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Effects of Temperature and Leaf Wetness Period on Brown Spot Disease of Soybeans. David J. Peterson, Graduate Research Assistant, Department of Biological Sciences, Western Illinois University, Macomb 61455. H. Herbert Edwards, Professor, Department of Biological Sciences, Western Illinois University, Macomb 61455. Plant Dis. 66:995-998. Accepted for publication 18 March 1982. Copyright 1982 American Phytopathological Society. DOI: 10.1094/PD-66-995.

The effect of temperature and leaf wetness period on the development of Septoria glycines infection of soybean (Glycine max) leaves in growth chambers and in vitro was studied under controlled environmental conditions. A minimum relative humidity of 95.6% was necessary for conidial germination and vegetative growth on potato-dextrose agar. On water agar (1.5%), more than 50% germination occurred at 2432 C after 24 hr of incubation and more than 80% germination at 1632 C after 36 hr. Similar germination percentages at 24 and 36 hr of incubation occurred on soybean leaf disks at 2428 C and 2028 C, respectively. No germination occurred at 36 C on either water agar or soybean leaf disks. Maximum germ tube growth of 667 µm occurred on water agar at 28 C after 36 hr. Germ tubes from 145 to 180 µm long developed on leaf disks after 36 hr between 24 and 32 C. A minimum leaf wetness period of 72 hr was required for leaf infection, but a longer period accelerated fungus development. Leaf infections occurred between 16 and 32 C, with optimum disease development at 28 C.