the least amount of surface or internal necrosis relative to the other cultivars following co-inoculation with US-1 and US-8 isolates ofP. infestans. Differences also were revealed among isolate combinations used for inoculation. The combination of a US-1 isolate from Prince Edward Island with a US-8 isolate from New Brunswick consistently produced the least amount of necrosis in tubers of the various cultivars. There was no evidence of oospore production in any of the tubers examined. AAFC Contribution No. 919     

Verification of sap flow by heat balance method on three potato cultivars

Summary A heat balance method for measuring sap flow in actively growing plants was tested on three potato (Solanum tuberosum L.) cultivars Monona, Norchip and Atlantic. The method is based on applying a known heat source to the stem of an intact plant: by measuring the axial and radial heat dissipation the magnitude of sap flow can be quantified. The technique shows promise as 24 h accumulated sap flow measurements were highly correlated (r²=0.935, n=36) with gravimetric measurements of transpiration and produced an average root mean square error (RMSE) of 20 g d⁻¹. The accuracy of the method improved as daily accumulated flow increased. Under low daily flow rates overall RMSE increased, supporting the need for a heat storage term in the heat balance equation. Short term flow rates were less significantly correlated (r²=0.764, n=1572) and produced an average RMSE of 2.89 g h⁻¹. Identification of these short term flow rates during high flow periods may be difficult. The presence of water potential gradients and the likelihood of re-hydration during high flow events may have also affected the short term performance of the method. At low flow rates, many different errors may be introduced.     

Breeding potato cultivars with tubers resistant toPhytophthora infestans

Summary Available information has been reviewed on tuber resistance toP. infestans, its inheritance and breeding procedures used to obtain potato cultivars superior in this character. P. infestans is worldwide the most destructive potato pathogen. Tuber resistance is an essential component of potato resistance as this pathogen is often responsible for tuber rot in storage, and infected tubers of susceptible cultivars provide an important way for its overwintering. In Europe many cultivars and advanced breeders selections have been obtained with tuber resistance toP. infestans, but in the last decades no progress is noted in the mean level of this resistance in cultivars. The expression of tuber resistance depends to a large extent on testing conditions and therefore it can be difficult to evaluate. Consequently making progress in breeding is not easy, and also cultivar assessment data from various countries sometimes differ considerably. It is concluded from published data that it should be easier to make progress in breeding potato cultivars with resistant tubers if more attention is paid to combining the various types of resistance which are already known, and if the genetic determination of durable resistance toP. infestans is better understood. Professor Dr. K.M. Świeżyński passed away on 27th July 2000     

Formation of microsclerotia ofVerticillium dahliae Kleb. on various plant parts of two potato cultivars

Summary Potato plants of cvs Element and Mirka were artificially infected withV. dahliae in two greenhouse experiments. Leaf blade, petiole, aerial stem, subterranean stem, stolon and root mass were separately harvested both when the canopy was still green and at maturity. After 4 weeks incubation, the plant tissue was air-dried and the numbers of microsclerotia per mg tissue and per plant were determined. The highest numbers of microsclerotia were observed in the haulm when harvest took place at maturity. Cultivar Element yielded significantly more microsclerotia in the haulm than cv. Mirka, whilst there were no cultivar differences in the microsclerotial production on subterranean parts. The petiole and the aerial stem contributed most to the total microsclerotial production, whereas roots were much more important for formation of microsclerotia than stolons.     

Reduced fungicide inputs to controlPhytophthora infestans in potato cultivars with high levels of polygenic resistance

Summary Final disease ratings, apparent infection rates and areas under disease progress curves were used to calculate lower fungicide rates for potato cultivars with high levels of polygenic resistance toPhytophthora infestans. Areas under disease progress curves offered more reliable indications than final disease ratings or apparent infection rates. Control of epidemic development was achieved using 20–80% of the recommended application rates of mancozeb on plots of cvs Brodick. Cara and Torridon compared with full application rates on cv. Maris Piper.     

Ring rot symptom development on potato cultivars and lines in southern Alberta

Summary Plants were grown in the field from seed pieces of potato cultivars injected with ring rot bacteria (Corynebacterium sepedonicum). Symptoms were produced with both 18 and 180 colony forming units (cfu) on cvs Norchip and Red Pontiac and with 180 cfu on cv. Belrus plants. Tuber symptoms were detected in all cultivars except Belrus and Teton. Tuber progeny produced plants that developed top symptoms on all cultivars except those of cv. Nooksack, Russet Burbank, and Teton. Tuber progeny of the second crop produced both plants and tubers with symptoms developing only in the cv. Nooksack. In another 3-year experiment, variability in the disease response of selected cultivars and lines was examined following knife-inoculation of tuber seed with a high level of ring rot bacteria. Significant correlations between top and tuber symptoms were detected, but they were not high enough to make unnecessary the examination of both top and tuber symptoms in ring rot disease selection studies. Contribution no. 3879111     

A comparison of laboratory and field reactions of a range of potato cultivars to infection withSynchytrium endobioticum (Schilb.) Perc.

Summary A comparison of reactions of potato cultivars toS. endobioticum indicated that the degree of susceptibility observed in laboratory tests correlated well with the reaction of cultivars in field tests. Some cultivars found to be less resistant (Resistance Grade 2) in laboratory tests produced winter spores in the field test, and 10% of those derived from cv. Ausonia germinated in distilled water. Compost from the field plot was estimated to contain 14–18 apparently viable spores/g. and this resulted in 100% infection of the highly susceptible cv. Arran Chief. The implications of these findings for descheduling sites previously scheduled due to wart infestation are discussed.     

Reaction toPhytophthora infestans of potato genotypes with race-specific resistance

Summary Phytophthora infestans isolates collected in Poland in 1987–1995 were evaluated on detached leaflets of a differential series consisting of potato genotypes possessing resistance genes R1–R11. On the same differentials was evaluated over three years a highly virulentP. infestans isolate MP 245. Isolate MP 245 and those from natural populations ofP. infestans were seldom or inconsistently pathogenic to R5, R8 and R9. When leaflets of the differentials were infected with MP 245 the lesions were often small and sporulation weak. A statistically significant interaction was found between years and pathogenicity of MP 245 to individual differentials. Attempts to train the isolate MP 245 for pathogenicity to R9 were unsuccessful. The variation in specific resistance of potato genotypes may complicate the evaluation of their resistance toP. infestans, as well as the evaluation of the virulence spectrum of natural fungus populations. This variation may be due to changes in specific pathogenicity of the fungus or in specific resistance of the potato. Some differentials have a possibly useful resistance toP. infestans.     

The susceptibility of tissue cell walls toErwinia enzymes differs among the potato cultivars

Summary In the cell wall matrix of higher plants, cellulose fibrils are embedded in a network of pectic substances, hemicelluloses and structural proteins. But plant pathogenicErwinia carotovora bacteria secrete a mixture of enzymes including pectinases, cellulases and proteases to degrade the cell walls of host plant tissue. Tuber tissue of 31 potato genotypes was exposed to a mixture ofErwinia enzymes before the degree of cell wall lysis was determined by a Neutral-red vital staining method. The data indicate that the resistance of cell walls to the action ofErwinia enzymes differed significantly among the potato cultivars and clones. The decrease in cell viability caused by the action of enzymes ranged from 7% for cv. Maxilla to >46% for cv. Arnika. An increased rate of N-fertilization coincided with enhanced susceptibility of cell walls to the action ofErwinia enzymes. There was a correlation between the amount of cell wall substances in tuber tissue and the resistance of cell walls to enzymic degradation.     

Effect of haulm treatments on the formation of microsclerotia ofVerticillium dahliae Kleb. on potato

Summary In four pot experiments, potato plants of cv. Element were artificially infected withV. dahliae. At an early and a late harvest haulms were killed chemically, by burning or by various other treatments, including cutting them into pieces of different lengths and keeping the debris on the soil surface or covering with soil. After 4 weeks the plant material was air-dried and the number of microsclerotia per mg was determined. At the early harvest, in two experiments, the chemical treatment yielded more microsclerotia than the cutting treatments. Covering colonised haulm tissue with non-sterilised soil was effective in inhibiting microsclerotia formation. Shorter haulm pieces led to fewer microsclerotia at the later harvest if the material was kept on the soil surface. The variation in microsclerotial yield and in treatment effects among the different experiments was large.     

Testing the resistance to silver scurf in commercial potato cultivars under controlled environmental conditions

Summary Optimum storage conditions to identify resistance to silver scurf among commercially grown potato cultivars were determined in a series of experiments. Inoculation of field-grown tubers with a conidial suspension ofH. solani (concentration 10⁴ conidia per ml of water) and incubation at 15°C with 95% RH for 1 month followed by 2 months at 85% RH produced the greatest differences in disease severity between potato cultivars. Results with glasshouse-grown tubers were comparable with those from field-grown tubers, indicating that the test could be used to identify resistance in wild species of potato and to understand the inheritance of this resistance.     

In vitro evaluation of resistance of potato cultivars toPhytophthora infestans

Summary The resistance of ten potato cultivars (Agria, Ajax, Désirée, Liseta, Kennebec, Majestic. Monalisa, Prima, Spunta and Tonda di Berlino) toPhytophthora infestans was analyzed in vitro using 8 fungal strains. An assay based on electrolyte leakage was used for screening leaves and tuber tissues with fungal culture filtrates. With almost all cultivars the resistance of leaves did not correlate with the resistance of tubers. Cv. Ajax appeared the least susceptible in both leaf and tuber tests, while the cv. Prima was the most susceptible in tuber tests.     

Influence of ambient forced-air ventilation rate on potato storage wastage

Summary Potatoes were stored in three insulated bins having ventilation rates of 0.0190, 0.0306 and 0.0417 m³/s·t. The results showed that in the climate of northern Italy potatoes can be stored successfully at ventilation rates of not less than 0.0306 m³/s·t resulting in product weight loss of less than 2.0% after 100 days' storage. The incidence of diseases and of sprouting was very low at rates of 0.0306 and 0.0417 m³/s·t, but greater at 0.019 m³/s·t because of the higher tuber temperature. Laboratory tests simulating ambient conditions confirmed the validity of a thermodynamic model simulating air-potato heat exchange.     

Evaluation in pot and field trials of resistance of potato cultivars and breeding lines to common scab caused byStreptomyces scabies

Summary The resistance to common scab of nine cultivars and eight advanced breeding lines of potatoes, fiveSolanum andigenum Juz. et Buk. clones, and three potato breeding lines selected for resistance to common scab, was investigated in a field and a pot trial. In the pot trial potaroes were planted in inoculated coarse river sand in plastic pots (150 mm in diameter) placed on irrigated subsoil in a glassfibre house. In a field trial potatoes were planted in an artificially infested field. The harvested tubers were scored individually for scab incidence according to two scales, viz. a 1–5 point scale for percentage surface covered with scab and a 1–3 point scale for lesion type. A scab index value was calculated for each tuber. A significant correlation coefficient of 0.82 was obtained for the results of the two tests.     

Effects of soil-borneRhizoctonia solani Kühn on yield and quality of ten potato cultivars

Summary The effects of soil-borneRhizoctonia solani on yield and quality of potato were studied by an extensive individual plant sampling procedure. From 1983 to 1986, stem canker and stolon pruning were examined in 10 768 plants growing on a sandy soil. Tuber yield and quality and haulm yield per plant were also recorded. The degree of attack byR. solani depended on the cropping frequency of potato. Severe and very severe stem and stolon attacks decreased fresh yield, dry matter yield and dry matter content of tubers and increased the number of deformed and small tubers, whereas the effect on haulm yield and stem number was small. Potato cultivars differed in susceptibility toR. solani, but yield response did not differ significantly between cultivars at the same levels of stem and stolon infection.