This disease is nowhere near the midwestern corn/soybean belt, and we don't know if it will come or how much damage it potentially could cause here. For obvious reasons, research is not allowed on this disease in the Midwest, and we don't know much about how or if it is capable of surviving or spreading in the Midwest.

The purpose of this article is to provide some basic information about soybean rust and to answer the question "What is soybean rust?"

Soybean rust is caused by two different rust fungi: Phakopsora meibomiae and Phakopsora pachyrhizi. P. pachyrhizi is the more aggressive of the two species and has been reported in Hawaii. The fungi that cause soybean rust are unrelated to the fungi that cause rust on corn. Different races of these pathogens are known to occur. Lesions most often develop on the bottom of leaves, in middle or late summer. The lesions are pinhead sized (1/12 to 1/6 inch in diameter) and tan to gray to brown in color, and have sharp edges that are bordered by leaf veins. Several small pustules (tiny bumps) may be observed in the lesions, whereas only one pustule develops in lesions from bacterial pustule.

Soybean rust lesions on soybean leaves. (Photo courtesy of Dr. Glen Hartman.)

What conditions favor development of soybean rust? According to the basic principles of plant pathology, three things are required for plant diseases to occur: a susceptible host, presence of the pathogen, and appropriate environmental conditions. It is assumed that most or all commercial soybean cultivars are susceptible to rust, although the cultivars grown in the Midwest have not been tested for rust resistance. The hope is that resistant soybean varieties either exist or can be developed. As noted, the soybean rust pathogen does not occur in the Midwest, and we hope it doesn't arrive. It is not seedborne, and the microscopic soybean rust spores would most likely be spread by wind and rain or contaminated objects. Where the pathogen is present, development of the disease is favored by extended periods of leaf wetness and temperatures roughly between 50 deg F and 80 deg F.

Soybean rust spores. (Photo courtesy of Dr. Glen Hartman.)

Where is this disease now and how much damage has it caused? Soybean rust has occurred in a number of different areas for a number of years, including Australia, Brazil, China, India, Japan, central and southern Africa, and Thailand. Yield losses of 10% to 90% have been reported for some of these areas. Its presence was reported in Hawaii in 1994. Soybean rust (the more aggressive type) was reported in South America for the first time in 2001, and again in 2002. Yield loss estimates for Paraguay and Brazil range from 10% to 50% for 2002, respectively, in some fields. This disease also has the potential to cause significant losses in the United States.

How can soybean rust be managed? Fungicides are a main method for control of soybean rust. In addition, work is being done to develop soybean varieties with resistance to rust. Dr. Glen Hartman, a research plant pathologist with the USDA/ARS in Urbana, Illinois, and collaborators are working with soybean rust in confinement facilities in Maryland. Additional research has been done in Thailand, China, and Zimbabwe. Soybean rust may not come to the Midwest and may or may not be a problem even if it is introduced here. We hope never to find out, but management strategies are available (albeit, they may be costly in terms of additional expenses of multiple fungicide applications) to reduce the impact of rust on yields.--Dean Malvick