About Asian Soybean Rust Fungus
Compliments of Dr. Melvin Newmann, University of Tennessee
Background
Asian soybean rust is an aggressive fungal disease that, under optimal environmental conditions, can reduce soybean yield by as much as 80 percent. It is a potentially damaging disease because the severity can double every 2 to 9 days depending on environmental conditions. It is expected to quickly become endemic to coastal regions in southern Texas and parts of Florida (Pivonia and Yang, 2004) and could cost United States soybean producers $240 million to $2.0 billion annually (Livingston, et al., 2004). A less aggressive species of soybean rust, Phakopsora meibomiae, also occurs throughout the world but is not considered yield limiting to soybean in the United States.
After Asian soybean rust was first recorded in Japan in 1902, the pathogen moved through Asia, Australia, and Africa before it was discovered in South America in 2000. Asian Soybean Rust has been moving northward through South America and has now been confirmed in many southern US states. This provides a good indication of how far and wide the rust spores can travel. Soybean rust spores could over-winter in extreme southern areas where temperatures do not go below freezing and then be blown north into other states during the growing season.
The over-wintering source(s) of primary inoculum for soybean rust in the United States is not known although at least 20 host species (including kudzu) are known to exist in the southern United States. Current research suggests the pathogen will over-winter in certain areas surrounding the Gulf of Mexico and in the Caribbean. Research also suggests that soybean rust will not over-winter in the midsouth states or other northern soybean growing areas. Rust spores will most likely be transported by wind or other mechanisms. Rust spores must be introduced, be deposited on a suitable host, and be exposed to sufficient free moisture and moderate temperatures to initiate germination before yield loss will occur.
The most severe epidemics of Asian soybean rust occur when soybean leaves are infected early in the growing season; however, leaves are susceptible at all stages of plant growth. Soybean rust pustules have been recorded on soybean as early as the V1 (first trifoliate leaf) stage of development and can reproduce on cotyledons, stems and pods.
Epidemics of soybean rust are greatly influenced by environmental conditions. Weather conditions during spore dispersal, deposition, and germination greatly influence the success rate of lesion development as well as the time it takes for disease severity to double (i.e., from 2 to 4% severity or 20 to 40% severity). Germination of urediniospores and subsequent host penetration and lesion development require a minimum of 6 to 7 hours of continual wetness with an optimum infection rate occurring if dew periods are ≤12 hours in duration with temperatures between 18 and 26.5o C during the wetness period (Melching et al., 1989). Spores can remain viable in the absence of moisture for durations of no more than 8 days (Kitani and Inoue, 1960; Melching et al., 1989). However, Paltil et al., 1977, reported that soybean rust spores could survive 50 days in the shade. Spore viability is negatively influenced by ultraviolet light. Exposure to sunlight reduces viability of spores compared to spores exposed to cloudy conditions (Melching et al., 1989).
