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.     

Variation in tuber pathogenicity ofPhytophthora infestans in the Netherlands

Summary Variation in aggressiveness to tubers among isolates ofPhytophthora infestans sampled from three potato growing regions in the Netherlands was compared. Variation in the ability to infect tubers of cv. Bintje was found between isolates of each of the three regional populations. The most aggressive isolate of the old population matched the average level of the new population in its ability to infect tubers. As a consequence, the commonly used reference isolate VK 6C can no longer be considered to be representative of the present population ofP. infestans. Therefore it is recommended that testing tuber resistance for the official list of potato cultivars with this isolate should be discontinued. Tuber infection and subsequent spread of the fungus in the tuber tissues were not found to be correlated. The components of tuber pathogenicity studied were not correlated to pathogenicity factors in the foliage, as measured under growth chamber conditions.     

The infection of potatoes byPhytophthora infestans during simulated washing and its control using a hot water treatment

Summary A single blighted tuber released on average 1.39 mg ofP. infestans mycelia and sporangia during simulated washing. In subsequent experiments up to 100% of tubers were infected when healthy tubers were washed in a suspension ofP. infestans equivalent to a concentration of 1.65 μg ml⁻¹, i.e. that calculated to be in a commercial washing plant after washing a potato stock with approximately 1% of tubers infected with blight for 4 hours. Immature tubers were more prone to infection during washing than mature tubers. Damaging tubers, irrespective of their maturity, increased the incidence of tuber infection. The viability ofP. infestans isolates was significantly reduced byin vitro immersion in water at 44°C for 5 minutes. The infection of daughter tubers dipped in a suspension ofP. infestans for 3 minutes was prevented when the suspension temperature was 44°C. There was no indication of tuber damage at this temperature.     

Resistance of Potato Tubers to a Highly Aggressive Isolate of Phytophthora infestans in Relation to Tuber Age

The goal of these studies was to evaluate the influence of tuber age on the expression of resistance to Phytophthora infestans in five selected potato genotypes (three cultivars and two clones) varying in tuber resistance and maturity type, and also to select the best time for testing potato tubers for resistance to blight. Resistance was evaluated by inoculating whole tubers at 13 different times as they progressively aged during growth and storage. Tuber age was expressed as weeks after planting. Tubers were rated from week 13 (late July) until week 43 (late February) for tubers planted on 27–28 April, over three consecutive seasons (2001–2003). Analysis of variance performed on tuber resistance data showed significant effects of genotype, year and tuber age. Significant contributions of genotype × year, genotype × tuber age, year × tuber age and year × genotype × tuber age interactions were also detected. A slight increase in tuber resistance with tuber age was observed for cvs Bzura, Sokół and Irga, while the opposite trend was observed for the clones 97-A-63 and DG 92-227. Changes in tuber resistance with age of tested genotypes were not related to their maturity type. The period of most stable expression of tuber resistance was observed when tubers were tested between the 16th and 28th week after planting.     

Impact of Different US Genotypes of <Emphasis Type="Italic">Phytophthora infestans</Emphasis> on Potato Seed Tuber Rot and Plant Emergence in a Range of Cultivars and Advanced Breeding Lines

Seed pieces of different potato cultivars and advanced breeding lines (ABLs) from north central US breeding programmes were inoculated with different genotypes of Phytophthora infestans (US-1, US-1.7, US-8, US-11 and US-14). The effect of these genotypes of P. infestans on seed piece rot severity after re-storage was assessed using an image analysis technique. P. infestans genotypes demonstrated variable ability to cause seed piece rot and to reduce plant emergence measured as final plant stand (%) and the relative area under the plant emergence curve (RAUEPC). The US-8 genotype of P. infestans was the most aggressive genotype, as indicated by tuber rot severity across all cultivars/ABLs tested, followed by US-14 in both years. The US-1, US-1.7 and US-11 genotypes were the least aggressive, causing only moderate seed piece rotting across cultivars/ABLs tested. Similar trends were observed in two field experiments, where the US-8 and US-14 genotypes delayed or reduced emergence. Values of final plant stand (%) and RAUEPC demonstrated that the cultivars/ABLs Atlantic, MSJ453-4Y and Torridon were the least susceptible across all P. infestans genotypes. In both experiments cv. Pike was the most susceptible. Other cultivars/ABLs demonstrated variable responses to different genotypes of P. infestans. No symptoms of P. infestans were observed on emerged plants up to 60 days after planting. The variability of susceptibility of tubers to different genotypes of P. infestans has implications for plant breeding efforts in that the major emphasis in the past has been to breed for foliar resistance, with limited emphasis on the reaction of the tuber. Results from this study suggest that highly aggressive genotypes of P. infestans such as US-8 may lead to severe tuber rotting and deterioration of tubers before emergence, but despite this observation the US-8 genotype is still predominant in North America.     

Comparison of resistance evaluation in potato variety assessment

Summary Results of varietal assessment in France, Germany, Great Britain. The Netherlands and Poland have been compared for two characters: resistance to potato virus Y (PVY) and tuber resistance toPhytophthora infestans. In total 75 varieties were evaluated in more than one country. All evaluations were expressed or could be transformed into a 9-grade resistance scale. Evaluations did differ considerably due to two main reasons: countries differed in severity of evaluation (mean resistance level ascribed to varieties) and the relative resistance level ascribed to individual varieties depended on the country. Differences between countries in severity of the evaluation could exceed two grades in resistance to PVY and three grades in tuber resistance toP. infestans. In the relative resistance ascribed to individual varieties the differences between countries were larger, reaching up to 6 grades in evaluation of resistance to PVY and up to 8 grades in evaluation of tuber resistance toP. infestans. To achieve consistent variety assessment results from different countries, it will be necessary to use agreed standard varieties and more consistent evaluation methods. Professor Dr. K.M. Świeżyński passed away on 27th July 2000     

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.     

Light use efficiencies of potato cultivars with late blight (Phytophthora infestans)

Summary Potato cultivars of different maturity classes and levels of resistance toPhytophthora infestans were grown under several disease intensities in three field trials. Seasonal courses of ground cover by green foliage and final tuber yields were determined. Light use efficiencies (LUE) were calculated from regression analyses of yield on cumulative light interception. Late blight reduced tuber yields by decreasing cumulative light interception without affecting LUE. No differences in LUE between cultivars or cultivar classes were detected. Therefore, the maintenance of green leaf area is important when breeding potatoes for optimal performance in the presence of late blight. The results support the hypothesis that the correlation between lateness and reported resistance of potato cultivars is due to the vigorous foliage growth of late cultivars.     

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     

Detecting resistance to the root-knot nematodesMeloidogyne hapla andM. chitwoodi in potato and wildSolanum spp.

Summary Various methods of screening for resistance to root-knot nematodes were compared and evaluated. Seedling populations ofSolanum spp., grown in clay pots and plastic tubes with silver sand and inoculated with juveniles ofMeloidogyne chitwoodi andM. hapla, showed large differences in the number of egg masses on roots 7 weeks after inoculation. The differences were reproducible when re-testing was done with cuttings and plants from tubers. No resistance toMeloidogyne spp. was observed with ten potato cultivars when grown in clay pots, plastic tobes or closed containers. Plants from tubers in growth pouches developed a large two-dimensional root system, and after inoculation with juveniles the infection process could be observed over 8 weeks. A method of infecting potato tuber tissue withMeloidogyne is described, using tuber slices in Petri dishes as a potential screening test for tuber resistance.     

Influence of storage temperature on rate of potato tuber tissue infection caused byPhytophthora infestans (mont.) de bary estimated by digital image analysis

Summary Potato tubers were inoculated with two biotypes ofPhytophthora infestans then stored at 3,7, 10 and 15°C. Image analysis quantified average reflective intensity (ARI) of diseased tissue from cut surfaces of sample tubers. Tuber tissue infection and infection rate were measured by calculating Mean ARI of samples. Average tuber tissue infection and infection rate was minimal at 3°C (P.i.-US8 orP.i.-US1). Tuber tissue infection increased at temperatures >3°C, from 220 Mean ARI seven days after inoculation (dai) to 190–150 Mean ARI 50 dai (depending on cultivar and biotype ofP. infestans). Rate of tuber tissue infection caused byP.i.-US1 at 7°C was about zero in cv. Snowden but greater than −0.2 ARI day⁻¹ (cvs Russet Burbank and Superior). Rate of late blight infection in tuber tissue generally increased with temperature from −0.2 ARI day⁻¹ (at 7°C) in all cultivars to a maximum of −0.8 ARI day⁻¹ (10°C).     

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.     

Production of phytoalexins, glycoalkaloids and phenolics in leaves and tubers of potato cultivars with different degrees of field resistance after infection withPhytophthora infestans

Summary The kinetics of accumulation of phytoalexins, glycoalkaloids and phenolics was studied in two potato cultivars differing in their degrees of field resistance when infected withPhytophthora infestans. Tuber slices and leaves of cvs Pampeana INTA (high degree of field resistance, free of R genes) and Bintje (susceptible) were infected with race C (complex race 1, 3, 5, 7, 11) ofPhytophthora infestans. Phytoalexins and phenolics accumulated in tuber and leaf tissues which had been inoculated. The levels of these compounds in the susceptible cv. Bintje were relatively low and similar to those found before inoculation. Leaves of cv. Pampeana INTA had a very high glycoalkaloid content, suggesting that glycoalkaloids may play a role in protection of leaves against the fungus. However, we could find no correlation between resistance and glycoalkaloid content of tubers. Our results suggest a major role of phytoalexins, phenolics and glycoalkaloids in the complex mechanisms of field resistance.     

Impact of Agronomic Strategies (Seed Tuber Pre-sprouting,Cultivar Choice) to Control Late Blight (<Emphasis Type="Italic">Phytophthora infestans</Emphasis>) on Tuber Growth and Yield in Organic Potato (<Emphasis Type="Italic">Solanum tuberosum</Emphasis> L.) Crops

In order to optimize potato crop management in organic farming systems, knowledge of crop growth processes in relation to limitations and reductions by nitrogen (N) and disease is necessary. This paper examines the effect of different prevention measures (seed tuber pre-sprouting, choice of cultivars: resistance to Phytophthora infestans; earliness of tuber initiation) against disease-related constraints on yields, depending on the N-mediated growing conditions of organic potato crops. Under conditions of a relatively high N supply, accelerating the early development and tuber initiation of potato crops by seed tuber pre-sprouting (yield increase of c. 18–23%) or the selective choice of cultivars with an earlier tuber initiation (yield increase of c. 0–21%) are the most effective strategies in combating late blight. They bring forward the crop development c. 7–10 days, escaping the negative impact of the fungus on tuber growth. Under conditions of a relatively low N supply, these strategies do not affect final tuber yield. The reason for the differences of the effect depending on N supply of the crops is, that the lower the N supply, the shorter the period of time over which tuber growth takes place, independent of whether P. infestans becomes severe or not. Tuber growth of organic crops low in N has mostly ceased by the time late blight becomes potentially important for limiting yield, with the consequence that preventative measures against P. infestans are meaningless. Surprisingly, the level of cultivar haulm resistance to late blight did not affect tuber yields in years with an early and strong late blight development. Probably, the positive effect of a longer resistance to the fungus (c. 1 week) was counterbalanced by a later tuber initiation (also c. 1 week). In years with a moderate late blight incidence, less susceptible cultivars were able to produce higher tuber yields on fields with a lower N availability (c. 17–20%), probably due to higher N use efficiency and a better match of N mineralization and N uptake. Under conditions of a relatively high N supply, the tested cultivars with a higher haulm resistance to late blight were not able to improve tuber yields.     

A Greenhouse Assay for Powdery Scab (<Emphasis Type="Italic">Spongospora subterranea</Emphasis> f. sp. <Emphasis Type="Italic">subterranea</Emphasis>) Resistance in Potato

The plasmodiophorid pathogen Spongospora subterranea f. sp. subterranea causes powdery scab disease of potatoes (Solanum tuberosum L.), which severely affects tuber quality. Adequate chemical control of the disease is lacking, so breeding resistant potato cultivars is important. Little is known about the mechanisms or genetics of host resistance, as the epidemiology of the pathogen makes research on the disease difficult. An environmentally controlled disease screen that allows the accurate and repeatable assessment of diseased tubers (rather than a ranking system) is required to study host resistance in more detail. A high-throughput greenhouse disease assay is described that has been used to assess varying levels of tuber disease resistance of potato genotypes for three consecutive Southern Hemisphere growing seasons (2001–2002, 2002–2003, 2003–2004). This assay allows the evaluation of symptoms on tubers to verify resistance, rather than inferring the tuber infection on the basis of root galling or zoosporangial root infection. The assay was used to determine the level of resistance of genotypes of both known and unknown resistance status. The assay clearly distinguished the susceptible and resistant standards, and the results were highly correlated over the three seasons. The most efficient ways of assessing tuber disease susceptibility of genotypes are discussed. The consistency of the results and the ability to screen many genotypes confirms the value of the assay as a tool for early-generation selection in a plant breeding programme.     

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.     

Susceptibility toPhytophthora infestans of field-and glass-house-grown potato tubers

Summary Tests for resistance toPhytophthora infestans were done twice in each of 2 years on field- and glasshouse-grown tubers of ten cultivars. The cultivar rankings, were almost identical for the means of the four glasshouse tests and the means of the four field tests. However, for the field tests but not for the glasshouse tests there were cultivar x harvest date and cultivar x year interactions. It is concluded that resistance can be assessed on glasshouse-grown tubers and that such assessments should prove more consistent over years than those done on field-grown tubers. R.L. Wastie died 16.1.96     

Foliar and tuber assessment of late blight (Phytophthora infestans (Mont.) de Bary) reaction in cultivated potato (Solanum tuberosum L.)

Summary Host plant resistance is an important component to the management of potato late blight,Phytophthora infestans (Mont.) de Bary. Assessment of potato lines (Solanum tuberosum L.) with various levels of resistance toP. infestans (US8, A2 genotype) were evaluated in field trials, greenhouse controlled environment chambers and inoculated tuber reactions. Five lines (AWN86514-2, B0692-4, B0718-3, Jacqueline Lee, and B0288-17) with strong foliar resistance to late blight were identified in these inoculated field trials. Greenhouse controlled environment chamber studies allowed resistant and susceptible lines to be distinguished, but the 1998 greenhouse results did not correlate well with field data. Four lines (A084275-3, Bzura, MSG007-1, and MSG297-4RD) evaluated by a digital image analysis technique demonstrated tuber resistance based upon average reflective index values in the inoculated tuber studies. Tuber resistance did not correlate with field foliar resistance. Based upon these results, field assessment of foliar reaction toP. infestans provides the best measure for assessing late blight resistance in potato. Tuber resistance to late blight can be identified among lines with varying levels of foliar resistance.     

Foliar reaction toPhytophthora infestans in inoculated potato field trials in Michigan

Late blight, caused byPhytophthora infestans, is the most important disease of potato worldwide and foliar resistance is an important component of managing late blight in the field. The objective of this research was to identify germplasm for use in breeding cultivars with foliar resistance toP. infestans. More than 500 clones were tested from 1997 to 2002 in inoculated (US8 genotype) field experiments conducted at the Michigan State University Muck Soils Research Farm in Bath, Michigan. All of the current commercial cultivars tested were classified as susceptible toP. infestans. The most resistant clones were A90586-11, AWN86514-2, B0718-3, Jacqueline Lee (MSG274-3), MSI152-A, MSJ307-2, MSJ317-1, MSJ453-4Y, MSJ456-2, MSJ456-4, MSJ461-1, MSK101-2, MSK128-1, NY121, LBR8, LBR9, Tollocan, and Torridon. Some of these resistant selections were from crosses with B0718-3, Jacqueline Lee, and Tollocan suggesting that the resistance toP. infestans was transmissible. These resistant clones will provide the opportunity to breed late-blight-resistant cultivars from a diverse pool of cultivated germplasm. Consistent foliar reaction toP. infestans over years suggested that the Michigan State University Muck Soils Research Farm is a valuable location for North American breeders to assess the reaction of potato germplasm to the US8 genotype of late blight.     

Resistance to late blight and soft rot in six potato progenies and glycoalkaloid contents in the tubers

Glycoalkaloids are anti-nutritional compounds commonly found in wildSolanum species used as resistance sources to major potato pathogens. It is therefore important for breeding purposes to know whether selecting for resistance using such species necessarily selects also for high glycoalkaloid contents in the tubers. To test this hypothesis, we used six partial progenies from crosses betweenSolanum tuberosum and accessions ofS. andigena, S. berthaultii, S. phureja, andS. vernei to investigate the possible correlation between resistance toPhytophthora infestans and/or toErwinia carotovora subsp.atroseptica and the concentration of glycoalkaloids in tubers. Concentrations of α-solanine and α-chaconine in the tubers segregated in each progeny, as did resistance to each pathogen. Some, but not all, clones from each progeny showed hypersensitive reactions to the isolate ofP. infestans used. Furthermore, clones within each progeny also differed for components of partial resistance toP. infestans, suggesting that all four wild species could be used as sources of both race-specific and partial resistance to late blight. With the exception of low, but statistically significant, correlations between concentration of α-solanine and two late blight resistance components (incubation period and spore production per unit lesion area) in progenies derived fromS. vernei, and despite a trend towards higher glycoalkaloid concentrations in the tubers of the clones most resistant to soft rot within progenies derived fromS. berthaultii andS. vernei, no consistent relationship between resistance to either disease and concentrations of α-solanine and/or α-chaconine was observed. These results indicate that neither race-specific nor partial resistance to late blight and soft rot in the accessions used as progenitors of resistance depend on high solanine or chaconine concentrations. These resistance sources could thus prove useful in breeding programs for improved behaviour againstP. infestans and/orE. carotovora.