Breeding for Multiple Root Rot Resistance in Snap Beans1

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Abstract
Inheritance of resistance to Pythium, Fusarium, and Rhizoctonia in snap beans, Phaseolus vulgaris L., was studied for three populations involving a common resistant parent (Cornell 2114-12) and 3 different susceptible parents. Pythium resistance was strongly associated with colored seed but resistance was found in some white seeded segregants and was widely influenced by degree of susceptibility of white seeded parents. In contrast to the widely accepted theory that colored seed and resistance to Rhizoctonia are tightly linked, Rhizoctonia resistance seemed to be independent of seed color. Heritability for Rhizoctonia resistance was 0.75 and 0.65 for broad and 0.32 and 0.29 for narrow sense heritability. Resistance to all three diseases was independent and quantitatively inherited. Correlations for resistance to Pythium and Fusarium in F4 with F3 selections (r = 0.557**) were in line with heritability expectations. In the Rhizoctonia selections, generation to generation correlation was high (r = 90**) among those with the best resistance but only moderate among those with moderate resistance. In view of the low narrow sense heritability, selection for resistance in later rather than earlier generations should be more effective.

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