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Influence of Temperature and Wetness Duration on Infection of Apple Leaves and Virulence of Different Isolates of Alternaria mali. Nenad Filajdi?, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695; T. B. Sutton, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Phytopathology 82:1279-1283. Accepted for publication 16 July 1992. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-1279.

The effect of combinations of nine different temperatures (436 C) and eight wetness periods (248 h) on the infection of Delicious apple (Malus domestica) seedlings by Alternaria mali was studied. Disease severity increased with increased wetness duration over all the temperatures tested and was greatest from 12 to 28 C. The influence of temperature and wetness duration on infection of apple seedlings by A. mali was described by the model: Yij = 6.4580 + 0.1853Ti + 0.0912Wj 0.0033T2i 0.0030W2j + 0.0194TiWj 0.0005T2iWj, in which Y = log10 (percentage of leaf area covered with lesions + 0.01), T = temperature (C), W = wetness duration (h), i = individual temperature treatment, and j = individual wetness duration treatment. The predicted optimum temperature for infection was 23.5 C. At this temperature, 5.1 h of wetness was required for light infection (0.2% leaf area covered with lesions). The model derived from laboratory data was tested in the field with healthy seedlings that were surrounded by inoculated ones. Conidia of A. mali were trapped during 14 wetness periods; infection criteria were met during 10 of these periods. No false negatives occurred; however, six false positives were recorded. A predictive model for first occurrence of Alternaria blotch developed in South Korea was evaluated under North Carolina conditions; it predicted first occurrence of the disease 1 wk earlier in 1990 and 5 days later in 1991 than disease was observed in the field. Eight isolates of A. mali from western North Carolina were tested for pathogenicity and virulence in the laboratory. All were pathogenic on Delicious seedlings but varied in virulence.