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Temperature and Wetness Duration Requirements for Apple Infection by Botryosphaeria obtusa. L. F. Arauz, 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. Phytopathology 79:440-444. Accepted for publication 8 November 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-440.

The combined effect of temperature and wetness duration on infection of apple by Botryosphaeria obtusa was studied on Delicious seedlings and Golden Delicious apple fruit. The optimum temperature for leaf infection was 26.6 C; at this temperature, 4.5 and 13 hr were required for the pathogen to cause light and severe infection, respectively. Lower temperatures required longer wetting periods for infection to occur, and no infection was observed at 8 C with wetness periods shorter than 48 hr. At 32 C, infection was reduced and a longer wetting period was required for infection than at 28 C. The optimum temperatures for fruit infection ranged from 20 to 24 C; 9 hr of wetting were required for light infection to occur. Infection of fruit required 38 hr of wetting at 8 C, whereas 28 and 32 C resulted in reduced fruit infection. Models were derived empirically to indicate the duration of leaf wetness (W) necessary, at a given temperature (T), for a specified level of infection to occur. For light leaf infection (< 1 lesion/ 1002 cm of leaf tissue), W = 3527.7T-2, and for severe leaf infection (> 10 lesions/100 cm2), W = 1165380.7T-1 + 70257.5T-2. For fruit infection, W = 14.8265.2T-1 + 2988.4T2. No infection occurred under field conditions in those instances where no infection was predicted. The leaf infection models accurately predicted the level of 84.7% of the infections obtained under field conditions. In 8.5% of the cases, less disease than expected for the particular combination of W and T was obtained; more infection than predicted was observed in 6.8% of the cases. Most of the incorrect predictions occurred for wetting periods where moderate infection was anticipated.