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.