VIEW ARTICLE

Ecology and Epidemiology

Changes in the Susceptibility of Developing Apple Fruit to Venturia inaequalis. W. F. S. Schwabe, Visiting professor, Department of Botany and Plant Pathology and the Pesticide Research Center, Michigan State University, East Lansing 48824; A. L. Jones(2), and J. P. Jonker(3). (2)Professor, Department of Botany and Plant Pathology and the Pesticide Research Center, Michigan State University, East Lansing 48824; (3)Research technician, Fruit and Fruit Technology Research Institute, Stellenbosch 7600, South Africa. Phytopathology 74:118-121. Accepted for publication 26 July 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-118.

In greenhouse studies, potted apple trees with fruit in various stages of development were inoculated with conidia of Venturia inaequalis, exposed to different durations of wetting, and rated after a suitable incubation period for the severity of scab infection on the fruit. Wetting duration required for infection increased with increased age of the fruit. At high inoculum levels, fruit showed light (about 2%) infection at infection indices (hours of wetting × mean temperature C) of > 120, 260, 370, 455, 525, and 590 when they were inoculated 1, 5, 10, 15, 20, and 25 wk after full bloom, respectively. A curve describing this relationship increased exponentially with time and was linearized with a square root transformation of fruit age. When the duration of wet periods was adjusted to account for changes in fruit susceptibility with age, it was possible to develop a linear regression model relating hours of wetness at 9.3 to 20.0 C to the severity of scab infection on the fruit.