Effects of Duration, Frequency, and Temperature of Leaf Wetness Periods on Soybean Rust

On soybean leaves at 20 C in the dark, uredospores of Phakopsora pachyrhizi began germinating 1,5 hr after dew was provided and reached a maximum level after 6-7 hr. Susceptible soybeans inoculated with viable uredospores developed no rust at dew periods less than 6 hr. At 6 hr, trace levels of primary rust lesions developed at 18, 20, 23, and 26.5 C. After 8 hr of dew at 18-26.5 C, lesion intensities were 10-fold higher than those at 6 hr at the corresponding temperatures. (…)