The latent period (LP) is a crucial component of partial resistance. Five cultivars, L94, Sultan (Su), Volla (VI), Julia (Ju) and Vada (Va), representing the known range in partial resistance and LP were crossed in a diallel, and the F1, F2 and F3 tested. The LP effectuated by the five cultivars is about 9, 101/2, 101/2, 13 and 151/2 days, respectively. The crosses Su ร L94, V1 x L94 and Ju x L94 had an F2 positively skewed. Their F2 means were similar or only slightly larger than the F1 means. The F2 frequency distributions in the crosses V1 x Su, Ju x Su and Ju ร VI were normal or nearly so with F1 and F2 means similar to each other and to the mid-parent value. The crosses involving Va as a parent again showed a positive skewness but with F2 means considerably larger than the F1 means.
Most F2's ranged from the low parent to the high parent values without transgression. In the crosses Va ร L94 (reported earlier) and Ju x L94 the parental values were not recovered among 216 and 154 F z plants, respectively. The cross Ju x Va showed transgression beyond the low parent, Ju.
From these data it is concluded, assuming no linkage, that seven loci are involved. The รท alleles (governing a longer LP) are thought to be distributed over the parents as follows : L94 .