Root rot resistance in common beans sought in plant breeding program

More than 1,200 introductions and commercial bean varieties were tested by inoculation with cultures of the two most prevalent organisms causing root rot in California. Only seven showed some tolerance to the Fusarium fungus, and none to the Rhizoctonia. These seven were crossed with ten commercial bean varieties in most of the 70 possible combinations. Each plant was, graded on severity score of 0-4, and the average scores were used for evaluation. None of the first-generation plants showed resistance. Second-generation plants were tested from 60 of the 70 combinations. These tests indicated that hybrids from three of the resistant lines gave very few plants with low disease scores. Second-generation plants of hybrids from the other four accessions had 16%-19% resistant plants, or a ratio of 13 susceptible to three resistant. These results could be explained by assuming that resistance is due to two independent genes, one dominant and the other recessive. To test this hypothesis, four third-generation combinations were tested. To confirm the assumption, only 1/16 of the third-generation progenies would have disease scores as low as the resistant parent, and 2/16 would segregate three resistant to one susceptible and would have relatively low average scores.