Management of Powdery Mildew in Summer Squash with Host Resistance, Disease Threshold-based Fungicide Programs, or an Integrated Program. M. T. McGrath, Department of Plant Pathology, Long Island Horticultural Research Laboratory, Cornell University, 3059 Sound Avenue, Riverhead, NY 11901-1098. H. Staniszewska, Department of Plant Pathology, Long Island Horticultural Research Laboratory, Cornell University, 3059 Sound Avenue, Riverhead, NY 11901-1098. Plant Dis. 80:1044-1052. Accepted for publication 5 June 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1044.

Powdery mildew resistant (PMR) yellow summer squash (Cucurbita pepo) hybrids PSX 2287 and HMX 1707 had significantly lower disease severities than the susceptible hybrids Goldbar and Superset!, respectively. The level of control obtained with host resistance was not as good as that obtained with fungicides (chlorothalonil applied weekly with either triadimefon or benomyl), except when fungicide resistance developed. Average disease severities on 11 September 1991 on adaxial/abaxial leaf surfaces were 3%/0.4% and 0%/0% for nontreated and fungicide-treated PSX 2287, respectively, and 22%/9% and 0%/0% for nontreated and fungicide-treated Goldbar. In 1993, the values were 4%/2%, 0%/0%, 12%/27%, and 0%/15%, respectively, on 30 September, and all Sphaerotheca fuliginea isolates from fungicide-treated plants were insensitive to both triadimefon and benomyl. An integrated program was best with PSX 2287: significantly more fruit were produced when fungicides were applied compared with nontreated plants because fruit weight was 40 to 51% higher during the last third of the harvest period. The fungicide-treated susceptible hybrids produced 12 to 34% more fruit than the fungicide-treated PMR hybrids, indicating an inherent difference in yielding potential. Disease severity and yield were similar for plants treated with fungicides on a preventive schedule (six to eight applications) or an IPM schedule with treatment initiated after reaching the threshold of one leaf with symptoms out of 45 old leaves examined (five applications). A disease threshold-based, mid-season fungicide program (three applications, the last one made 3 to 5 weeks before the last harvest) applied to Goldbar was effective because disease development late in the growing season was not associated with a significant reduction in yield compared with a full-season fungicide program.

Keyword(s): cucurbits, economics, integrated pest management