Partial resistance toPhytophthora infestans in fourSolanum crosses

Thirty progeny from each of fourSolanum crosses were evaluated in the field at Mount Vernon, WA, in 1996 and 1997 for partial resistance toPhytophthora infestans. Of the four parents, three have high levels of partial resistance toP. infestans; one derived from somatic hybridization ofS. bulbocastanum, the other two from traditional breeding efforts for multiple disease resistance. Data were collected from each cross to estimate area under the disease progress curve (AUDPC), days to 5% disease severity threshold (DT₅), and sporangia production (SP). All of these variables differed significantly among the progeny within each cross in each year. Correlation analysis indicated that DT₅ was highly correlated with AUDPC for all four populations for both years. Log-transformed SP was significantly (P>0.001) correlated to AUDPC values for one population in both years, but the significance of the correlation was variable between years for the remaining three crosses. The variable DT₅, which is composed of three components (infection efficiency, latent period, and lesion growth rate), was the most important in identifying progeny with partial resistance to late blight in all four crosses in this study.