Quantitatively Inherited Reactions of Alfalfa toPeronospora trifoliorum

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Abstract
Infection-type (IT) data were obtained by inoculating plant populations derived from all possible self- and cross-pollinations, and some F2 populations, of 6 diploid clones of M. sativa with 3 monoconidial isolates (I-5, I-7 and I-8) of P. trifoliorum [downy mildew]. Host effect on intensity of conidial production appeared to be inherited quantitatively. Analyses of variance indicated that primarily additive effects were involved in response to isolate I-8, while additive and nonadditive effects were involved in response to isolates I-5 and I-7. Comparisons of variances and covariances in IT data indicated that genes conditioning a reduced IT usually were dominant. In some cases, however, genes that conditioned a higher IT were expressed although genes that conditioned a reduced IT were present, which suggested epistasis.