Variation in Sclerotinia sclerotiorum Bean Isolatesfrom Multisite Resistance Screening Locations

Otto-Hanson, L., Steadman, J. R., Higgins, R., and Eskridge, K. M. 2011. Variation in Sclerotinia sclerotiorum bean isolates from multisite resistance screening locations. Plant Dis. 95:1370-1377. There is no complete resistance to Sclerotinia sclerotiorum, cause of white mold in dry bean (Phaseolus vulgaris). Variable resistance expression is one problem in screening for improved white mold resistance. With no previous information in the literature, pathogen variation in multisite screening nurseries was evaluated as one cause of diverse resistance expression. In all, 10 isolates of S. sclerotiorum used in greenhouse screening and 146 isolates collected from nine white mold field screening nurseries in major bean production areas in the United States were compared using mycelial compatibility groupings (MCGs) and an aggressiveness test. These 10 greenhouse screening isolates formed six MCGs. Among 156 field and greenhouse isolates, 64 MCGs were identified and 36 of those were each composed of a single unique isolate. Significant differences in isolate aggressiveness were found between some isolates in different MCGs but the isolates within an MCG did not differ in aggressiveness. High isolate variation found within and between field locations could influence the disease phenotype of putative white mold resistant germplasm. We next compared genotype and phenotype of isolates from screening nurseries and those from producer fields. Variability found in and among screening locations did reflect variability found in the four producer fields sampled. White mold resistance screening can be improved by knowledge of isolate genotypic and phenotypic characteristics. Sclerotinia sclerotiorum (Lib.) de Bary, cause of white mold on dry bean, is among the most devastating and promiscuous necrotrophic fungal plant pathogens. S. sclerotiorum has a wide geographic distribution and diverse host range consisting of over 400 plant species, causing severe yield losses in many important agronomic crops, but it also includes many weed species among its hosts (15). Some other important crops affected are sunflower, soybean, oilseed rape, peanut, dry pea, and lettuce (4,5). Management of S. sclerotiorum can be problematic. Planting a white-mold-resistant cultivar would be the most cost-effective and least intensive management strategy for dry and green bean Phaseolus vulgaris producers. A major detriment to the resistance control strategy for this pathogen is the lack of high levels of broad resistance in the majority of its host plants (3,5,34). Because disease outbreaks are usually sporadic in fields (especially ascosporeinitiated disease) and the dispersal of the pathogen in a field is usually aggregated, screening putative sources of host plant resistance under field conditions can be challenging, especially without irrigation and artificial inoculation (5). Dry bean producers in the Northern Great Plains of the United States and in Michigan have rated white mold as the most serious bean disease (27,39). Bean cultivars grown in temperate regions under irrigation can suffer greater losses under S. sclerotiorum attack (e.g., the pinto bean, which is the most important dry bean class in the United States; 28). Similar to other host species of S. sclerotiorum, a lack of broad resistance sources slows progress in breeding for improved bean lines. Kolkman and Kelly (23) found that genetic resistance in dry bean is quantitatively inherited and complex because it consists of both disease avoidance and physiological mechanisms. Avoidance is manifested in upright and less dense canopies that create a less conducive microclimate for S. sclerotiorum to colonize blossoms and infect the stems (23,29,33). Greenhouse or laboratory screening methods detect physiological mechanisms better than field tests because they directly challenge the resistance of a bean line, and can identify “escapes” that had been identified in the field as resistant. Multisite screening is a collaborative effort in which bean breeders and pathologists from major production areas in the United States each screen new putative sources of white mold resistance, with the same evaluation methods for greenhouse and field tests (30,35–37). Screening for broad disease resistance across multiple locations incorporates variable environments for disease development or variable aggressiveness of the pathogen population across locations (8). Variable environments can be used to test for genotype–environment interactions, with the across-site requirement that nurseries are managed to promote white mold. In general, disease is favored when bean crops are managed to maximize yield (34). Kull et al. (26) suggested that the range in aggressiveness of S. sclerotiorum isolates in agricultural populations may impact cultivar performance. Pathogen variability could contribute to the low correlation between greenhouse and field screening tests across sites, and variability in isolate aggressiveness has been associated with problems in evaluating and breeding for resistance in host crops (7,24,25). Our isolates collected from multisite nurseries provide opportunities to study pathotype aggressiveness among different screening locations. In previous S. sclerotiorum variation studies, mycelial compatibility groupings (MCGs) were a tool that has been used to determine the genetic variation among pathogen isolates. MCGs have been used to determine pathogen variability in isolates of S. sclerotiorum from cabbage, canola, giant buttercup, cauliflower, soybean, and dry bean in the United States, Canada, Australia, and New Zealand (7,9,11,13,17,19,21,22). The MCG system is an assay of self and nonself recognition controlled by multiple loci (5,6) and is a good test for intraspecific heterogeneity in S. sclerotiorum. Researchers have suggested that MCGs are genotypically unique (i.e., they represent genetically different individuals; 11,22) but have not been reported to relate to a phenotype. Factors for MCGs were found to be stable and unchanging through successive sexual generations and after serial culturing, and the correlation between an MCG and a DNA fingerprint supports the synonymous relationCorresponding author: L. Otto-Hanson, E-mail: [email protected] Accepted for publication 28 April 2011. doi:10.1094 / PDIS-11-10-0865 © 2011 The American Phytopathological Society