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Issue No. 4, Article 7/April 15, 2005

Outlook for Soybean Rust in Illinois

Soybean rust is a major soybean production topic in April 2005. Asian soybean rust caused by the fungal pathogen Phakopsora pachyrhizi is a fungal disease that can cause defoliation and significant yield losses. When this disease will arrive in Illinois is a major question on the minds of many, but several plans and operations are in place to provide information on when and how we should respond to this threat.

Fortunately, we have had several years to gather information on Asian soybean rust in South American and Africa. However, there are still many questions about what to expect and how to plan for managing soybean rust in Illinois. In fact, the speculation may exceed the facts that are currently available. Many questions will not be answered until this disease arrives in Illinois and nearby states. Among the questions are when and how many spores may be dispersed by wind to Illinois at critical times, and how favorable the environment in Illinois is for soybean rust.

We know the states and counties where soybean rust was found in November and December 2004. Many expected that by mid- to late March we would have information on where soybean rust has survived over the winter in the continental U.S. and that soon after that, we would get some idea of the distribution and severity of soybean rust on kudzu, soybean, and perhaps other hosts in the southern U.S. These predictions have turned out to be mostly accurate. We now know that soybean rust survived over the winter in Florida and has been confirmed in three counties, as shown on this map. These are the only confirmed findings of soybean rust to date in the U.S., and the disease has not yet been confirmed on soybean in 2005. Kudzu and other potential hosts have been growing leaves along the Gulf Coast from Florida to Louisiana and Texas for several weeks, and soybean rust has not been found outside of Florida to date in spite of extensive scouting efforts. More information on where scouting has been conducted and where soybean rust is confirmed can be found at the USDA Web site.

Some assumptions suggest that the severity and incidence of soybean rust must reach at least a moderate level in states such as Arkansas, Louisiana, and/or Tennessee to act as a sufficient source of spores to be blown into and initiate disease in more northern soybean production areas, including Illinois. Thus, the reports from the South will likely be used as an indicator of subsequent risk in the northern states. In addition, extensive efforts have been made this winter to develop a forecasting system for dispersal of soybean rust from source areas to other areas that may be vulnerable. The forecasts are updated daily and are an interesting and potentially valuable tool to help prepare and to predict where and when soybean rust may occur. The forecasts are still experimental and can be found here. Be sure to read the information on the Web site to help with interpretation of the maps.

Although rust spore deposition, with spores originating in southern states, appears to be critical to start soybean rust epidemics in Illinois, the environment must also be suitable to initiate and propagate epidemics. Long dew periods with free water on leaf surfaces (over 6 hours) and moderate temperatures (66-82°F) favor soybean rust. In Brazil, soybean rust epidemics in different regions have been associated with the amount and frequency of rainfall. For example, in a single growing season in Brazil, wet areas had high disease severity, while dry areas in the country had low regional disease severity. In parts of Brazil where soybean rust has frequently been severe, the average amount of rainfall in the main growing season is about 24 inches (55 rainy days), whereas in Urbana, Illinois, from June to August, the average is about 11 inches (24 rainy days). Soybean rust could be similar to southern rust of corn, which is an infrequent problem In Illinois that mostly occurs late in the growing season.

What can we expect in Illinois for 2005? Some think soybean rust will arrive in the state in 2005 and cause crop loss, but others think it may not reach significant levels until 2006 or later. The amount of damage this disease can cause in different parts of Illinois is unknown and will likely vary by year and location. Risk analysis based primarily on average historical climatic conditions suggests that the most frequent problems with soybean rust in the U.S. soybean production areas will occur in the southern and eastern regions. Sentinel plots for soybean rust monitoring, which will be distributed across Illinois and throughout the soybean-producing states, will provide information on where and when soybean rust is occurring. Soybean rust can be a very serious disease and should be monitored and prepared for appropriately.

A key time to begin scouting for soybean rust and to consider fungicide applications appears to be in the late vegetative or early reproductive stages, especially after the disease forecasts and scouting reports from sentinel plots and states to the south of Illinois indicate that movement of soybean rust into Illinois is likely. Initial scouting should focus in the lower parts of plants, where the disease usually occurs first. The early lesions and pustules can be seen most clearly with a hand lens at 20X magnification. We have much to learn about this disease in the United States, but soybean rust can be managed at a cost. The Web sites noted in this article and links to more information on soybean rust can be found on the new and developing Illinois Soybean Rust Web site. --Dean Malvick

Dean Malvick

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