Inheritance of disease resistance in the common bean

Investigations of the inheritance of resistance to the bean anthracnose fungus indicate a single factor difference between resistance and susceptibility where only the alpha strain of the fungus is concerned in the cross. Where both the alpha and beta strains are concerned a two-factor difference is indicated and a 9:7 ratio in F2 is obtained. In both instances resistance is dominant over susceptibility. F1 and F2 results of crosses involving the inheritance of susceptibility to the bean mosaic organism indicates a partial dominance of susceptibility over resistance to this disease. A two-factor hypothesis is advanced to account for the inheritance of resistance and susceptibility, Such an hypothesis is borne out by the totals of the observed F2 ratios between resistant and susceptible plants and by the fact that approximately one sixteenth of the F2 plants were severly infected with mosaic and bred true for this character in the F3. Inoculation results of F2 hybrids from crosses involving the inheritance of susceptibility to root rot indicate susceptibility ot this disease to be dominant over resistance. That this was the case had been indicated by the fact that a few F1 plants, which had been grown on infested ground, showed good infection. A tentative two-factor hypothesis to account for the inheritance of susceptibility to root rot, with a 9:7 ratio in the F2 between susceptible and resistant plants, is advanced to explain the results obtained. The fact that the susceptible plants were in the majority in the F2 and that a large number of the F3 families from these plants did not breed true while the F3 families from resistant F2 plants in almost all cases bred true, is in accord with the hypothesis advanced. The necessity of establishing an arbitrary dividing line between resistant and susceptible forms adds considerable difficulty to the proving of any hypothesis to account for the inheritance of susceptibility to bean root rot. A number of promising strains of beans have been isolated which show resistance to anthracnose. A few hundred heavily podded F2 types have been selected which show resistance to both root rot and mosaic and which should also be resistant to the beta strain of the anthracnose fungus. Further testing should isolate some very desirable resistant commercial types from these selections.