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Soybean Plant Health

Soybean Rust Disease Cycle

Asian Soybean Rust

Figure 1. Rust spores are light and easily disseminated by winds over large geographical areas. The pathway shown here is similar for that of other rust diseases found on grains, and serves as a model for likely soybean rust movement from south to north.

Phakospora pachyrhizi, like all rust pathogens, requires green living tissue at all times. To survive winter in the absence of a soybean crop, the soybean rust pathogen must find another living host. Thus, the soybean rust pathogen will survive in the U.S. only where freezing temperatures do not occur. Kudzu, a common perennial legume, is thought to be a potential overwintering host in these areas.

In Wisconsin, yellow sweetclover would be a possible candidate for an overwintering host of the soybean rust pathogen, except that since the foliage of this perennial host is usually killed by freezing temperatures, the rust fungus would also perish. So unless winter temperatures increase dramatically, Phakospora pachyrhizi is not expected to survive in Wisconsin.

The absence of an overwintering source of inoculum means the soybean rust pathogen must be reintroduced to Wisconsin each growing season. Spores of the soybean rust pathogen are transported readily by air currents and can be disseminated rapidly hundreds of miles in 2 to 3 days (Figure 1). Weather conditions will determine when and where the spores travel from south to north.

Environmental conditions needed for the infection cycle
The environment plays a major role in the incidence and severity of rust. For spore germination and infection to occur, the leaf must be wet for 6 to 8 hours. Temperatures need to be between 59° and 86° F, and humidity levels should be in the range of 75 to 80%. Under these environmental conditions, pustules are formed and spores are produced within 10 to 21 days. This can be as little as 6 to 7 days after infection when temperatures are optimal at 72 to 81° F.

Hosts of Phakospora pachyrhizi
The fungus is reported to infect 95 plant species of which most are in the legume family. Hosts of most economic importance are soybean and all types of common bean including snap bean and dry edible beans. Host range will play an important role in the epidemiology and economic importance of the soybean rust pathogen in North America.

In Wisconsin, other potential hosts are soybean, snap and kidney bean (Phaseolus vulgaris), pea (Pisum sativum), American bird's-foot trefoil (Lotus unifoliolatus), crimson clover (Trifolium incarnatum), Korean clover (Kummerowia stipulacea), white clover (Trifolium repens), purple crownvetch (Coronilla varia), Chinese lespedeza (Lespedeza cuneata), lupine (Lupinus spp.), rattlebox (Crotalaria spp.), yellow sweetclover (Melilotus officinalis), ticktrefoil (Desmodium spp.), and winter vetch (Vicia villosa). Corn and other grain crops are not hosts.

Rust is a multi-cyclic disease. After the initial infection is established, the infection site can produce spores for 10 to 14 days. Under optimal conditions, a plant can go from the first signs of infection to severe defoliation in 1 to 2 weeks.

Information from this site can be copied and distributed for educational use. Please credit the source with our name and URL: University of Wisconsin-Madison, Departments of Agronomy, Entomology, and Plant Pathology at  www.plantpath.wisc.edu/soyhealth.