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Influence of Rain Events on Spatial Distribution of Septoria Leaf Spot of Tomato. S. K. PARKER, Graduate Research Assistant; Department of Plant Pathology, Iowa State University, Ames 50011. M. L. GLEASON and F. W. NUTTER, JR., Associate Professors; Department of Plant Pathology, Iowa State University, Ames 50011. Plant Dis. 79:148-152. Accepted for publication 28 October 1994. Copyright 1995 The American Phytopathological Society 148. DOI: 10.1094/PD-79-0148.

Tomato seedlings spaced 30 cm apart were placed 30-180 cm from an inoculum source in the field for 24-hr periods on five rain dates and five non-rain dates. The mean number of lesions per leaf on plants exposed on rain dates ranged from 70 to 1,680, whereas only 0.0-0.46 lesions per leaf developed on plants exposed on non-rain dates. The mean number of lesions per leaf was linearly related to millimeters of rainfall during exposure periods (b0 =224, b1 = 528, R2 = 0.86, P < 0.0001). Percent defoliation within rows of tomato plants increased at the apparent infection rate of 0.282 units per day following spray inoculation of the center plant in each row. The study confirms that rain is a primary mechanism for spore dissemination and an important factor contributing to the development of Septoria leaf spot epidemics in tomatoes.

Keyword(s): Lycopersicon esculentum, disease forecasting, disease gradients