Across Iowa, soybean farmers are preparing for the possible arrival of Asian soybean rust this growing season. Asian soybean rust is a serious disease of soybean plants. However, home gardeners should be aware of this plant disease too. Besides soybeans, Asian soybean rust can also affect more than 30 other species, including green, kidney, lima and butter beans, although its severity on these crops is uncertain.
What is Asian soybean rust?
Asian soybean rust is a fungal plant disease that affects soybean and several other crops and weeds in the bean family. On beans, rust appears as small, angular tan to reddish-brown spots on the lower surfaces of leaves. Spots are often clustered over the leaf veins. Spots may merge together to form larger lesions, and within each lesion may be several small volcano-like bumps or pustules on the underside of the leaf, from which spores are produced. The leaves of severely affected plants turn yellow and fall off, causing significant yield loss. The fungus is spread from plant to plant by tiny, dust-like spores that are carried by the wind. Wet conditions are necessary for infection to occur. Warm, but not hot, wet weather favors the disease.
On soybeans, Asian soybean rust can cause up to 80 percent yield loss under extreme conditions, and proper management is critical. There is little information available regarding how serious the disease may be on garden beans.
A similar disease, common bean rust, already occurs on beans in the Midwest. Common bean rust is caused by a different fungus than Asian soybean rust. Common bean rust causes round, reddish-brown spots with pustules that form on both upper and lower surfaces of the leaves. Spots are often surrounded by a yellow halo. Common bean rust favors cool, wet weather in the spring and fall.
Asian soybean rust first appeared in the continental U.S. in Louisiana in November 2004. Shortly thereafter it was also found in eight other southeastern states, as far north as southeastern Missouri. It is thought to have been carried to our country from farther south via hurricanes. Soybean rust has been found in Brazil since 2000. In the U.S., it has been found on soybean and kudzu, a weed that is common in the South.
Will Asian soybean rust reach Iowa this growing season?
Several factors must come together in order for Asian soybean rust to cause problems in Iowa this growing season. First, spores must be blown from the south where the fungus can survive the winter. Only the very southern parts of Texas and Florida are thought to be mild enough during the winter for the fungus to survive. It is thought that if the fungus overwinters in Texas, Florida or Mexico, the disease may travel north at a rate of 20 to 30 miles a day during the growing season, arriving in Iowa as early as July.
Secondly, environmental conditions must be favorable for infection. In Brazil, infection has been shown to be favored by warm, rainy weather. The fungus cannot infect a plant unless it has sufficient moisture and the temperature is high enough. Having the spores alone is not enough to cause disease.
What can gardeners do to protect their bean plants?
Soybean farmers are gearing up to spray fungicides at the first sign of rust, as this is the only management option available to them. Gardeners also have few options for management of Asian soybean rust, although it is uncertain how severe the disease may be on garden beans. Resistant varieties of beans are not available, and cultural practices will probably have little effect on the disease. Fungicides that are labeled for common bean rust or other foliar diseases will likely be effective against Asian soybean rust on garden beans. It is important to apply the fungicides as soon as symptoms are noticed, and to follow label directions regarding application rates and length of time that must elapse before harvest.
As the growing season begins, both farmers and gardeners will anxiously await the possible arrival of Asian soybean rust.