Effects on Yield
Soybean rust has the potential to cause huge yield reductions. Thailand frequently experiences losses up to 50% in infected soybean fields. Frequent endemic outbreaks in southern China also cause losses of up to 50%. Soybean rust has only been in South Africa for a few years, but yield losses in infected fields have ranged from 10-80%. Zimbabwe has reported losses of 60-80% in commercial fields.

Losses of up to 40% are possible in soybean fields in the southeastern United States, now that the rust has been found in North America. Losses in other parts of the United States will depend on the timing and severity of the infestation due to the dispersal of inoculum blown in from southern regions.

Soybean rust has been observed to have periodic outbreaks which are more severe than an average year. China had a severe outbreak in 1975, and India had epidemic outbreaks in the 1970s and again in the late 1990s. Soybean rust appears to be exhibiting a 4-year cyclic pattern in Australia, which is probably related to a cyclic weather pattern in the area. Cyclical weather patterns probably influence the severity of outbreaks in many regions where soybean rust is present.

Management
Managment of soybean rust can occur in several ways, the most effective of which are usage of resistant soybean varieties and/or application of chemical control measures.

Planting resistant or tolerant varieties may be the best way to manage soybean rust in the future, however resistant germplasm is rare. Researchers in India screened 3,300 entries in 1971 and 4,066 in 1974 and found only 6 resistant cultivars. Research in China found 64 resistant entries out of 8,000 screened, and testing by the Asian Vegetable Research and Development Center (AVRDC) in Taiwan found 20 resistant out of 9,000. No immune germplasm has ever been found.

R.L. Bernard at the Univerity of Illinois released 3 resistant lines in the 1980s, but they are no longer in commercial use. Currently, no commercial U.S. soybean cultivars are reported resistant to soybean rust, though breeding for resistance has been resumed.

Chemical Control
Chemical control using fungicides has been shown to be effective at controlling soybean rust in many countries, and may be the best short-term alternative until adapted, resistant soybean varieties can be released. In regions favorable to rust development, the rapidly repeating life cycle of the soybean rust fungus requires that detection be early and fungicide application be made immediately in order to effectively control the infection. Research at AVRDC has shown that as few as 21 days are required for soybean rust to develop from less than 5% to 90% disease severity.

Since the disease tends to hit more mature plants, initial fungicide application is normally done at flowering, then a follow-up application 21 days later. These applicatons are a common practice in southern China. In high-risk regions such as Africa, a third follow-up application is sometimes used, again 21 days after the previous.

(information courtesy ISU and the Crop Adviser Institute