Symptoms

Asiatic soybean rust causes superficial, tan to reddish brown, lesions that will be observed on plant tissues. Lesions will contain one to three rust pustules which are raised on the leaf surface. The lesions may have an angular appearance and be limited by leaf veins. Rust pustules may appear on cotyledons, leaves, petioles, stems, or pods, but are most likely to be observed as raised pustules on the under side of the leaf. The pustules are quite small (about the size of a pin head) and can be confused with another disease, bacterial pustule. Bacterial pustule, however, is rare in commercial soybean varieties, since most if not all are resistant to this disease. A hand lens may aid in seeing the raised nature of the pustule. Also, placing leaves in a plastic bag with a moist paper towel for twenty four hours may cause the pustules to erupt, thus making identification easier. Each pustule contains hundreds of spores. Spores are elliptical to obovoid in shape, colorless to yellowish or yellowish brown and minutely and densely spiny (Fig 6.). Infected plants will senesce early and have smaller seed with reduced yield.

Life Cycle, Disease Cycle, and Epidemiology

Windblown spores infect susceptible tissue at temperatures of 45 to 83 degrees (optimum 68-75). Tissue must remain wet for approximately 6 hours for infection to occur. Pustules will develop within a week to ten days after infection, and spores production may begin in as little as 10 days depending on environment. The pustules release large numbers of spores which become airborne and infect other tissue repeating the life cycle. Spores are the survival structures for this fungus and they will not survive freezing weather. The role other hosts (kudzu, winter vetch, lima beans, dry beans, and lupines) might play is not known, but fields with kudzu along borders may be good places to start scouting. Soybean rust may be able to survive in southern Florida and Texas, but the soybean acreage in these states is limited at this time. Several species of common bean are also hosts for this fungus, but fungicide sprays to control other diseases on these vegetable crops may limit rust development. In 2004, soybean acreage in Florida, Texas, and Mexico was less than 1 million acres, although Louisiana and Mississippi do have significant acreages of soybean. The potential for high concentrations of windblown rust spores coming from soybean in the Gulf of Mexico is not terribly high, the extent of and acreage of other hosts for soybean rust is not fully understood. For rust to be damaging first infections will probably have to occur before the R3 stage of soybean development, but severe defoliation has occurred in fields when high concentrations of rust spores are deposited on a field.

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