Powdery mildew field infection on leaves and stems of Cucurbita pepo accessions

The response of 170 Cucurbita pepo accessions to natural field infection with powdery mildew (Erysiphe cichoracearum and Sphaerotheca fuliginea) on leaves and stems was evaluated and considered with regard to the possible connection between powdery mildew severity on stems with resistance at the whole plant level. Infection degree of powdery mildew was assessed by evaluating the leaf surface (L) and stems and petioles (S). The "area below curve" (ABC) of the disease infection progress was calculated separately for leaf blades (ABC-L) and stems and petioles(ABC-S). The 170 C. pepo accessions (zucchini, vegetable marrow and pumpkin morphotypes) were highly susceptible to infection by powdery mildew. The mean value of the ABC-L for this set was 3070.5, while the mean value of the ABC-L for two control cultivars was 978.1. The mean value ABC-S was 929.5 for the whole set and 656.3 for the control cultivars. Statistically significant differences for resistance were detected among groups of morphotypes. The differences in infection degree on stems and petioles among morphotypes were not statistically significant, however, these data followed the same trend as those obtained on leaves. A clear relationship between disease severity on leaves and on stems among accessions was not found.