Location: Sunflower Research
2013 Annual Report
The development of common bean cultivars with partial resistance and/or avoidance to white mold (WM) caused by Sclerotinia sclerotiorum would benefit producers by reducing yield loss and reducing input costs for fungicides. One goal is to identify bean germplasm supplied by bean breeders from across the USA with broad and/or specific partial resistance to WM. There were six field tests conducted in six locations. Nebraska, North Dakota and Wisconsin did not have results due to weather. These problems that resulted in no data were disappointing, but demonstrate the importance of testing in multiple locations. In the field tests, all nine lines were significantly more resistant than the susceptible cultivar Beryl. The results from the three fields that reported WM disease substantiate that two bean lines, A195 and VRW32, developed with NSI support, has resistance similar to G122 (resistant check). The remaining seven lines, also developed by NSI support, were rated as having intermediate resistance in navy, small red, pinto and great northern seed classes. The greenhouse straw test results were used to identify 11 bean lines with resistance similar to G122. There were four lines that showed intermediate resistance in the field but were susceptible in the greenhouse test. These lines exhibited escape or avoidance mechanisms which can be useful where WM is not severe. Navy, small red and pinto lines exhibited resistance in the field and greenhouse. Pinto, small red, great northern and cranberry seed classes showed resistance in the greenhouse tests. Progress breeders have made in incorporating WM resistance into dry bean lines with commercial potential validates use of multisite screening and National Sclerotinia Initiative support to breeders over the last 10 years. Previous snap beans were released with WM resistance supported by NSI.
The second goal is to assess the variation in dry and snap bean isolates of S. sclerotiorum to guide resistance screening. The genetic diversity of 180 S. sclerotiorum isolates representing bean growing regions of the USA as well as Australia, France and Mexico was analyzed using Mycelial Compatibility Grouping (MCG) as well as 16 microsatellite markers that are polymorphic for the isolates we have collected. This process involves generating the complete sequence of simple sequence repeats (SSRs) in the isolates. Cluster analysis using Euclidean distance placed 180 isolates in 15 large clusters and 38 sub clusters. This result indicates a high level of diversity among these isolates. A substantial variation was observed between clusters while most of the isolates within a cluster were similar for all microsatellite markers. Isolates from each single cluster may belong to the same or different MCGs. Isolates from clusters 2 and 11 have the same MCG (these results support the hypothesis that isolates within a single MCG share the same microsatellite allele and may be clonal) while isolates from all other clusters have different MCGs (these results support the hypothesis that MCG is a phenotypic marker system controlled by multiple loci and is not always associated with groups of identical or closely related microsatellite haplotypes). Isolates from clusters other than 2 and 11 have shown polymorphism between isolates of the same cluster and may explain different MCGs found in the same cluster. In total, 145 microsatellite haplotypes were found within the 180 isolates analyzed to date. The remaining 170 isolates will be haplotyped this summer and compared to aggressiveness for detection of associations with location, cluster or MCG. Fungicide sensitivity of these isolates will be tested this fall.