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Powdery Mildew of Tomato: The Effect of Planting Date and Triadimefon on Disease Onset, Progress, Incidence, and Severity. J. C. Correll, Department of Plant Pathology, University of California, Berkeley 94720; T. R. Gordon(2), and V. J. Elliott(3). (2)Department of Plant Pathology, University of California, Berkeley 94720; (3)Department of Plant Pathology, University of California, Berkeley 94720, Present address: USDA/ARS Research Laboratory, Oxford, NC 27565. Phytopathology 78:512-519. Accepted for publication 30 September 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-512.

Powdery mildew epidemics of tomato, caused by the pathogen Leveillula taurica (= Oidiopsis taurica), were monitored in experimental field plots. Disease onset, incidence, severity, and disease progress were measured in relationship to host growth during several successive planting dates in both 1984 and 1985. Disease was assessed by counting individual lesions on randomly sampled leaves and whole plants. Canopy defoliation was measured by counting the number of necrotic leaves on individual plants. Disease onset occurred at an earlier stage of crop development with each successive planting date; the data indicate that disease onset was not related to the physiological age of the crop nor any specific meteorological conditions. In general, disease distribution on a single plant was very aggregated, with considerably more lesions observed on older leaves than younger leaves for all sample dates. In the field, both immature and mature host tissue was susceptible to infection by L. taurica. When whole plants were the sampling unit, individual plants within a treatment from throughout the experimental plot had a similar disease incidence and severity for any given sample date. Disease progress curves fit both a logistic and Gompertz model reasonably well (r2 values ranged from 0.60 to 0.99). The fungicide triadimefon was effective at controlling powdery mildew epidemics. Overall, powdery mildew caused a significant increase in defoliation over triadimefon treatments in the late plantings. However, even when powdery mildew disease was severe, it did not cause any reduction in yield of two susceptible fresh market tomato cultivars harvested at the “mature green” stage of development.