In the last two seasons, soybean rust has shown limited dispersal during each growing season in the southern United States. During the summer months, it mainly spread in the Gulf Coast states. Near the end of the growing season, the disease took off and was found in many states, including three new states, Illinois, Indiana, and Virginia. Dry weather conditions the past two summers have been considered as a major reason for the slow development of this disease in the growing season. However, results reported at the 2nd National Soybean Rust Symposium held last year indicate that the rust fungus itself may have something to do with it. The fungus quickly moved a great distance, but the disease did not.

Spore trapping results

Two independent studies reported at the meeting showed that the fungus moves great distances in a growing season. A study led by the United States Department of Agriculture at the University of Minnesota detected soybean rust spores by using a molecular method from rain drops collected nationwide. Rust spores were detected many times in each state in northern soybean production regions during the entire growing season. Another spore trapping study led by the University of Arkansas and sponsored by Syngenta showed that soybean rust spores can be found throughout the soybean production region, including Iowa, which was not in a dry condition in 2006. These studies were done in two seasons and clearly show that the rust fungus moves. So why was disease not found in Iowa and surrounding states when the summer was not dry, actually relatively wet in late summer with a lot of sudden death syndrome and white mold? Obviously disease movement is affected by the biology of soybean rust fungus.

Limited spore survival

A study by the University of Illinois found that soybean rust spores have a relatively short life span when exposed to solar radiation. In layman's terms, it shows that soybean rust spores die out when exposed to sunlight for two days. In other words, if spores fly in a cloudless sky for two days, they would not be able to cause disease even if they landed on soybean plants in Iowa.

Light effects

Results of a preliminary study by Iowa State University and University of Florida researchers suggest that movement of soybean rust may be sensitive to light. The study examined the effects of light on infection of the disease, a factor previously unnoticed. Results showed that the percentage of soybean rust infection was 95 percent for shaded treatment compared with 25 percent for the no-shade treatment. The results were consistent with field observations. In Florida, soybean rust was found mainly in kudzu leaves under shade trees and not much in open ground. In South America, the disease was always first found in the lower portion of the canopy, although the top leaves should have greater chance for infection because they have more dew and greater spore deposition than lower leaves. If results of this study are confirmed by future experiments, it also would explain why rainy days are associated with the outbreak of soybean rust in South America and Asia, or it suggests that frequent and lengthy rains with cloudiness would be critical to movement of the disease in the U.S. soybean production region.

X. B. Yang is a professor of plant pathology with research and extension responsibilities in soybean diseases. Ana Paula Dias and Linus Li are doctoral students in plant pathology.