White mold, (Sclerotinia sclerotiorum), is an aggressive pathogen of beans and is capable of inflicting devastating damage on yield. Finding resistance is a major concern to bean breeders. The scarlet runner bean (Phaseolus coccineus) is generally known to have greater resistance to white mold than does the common bean, (P. vulgaris). Since it is possible to cross these two species, we have started to examine the NPGS core collection of P. coccineus for resistance to this pathogen. A straw test was used to measure physiological resistance of bean stems to white mold. A rating of one equates to a small lesion, resulting from contact with inoculum, and a rating of nine describes total plant collapse. Controls that were used were two common beans, 91G, a commercially produced, blue lake type snap bean and ExRico, a small, white dry bean. The bean 91G received a straw test rating of 8.3, which correlates to a field test rating of 8.5. ExRico rated 7.4 with the straw test and had a field test score of 6.5. Within the P. coccineus collection we found very strong resistance, with straw test values of 1 and 2 in several individual plants and in some accessions. Accessions that had individuals that displayed the strongest resistance of all the plants tested were: PI201299, PI361302, PI406938, and PI535278. These accessions appeared to be segregating for white mold resistance. Accessions showing the best average resistance were: PI313221, PI361372, PI361539, and PI583553. Because P. coccineus is outcrossed, we expected to find variation within accessions for white mold resistance. Some accessions had uniformly high levels of resistance, while other accessions showed variability.
061 Examining the Phaseolus coccineus Collection for White Mold Resistance