An Empirical Model for Predicting the First Symptoms of Sooty Blotch and Flyspeck of Apples. E. M. Brown, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. T. B. Sutton, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. PLANT DIS. 79:1165. Accepted for publication 18 May 1995. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-1165.

Measures of leaf wetness duration were related to the appearance of symptoms of sooty blotch (caused by Peltaster fructicola, Leptodontium elatius, and other fungi) and flyspeck (caused by Zygophiala jamakensis) on apple (Malus x domestica) during the 1987 through 1994 growing seasons. Hours of leaf wetness of 4 h duration or greater accumulated from the first rain that occurred 10 days after petal fall provided the best measure of the time of symptom appearance. Over the 8 years of the study, symptoms appeared after an average of 273 h of leaf wetting (range 209 to 310 h). No consistent temperature effects on the time of first symptom appearance were found. A threshold value of 200 to 250 h of leaf wetting is suggested to initiate fungicide applications for the control of sooty blotch and flyspeck.