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A Model for Predicting Ascospore Maturation of Venturia inaequalis. J. R. James, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27650, Present address of senior author: CIBA-GEIGY Corp., P.O. Box 18300, Greensboro, NC 27419; T. B. Sutton, assistant professor, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Phytopathology 72:1081-1085. Accepted for publication 15 October 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1081.

A model was developed to predict pseudothecial development of Venturia inaequalis from the time asci begin to develop until ascospores mature. An equation, y = 0.0031 + 0.0546 (TEMP) 0.00175 (TEMP)2 (equation 1) in which y = daily change in pseudothecial development and TEMP = temperature (C), was developed from laboratory incubation studies to predict daily change in pseudothecial development when moisture was not a limiting factor. February 1 was used as a biofix date (biological reference date) to initiate the model in North Carolina. Pseudothecial development was best described by using equation 1 and daily average temperature and threshold levels of rainfall ≥ 0.25 mm or hours of 100% relative humidity ≥ 12 as indicators of leaf wetness. If daily average temperature ≤0 C, or if rainfall ≤0.25 mm and hr of 100% relative humidity ≤ 12, there was no predicted increase in stage of development. Predicted stage (st) of development = Σy + st 5, in which st 5 is the overwintering dormant stage and st 12 is when the ascospores are mature. The ys are summed over time. Equation 1 was also evaluated on an hourly basis by dividing all regression coefficients by 24, yielding the equation y = 0.00013 + 0.0022 (TEMP) 0.0000729 (TEMP)2 (equation 4). Equation 4 best described pseudothecial development by using hourly temperature and relative humidity ≥ 85% as the threshold moisture level. Equations 1 and 4 were tested at three overwintering sites in NC during a 3-yr field study.

Additional keywords: apple scab, epidemiology, Malus sylvestris.