INHERITANCE OF RESISTANCE TO UROMYCES-VIGNAE IN COWPEA AND THE CORRELATION BETWEEN RESISTANCE AND SENSITIVITY TO A CULTIVAR-SPECIFIC ELICITOR OF NECROSIS

Microscopic examination of rust-inoculated progeny of a cross between a resistant and a susceptible cowpea cultivar revealed infection sites in F1 and F2 plants that differed in fungal growth and plant responses from those observed in the parents. Frequencies of different categories of infection sites and the general correlation in each plant of the categories recorded for both monokaryotic and dikaryotic infections suggested that resistance to both the monokaryon and dikaryon were governed primarily by two, partially dominant, resistance genes. Six of the possible nine genotypes in F2 plants could be distinguished microscopically in contrast to only two by macroscopic assessment. Correlations between cytological phenotypes and predicted genotypes were generally confirmed by selected selfings and backcrosses to the susceptible parent, although a few discrepancies suggested phenotype modification by other factors. The number of dead cells induced in F2 plants by injection of intercellular washing fluids from monokaryon-infected, susceptible tissue was similar for all plants predicted from their cytological phenotypes to have one or more genes for resistance and was higher than that induced by water injection. The frequency of plants that showed significant sensitivity to the fluids differed between cytological phenotypes and declined with increasing susceptibility to the fungus.