Influence of Spore Density, Leaf Age, Temperature, and Dew Periods on Septoria Leaf Spot of Tomato. WADE H. ELMER, The Connecticut Agricultural Experiment Station, Box 1106, New Haven 06504. FRANCIS J. FERRANDINO, The Connecticut Agricultural Experiment Station, Box 1106, New Haven 06504. Plant Dis. 79:287-290. Accepted for publication 2 December 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/PD-79-0287.

In order to determine the effects of spore density, leaf age, dew period, and temperature on the susceptibility of tomato to Seploria lycopersici, growth chamber studies were conducted. Depending on the leaf area, 3-5 ml of water containing 102, 103, 104, 105, and 106 spores per milliliter were brushed onto the third true leaf up to the seventh true leaf. The log of the disease severity (lesions per square centimeter of leaf) was linearly correlated with the log of the inoculum density (spores per square centimeter of leaf). Older leaves were more susceptible than younger leaves to Septoria leaf spot when inoculum densities were high (104 spores per square centimeter of leaf), but younger and older leaves were uniformly susceptible when inoculum densities were low (101 to 102 spores per square centimeter of leaf). When older leaves were brush inoculated (104 spores per square centimeter of leaf), the number of lesions that developed was highest at temperatures between 20 and 25 C, but lesions still developed at 10 C. When brush-inoculated leaves were incubated for 10 days at 50-60% relative humidity without any dew period, the number of lesions that developed did not significantly differ from inoculated leaves that had received 16 hr of dew. Disease severity increased when the dew period extended beyond 20 hr. When leaves were spray inoculated, no lesions developed when dew was absent, but lesions did develop on leaves that were given 24 hr of dew. Our findings suggested that a low incidence of Septoria leaf spot may develop under unfavorable conditions and that leaf abrasion may allow spores to infect in the absence of dew.

Keyword(s): Lycopersicon esculentum, foliar diseases, leaf wetness