Effects of Duration, Frequency, and Temperature of Leaf Wetness Periods on Soybean Rust. J. S. Melching, Research Plant Pathologist, USDA-ARS, Foreign Disease-Weed Science Research, Ft. Detrick, Bldg. 1301, Frederick, MD 21701. W. M. Dowler, D. L. Koogle, and M. H. Royer. Research Leader, Biological Laboratory Technician, and Research Plant Pathologist, USDA-ARS, Foreign Disease-Weed Science Research, Ft. Detrick, Bldg. 1301, Frederick, MD 21701. Plant Dis. 73:117-122. Accepted for publication 24 August 1988. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1989. DOI: 10.1094/PD-73-0117.

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. Increasing dew duration from 12 to 16 hr resulted in no significant increase in rust intensity, even at the most favorable temperatures (18–26.5 C). No lesions developed at 9 and 28.5 C, even with dew periods as long as 20 hr. Uredospores on unwetted soybean leaves progressively lost infectivity during sunshine conditions, but exhibited enhanced infectivity during 1 or 2 days on dry foliage under cloudy conditions. After 8 days on dry foliage, no uredospores caused lesions following a 12-hr dew period at 18 C. Spores on leaves exposed to 4 or 6 hr of dew followed by drying for up to 4 days were able to infect when a 12-hr dew period was provided, but were 50% or less as infectious as similar spores that had not been exposed to brief initial wetting. Uniformly inoculated soybean plants given initial dew and then receiving dew every third, sixth, or ninth day after inoculation until maturity developed fewer lesions per square centimeter and higher yields with decreasing frequency of dews. Rust reduced numbers of filled pods, numbers of seeds per pod, and mean seed weight.

Keyword(s): Glycine max.