Inheritance Of Resistance To Fusarium Root Rot In Phaseolus Vulgaris L And Phaseolus Coccineus L

Progenies of two bean crosses were studied for inheritance of resistance to dry root rot caused by Fusarium solani f. phaseoli. O.S.C. 22 (P. vulgaris) was used in both crosses as the susceptible parent. A Mexican line (P. vulgaris) and a line 2014 of the runner bean (P. coccineus) were used as the tolerant (partially resistant) parents. Artificial inoculation was accomplished by dipping seeds in a thick emulsion of macerated agar media. A disease rating system of 0-5 was used, with 0 indicating no infection and 5 very severe infection. The disease average of all plants within a given seed lot was taken as the best indication for tolerance or susceptibility. The parents and their progenies were tested in both greenhouse and field. Differences between the reciprocal crosses of 22 x Mexican were not observed. The reciprocal cross of 22 x 2014 was not maintained due to the difficulty of securing seeds when 2014 was used as the female parent. In both crosses the F1 showed that tolerance was recessive. The observed ratios in the backcross generation of the cross 22 x Mexican fit the theoretical ratio of 1 tolerant to 7 susceptible indicating the action of three major genes. The F2 generation, in both crosses, showed a wider spread of disease ratings than either parent and occupied most, if not all, disease classes. The F2 generation of the cross 22 x 2014 indicated the possible action of two major recessive genes conditioning tolerance. The F2 of the cross 22 x Mexican exhibited no close fit to definite genetic ratios. Analysis of F3 data for the interspecific cross 22 x 2014 indicated that two or three major recessive genes conditioned the tolerance of P. coccineus. Difficulties of obtaining definite genetic ratios in this cross may be due to modifying genes, as well as to lack of exact control of environment in the various tests. The F3 progeny of 22 x Mexican indicated the possible action of three major recessive genes, or perhaps fewer major genes and several modifiers conditioning resistance. It is recognized that the results reported in this study represent an approximation for the inheritance of resistance to Fusarium root rot. There was no significant relation between root vigor of P. coccineus and resistance to root rot. Pigmentation in the cross 22 x Mexican indicated a simple inheritance of one dominant gene for pigmented over green. Seed color inheritance in the same cross indicated the interaction of two major genes and gave in the F2 a ratio of 9 black: 3 brown: 4 white. A very high association was observed between seed color and plant color. No clear relation between pigmentation and resistance to root rot was found. The recessive nature of resistance, the approximate estimation of genes involved, and the apparent freedom from serious linkage problems form a basis upon which to develop a breeding program designed to incorporate tolerance to root rot with horticulturally desirable characters in present day varieties of beans.