Partial resistance to Phytophthora infestans in four Solanum crosses

Thirty progeny from each of four Solanum crosses were evaluated in the fiel d at Mount Vernon, WA, in 1996 and 1997 for partial resistance to Phytophth ora infestans. Of the four parents, three have high levels of partial resis tance to P. infestans; one derived from somatic hybridization of S. bolboca stanum, the other two from traditional breeding efforts for multiple diseas e resistance. Data were collected from each cross to estimate area under th e disease progress curve (AUDPC), days to 5% disease severity threshold (DT 5), and sporangia production (SP). AU of these variables differed significa ntly among the progeny within each cross in each year. Correlation analysis indicated that DT5 was highly correlated with AUDPC for all four populatio ns for both years. Log-transformed SP was significantly (P<0.001) correlate d 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 effici ency, latent period, and lesion growth rate), was the most important in ide ntifying progeny with partial resistance to late blight in all four crosses in this study.