Inheritance of resistance to late blight in foliage and tubers of progenies from parents differing in resistance

Summary TenSolanum tuberosum genotypes differing in resistance to late blight (Phytophthora infestans (Mont.) de Bary) in foliage and tubers were intercrossed to determine the inheritance of disease resistance in their progenies. Plots of 10–15 clones per progeny were established in each of 2 years and resistance assessed by field or laboratory tests. The parental genotypes were similarly tested each year. The parents differed in general combining ability (GCA) for both foliage blight (FB) and tuber blight (TB). The parental and GCA scores were significantly correlated for both aspects of the disease, but the correlations between foliage and tuber scores for parents and for GCAs were not significant. Three parental genotypes were highly resistant in both foliage and tubers, and the genotype with the highest GCA for resistance to both FB and TB (cv. stirling) is recommended as the best parent. There was no evidence of strong genetic correlation between both aspects of resistance, and it is suggested that both be selected for in a breeding programme.     

Assessing general combining ability for late blight resistance and tuber characteristics by means of glasshouse seedling tests

Summary Screening tests to detect resistance to late blight in both foliage and tubers were done on glasshouse-grown seedling progenies in parallel with visual assessments by three experienced potato breeders of the yield and quality of glasshouse-grown tubers of the same progenies. There were large differences between the parents of the progenies in their general combining ability (GCA) for both foliage and tuber blight, despite some variation due to specific combining ability for foliage blight. There were also differences between parents in their GCAs for visual preference scores, but these GCAs and those for blight resistance were not correlated. The blight-resistant cv. Stirling had the best combination of high GCAs for all three attributes. The use of these and other seedling progeny tests in a multitrait genotypic recurrent selection scheme is discussed.     

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.     

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.     

Late blight resistance in the British Neo-Tuberosum potato population

Summary Development of the British Neo-Tuberosum population included selection for resistance to late blight. High levels of foliage resistance are most often found in later-maturing clones but some early-maturing clones have good resistance. Resistance of foliage to Race 4 and to a complex race is associated. Some clones give variable results in tests for resistance in tubers. While those consistently displaying resistance are most frequent among those with resistant foliage, some have rather susceptible foliage. Tuber resistance appears most common among early maturing clones but this could be misleading, perhaps being due to tubers of later-maturing clones tending to be immature when tested (resistance often increasing as tubers mature). However some ‘lates’ are resistant, suggesting that genes for resistance in immature tubers occur. Selections are being used experimentally as parents in breeding for blight-resistant cultivars, and controlled breeding within Neo-Tuberosum to produce superior parents has commenced.     

Effects of Mulch and Potato Hilling on Development of Foliar Blight (<Emphasis Type="Italic">Phytophthora infestans</Emphasis>) and the Control of Tuber Blight Infection

Foliar and tuber blight caused by Phytophthora infestans accounts for significant losses in potatoes in field and storage. Nevertheless, limited research has been published on the effects of cultural practices on late blight control. Field experiments were conducted in two years on Howard gravely loam soil in New York State to evaluate the effectiveness of mulching using oat straw and hilling in preventing tuber blight infection for cvs Allegany and Katahdin. Potato hilling and mulching had little effect on foliar blight development. The cultivar affected the disease development in the foliage, with cv. Allegany showing lower foliar late blight than cv. Katahdin. Tuber blight incidence averaged 25% for cv. Allegany and 3% for cv. Katahdin in hilled plots, while in the mulched plots the incidence of tuber blight averaged 33% for cv. Allegany and 10% for cv. Katahdin. The straw hay mulch was ineffective in tuber blight control. Tubers set at a soil depth of more than 7 cm had lower tuber blight incidence than shallow tubers set at a depth of less than 7 cm. In both years, hilling provided partial protection of tubers but its effectiveness was limited in the presence of favourable conditions for late blight development. Even though large hills had proportionally a lower tuber blight incidence than medium-sized hills, the difference between the different hill sizes was not significant. These studies suggest that the use of cultivars with foliage resistance to late blight in combination with cultural practices may partially reduce the incidence of tuber blight. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation for endorsement by Cornell University or the US Department of Agriculture.     

Frühselektion auf relative Krautfäuleresistenz an Sämlingen der Kartoffel und deren spätere Kraut- und Braunfäuleresistenz

Zusammenfassung Im Gew?chshaus wurden 838 hoch krautf?uleanf?llige bis hoch resistente Kartoffelidiotypen als S?mling auf relative Krautf?uleresistenz (KR_G), als Einzelstaude und A-Klon im Scheibentest auf relative Braunf?uleresistenz (B_S) und als A-Klon auf relative Krautf?uleresistenz im Feld (KR_F) untersucht. Durch rechtzeitiges Abschneiden am Stengelgrund überlebten auch stark befallene S?mlinge. Bei scharfer S?mlingsselektion (KR_G≥8 behalten) blieben 24% der Ausgangszahl erhalten, aber nur 52% der vorhandenen resistenten Idiotypen mit KR_F≥5 und B_S≥6,4. Wurden nur S?mlinge mit KR_G≥7 behalten, so gingen immer noch 8% der sp?ter erkennbaren Resistenztr?ger für Kraut- und Braunf?ule verloren, jedoch ergab sich eine Einschr?nkung des Zuchtmaterials durch die Frühselektion auf 57% der Ausgangszahl. Es kam starke ontogene Pr?disposition des Krautes sowohl in Richtung erh?hter Anf?lligkeit als auch erh?hter Resistenz vor. Neunundrei?ig Braunf?uleresistenztr?ger waren als S?mling hoch krautf?uleanf?llig. Scharfe Selektion der S?mling ist unangemessen. Für die Sortenauslese wird die Frühselektion auf Krautf?uleresistenz nicht empfohlen.

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Summary The reliability of blight resistance assessments made at the seedling stage and subsequent loss of tuber blight resistance were studied in 838 seedlings with different relative resistance to foliage blight. The criteria were assessed after Darsow (1989) in a field experiment in which the effect of maturity class was largely eliminated. Seedlings with 4–6 developed leaves were inoculated in the glasshouse with 3–3.5×10⁵ zoospores of a mixture of pathotypes. Multiple assessments (NB 9=high resistance, 1=greatest susceptibility) and immediate removal of shoots at the stem base allowed those forms most susceptible to foliage blight to be regenerated for testing in subsequent years (single plants, A-clone). Table 1 gives the crosses, scores for resistance of the parents and the numbers of seedlings tested per progeny. The percentage distribution of foliage blight resistance in the field (KR_F) related to that of the seedlings (KR_G) is given in Fig. 1. Potato plants which were very susceptible at the seedling stage could nevertheless grow in the field and exhibit good foliage blight resistance. The reverse was also observed. The distribution after weak selection for the development of blight on the seedling in the field is given in Table 2. There was an unavoidable loss of 10% and 14% in those rotted seedlings that possessed foliage and tuber blight resistance respectively (Table 3). High tuber blight resistance (B_S) occurred in all classes of seedlings with foliage blight resistance; likewise susceptibility to tuber blight occurred in seedlings with foliage blight resistance. From seedlings with KR_G≥7, 204 out of a total of 222 retained their genotypes with KR_F≥5 and B_S≥6.4. A reduction of 43% in the final number of seedlings was accompanied by a loss of 8% of those that possessed combined resistance. Of the seedlings which had been harvested and pre-tested, 277 out of 481 later had insufficient levels of resistance. Early selection will only be advantageous by careful selection of progenies with increased levels of resistance. The interaction of resistance with environment and developmental stage does not at present commend the application of this approach to varietal breeding.

     

Assessing progenies of potato for resistance to early blight

Summary Progenies from crosses between cultivars varying widely in resistance to early blight (Alternaria solani Sor.), were assessed for resistance as true seedlings in a glasshouse in Scotland. The resistance of a representative sample of surviving genotypes from each progeny was compared with samples of the same progenies not previously exposed to the fungus, both in the glasshouse in Scotland and in the field in Israel. The exposed population was more resistant. Resistance was identified more effectively in adult plants from tubers in the glasshouse than in true seedlings and agreement between glasshouse and field assessment was better when progenies were compared rather than individual genotypes. The mid parent and progeny mean scores of the unexposed population were correlated at both sites, thus confirming that the resistance is heritable. Selecting resistant individuals at the seedling stage is suggested as a useful tool for resistance breeding, having first chosen the best parents for crossing.     

Screening for resistance to early blight (Alternaria solani) in potato (Solanum tuberosum L.) using toxic metabolites produced by the fungus

Summary The phytotoxic components of the culture filtrate of the fungusAlternaria solani Sor, which causes early blight in potato, were used in this study to differentiate between resistant and susceptible genotypes selected to represent a range of reactions when screened in the field. Detached leaflet assays, using spores and toxic metabolites from the culture filtrate, were compared with field ratings and a whole-plant glasshouse test. Rank correlations between several detached leaflet tests were highly significant (P<0.01) but the correlations between these tests and the glasshouse and field tests were poor. The disease ratings obtained in the various tests were clustered into four groups and assigned to resistance classes. For many of the genotypes there was a good correspondence between resistance classes for all test methods. Divergent results between tests were associated with foliage maturity characteristics of the genotypes. Contribution no. 3879050 of the Lethbridge Research Station.     

Assessment of the field resistance of potato genotypes with major gene resistance to late blight (Phytophthora infestans (Mont.) de Bary) using inoculum comprised of two complementary races of the fungus

Summary Two races ofPhytophthora infestans, differing in virulence against four major (R) genes for resistance, were used to assess the field resistance of differential (R) genotypes, both singly and mixed together in equal proportions. The mixed inoculum led to an over-estimation of the foliage resistance of genotypes with which only one of the races was compatible, by up to two scoring categories of a 1–9 scale in field trials and, to a lesser extent, in glasshouse tests depending on the differential and the test. Assessment of tuber resistance was not significantly affected. The R₁₀ and R₂ differentials showed a higher level of field resistance in the foliage and tubers, respectively, than the other genotypes. It is concluded that assessments of genotypes which may possess R genes, using mixed inoculum, should be interpreted with caution, and apparent high levels of field resistance confirmed through genetic studies of the inheritance of the resistance.     

Relationship between resistance to late blight in potato foliage and tubers of cultivars and breeding selections with different resistance levels

Due to changes in the pathogen population, late blight of potatoes caused byPhytophthora infestans has become a more difficult disease to manage and there is a need for new potato cultivars with greater and more stable disease resistance. Field studies are required to identify and characterize host resistance to late blight in both foliage and tubers but, as epidemics vary from year to year, combining multi-year results can be difficult. Residual maximum likelihood (REML) was successfully used to analyze data which demonstrated good correlations between foliar and tuber responses to late blight (US-1 genotype). In addition, studies on post-harvest inoculated tubers provided more reliable disease responses than field studies, and the disease resistance with this method correlated well with foliar disease responses. This tuber test would be a useful method of pre-screening selections for disease response prior to testing in the field.     

Resistance to late blight (Phytophthora infestans (Mont.) de Bary) of potato plants regenerated fromin vitro selected calli

Summary Cells of seven potato cultivars were selectedin vitro with culture filtrate (CF) ofPhytophthora infestans (Mont.) de Bary. Regenerated plants were tested for late blight resistance. The aim of the study was to check the efficacy of CF in the selection of potatoes for resistance to late blight and to evaluate the effects of additive factors on general resistance. One selection cycle, applied to a cell culture system, selected cells resistant to toxic metabolites of CF.In vivo screening of clones regenerated from selected cells was done in two steps: on whole plants, assessing foliage late blight, and on detached leaves, assessing single factors of horizontal resistance. In general, the frequency of resistant variants selected with CF did not differ from that of resistant somaclonal variants. Nevertheless, high concentrations of CF in the growth medium seemed to induce an improvement of some partial resistance factors as compared with the source plants.     

Comparison of the NegFry decision suport system with routine fungicide application for the control of potato late blight in Ireland

Summary Field experiments between 1996 and 2000 compared the efficacy of fungicide programmes applied in accordance with the NegFry and Met éireann (Mé) decision support systems (DSS) for the control of late blight with 7- and 10-day routine fungicide programmes. The Mé DSS reduced fungicide use by 68% and 54% respectively while NegFry reduced fungicide use by 49% and 27% compared with the 7- and 10-day programmes. The NegFry DSS was similar to the 10-day routine programme in terms of late blight control, quality and marketable yield. A similar result was found when the NegFry DSS was compared with a 7-day routine mancozeb programme (included for two seasons only). Within the NegFry DSS the use of fluazinam resulted in improved yield, foliage blight and tuber blight control compared with mancozeb, but this benefit was significant for tuber blight only. The Mé DSS resulted in inferior disease control, yield and quality.     

Sites of entry ofphytophthora infestans (Mont.) de Bary into potato tubers in assessment of tuber blight resistance

Summary During assessment of tuber blight resistance using three test methods, the sites of entry ofPhytophthora infestans into the tuber was estimated for 41 cultivars. The trials were conducted using a mix of three highly virulent isolates at Gro? Lüsewitz 1991–1993. The influence of date of harvest and intermediate storage before inoculation was examined with two cultivars in 1993. Two levels of soil moisture and five densities of inoculum were tested in 1994. Methods of assessment of tuber blight resistance, cultivars, years and all their interactions showed a significant effect on sites of entry. Eye infections identified the highest the level of resistance and correlated best with tuber blight resistance in net bag test, r=-0.58. Soil moisture and temperature before harvest varied the ontogenic development of resistance and the relation of sites of entry of the pathogen from July to September. Intermediate storage changed the resistance and the preferred site of cultivar-specific infection. Interactions cultivar x soil moisture and cultivar x density of inoculum were significant and should be considered in the resistance assessment of cultivars.     

Assessment of new potato cultivars in Europe: a survey

Summary In 1994, a postal questionnaire was sent to 41 institutes in 27 countries to obtain information on the national potato cultivar testing programmes in Europe. Responses were received from 19 countries. Foliage, tuber and yield characteristics were assessed in all countries but the number of sites and replicated varied considerably between the countries. Cooking and taste qualities were measured in most countries using relatively simple methods but processing characteristics were tested less commonly. Susceptibility to the most common diseases, i.e. late blight, common scab, leafroll and PVY^o, was assessed in over 75% of the countries while resistance to another 21 disease and 4 potato cyst nematode pathotypes was assessed by various countries, depending on their national importance. Many of the assessments were based on records of natural infection although controlled inoculation tests were done more frequently for the more important diseases.     

Soft-rot resistance combined with other tuber characters in long day-adaptedSolanum phureja

Summary Offspring were produced from a cross between two long-day-adaptedSolanum phureja clones which carried resistance to tuber soft rot (Erwinia carotovora subsp.atroseptica). In tests carried out on the produce of field-grown plants raised from tubers, over fifty per cent of the 173 offspring were found to be highly resistant. Assessments were also carried out of tuber yield, mean tuber weight, tuber number, shape, regularity, flesh colour, texture of the steamed flesh, fry colour, after-cooking blackening, sprout length after storage and overall dormancy. There were statistically significant differences between clones for all characters (P<0.001). Twelve of the clones were selected on the basis of high resistance, yield, tuber weight, regularity of shape and absence of after-cooking blackening. The value of resistant long-day-adapted diploid material for commercial breeding is discussed.     

Field validation of four decision support systems for the control of late blight of potatoes in Ireland

Summary Field experiments using two cultivars (Rooster and King Edward) were carried out between 2001 and 2003 to compare the efficacy of the NegFry, Simphyt, ProPhy and Plant-Plus decision support systems (DSS) with routine 7-day fluazinam treatment for the control of potato late blight. Compared with the routine control, the NegFry and SimPhyt programmes resulted in a 58-44% reduction in application frequency. The ProPhy and Plant-Plus programmes resulted in more modest savings of between 10 and 25%. The date of disease onset was not significantly different between routine programmes and DSS programmes irrespective of the cultivar. None of the DSS’s resulted in inferior foliage blight control when compared with the routine application of fluazinam. DSS programmes generally out-yielded the routine treatment but differences were not significant. All DSS programmes give equivalent tuber blight control to the routine programme even with a very tuber blight susceptible cultivar.     

Biochemical and morphological characterization of potato clones differing in their resistance to late blight

Summary Morphological and biochemical parameters that could be involved in resistance to late blight were studied in non-infected and in infected potato hybrids resulting from a cross betweenSolanum phureja, resistant toPhytophthora infestans, and a susceptibleSolanum tuberosum. Some morphological differences between resistant and susceptible hybrids, indicating a positive correlation between stem diameter and phloem thickness in the stem and resistance toP. infestans, were observed. The lignin content in the leaves of the resistant hybrid rose upon infection byP. infestans. In the leaves of the susceptible hybrid, a diminution of the lignin content could be observed upon infection byP. infestans. In the same context, peroxidasic activity raised upon infection byP. infestans in both resistant and susceptible hybrids. Further characterization of the hybrid clones based on the polymorphism of peroxidases was attempted using isoelectric focusing.     

Resistance to <Emphasis Type="Italic">Potato virus Y</Emphasis> in a Multitrait Potato Breeding Scheme without Direct Selection in Each Generation

In an experimental breeding scheme to improve late blight (Phytophthora infestans) and white potato cyst nematode (Globodera pallida) resistance of tetraploid potato over three generations of crossing and selection, 15 clones survived the final selection, and these were derived from 15 great-grandparents. There was no direct selection for resistance to Potato virus Y (PVY), but 14 out of the 15 great-grandparents were resistant to PVY and three had extreme resistance. Thirteen of the 15 descendants had PVY resistance and one extreme resistance. This was within the range expected for a random (unselected) sample from the genotypes of the great-grandparents. Hence, we found no evidence for any positive or negative association between PVY resistance and the attributes selected. The conclusion is that laborious selection is not required in every generation when many parents have PVY resistance, including some with more than one copy of a PVY resistance gene or resistance at more than one locus. However, in the future, determining the major virus resistance genes present in potential parents in each generation using diagnostic molecular markers would prevent susceptible × susceptible crosses being made and maximise the number of resistant × resistant ones.     

Stability of late blight resistance in population B potato clones

Summary Increased virulence associated with fungicide resistance and variablePhytophthora infestans populations has been recorded in many potato growing regions with enormous economic effects. The current emphasis on disease management in East Africa includes the use of potato varieties with durable resistance to late blight. Seven promising clones from Population B with quantitative resistance (no R-genes), two advanced clones from Population A (with Rgenes) and three control varieties were grown for three cropping seasons in order to determine their reaction and stability of late blight resistance. Late blight occurrence was detected in all cropping seasons. The analysis of variance of disease data (AUDPC) for genotypes, locations x seasons was highly significant (P<0.001) indicating the differential response of the geno-types and the need for stability analysis. The Additive Main Effects and Multiplicative Interaction (AMMI) statistical model, showed that the most stable genotypes were 392127.256, 381471.18, 387121.4 (resistant) and 391049.255 (susceptible to late blight). Within environments, the ranking of genotypes was not consistent. The variety Kabale was ranked as the most susceptible genotype while clone 381471.18 and Rutuku were ranked the most resistant. Selective deployment of resistant and stable varieties is critical in minimizing economic loss and damage attributed to late blight in low input farming systems.