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Influence of Cultivar Resistance, Initial Disease, Environment, and Fungicide Concentration and Timing on Anthracnose Development and Yield Loss in Pickling Cucumbers. D. C. Thompson, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; S. F. Jenkins, professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Phytopathology 75:1422-1427. Accepted for publication 3 July 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1422.

Resistant cultivars of Cucumis sativus reduced the rate of anthracnose development caused by Colletotrichum lagenarium and allowed disease thresholds to be used to initiate fungicide applications. Action thresholds of 0, 1, 10, and 20% diseased tissue were used to initiate chlorothalonil applications on a 7-day or a weather-modified (WM) 7- 14 day schedule. The WM schedule had three determinants, evaluated weekly, that initiated a spray application when any one of them occurred. The determinants were: no fungicide in the past 14 days; a rain event of 1 cm or greater in the past 7 days; and a 40% or greater probability of rain in the next 2 days. Chlorothalonil rates were 2.34, 2.34, and 1.17 kg/ha for cucumber cultivars Earlipik 14, Calypso, and Calico, respectively. Two spring, and four fall, crop environments and two levels of initial disease provided a wide range of conditions and disease development. Disease developed slowly in the spring crops and caused no yield loss. In the fall crops, yield loss was greatest; there were early and rapidly developing epidemics that within each test had the greatest area under the disease progress curve. Yield loss was proportional to cultivar susceptibility. Inoculation of the central four to six plants resulted in less disease development than inoculation of the entire plot. Initial disease levels of 0.01 and 0.1% diseased tissue were not consistent in their effect on yield loss. The 10% action threshold resulted in some yield loss. In two of four fall crops, yield loss resulted only when no fungicide was applied. The WM application schedule had little effect on disease or yield, and saved from zero to one spray applications. Results of correlation analysis followed by factor analysis indicated that a high temperature (>32 C)- low disease determinant might have been useful in the WM schedule.

Additional keywords: action thresholds.