Based on our experience with corn rusts, we suspected that disease outbreaks occurring inFlorida and southern Texas in March and April would be an indication that rust spores would likely be available to move to Gulf Coast states. The earlier an outbreak occurs in the South, the higher the risk for northern soybean production regions. Since the first detection in kudzu in central Florida in late February, no outbreak was observed in the last two months. The first detection outside Florida on volunteer soybeans was confirmed in southern Georgia.
It may be too early to predict the outbreak risk of this disease in the northern soybean production region because of some uncertainties. One is the high dependence this disease has on weather conditions, which is difficult to predict; a widespread outbreak of the disease does not guarantee occurrence of severe outbreaks. The later the detections of soybean rust in southern states beyond Florida, the lower the risk of severe outbreaks to northern regions. The United States Department of Agriculture's (USDA) APHIS soybean rust site suggests scouting in the eastern regions.
Another uncertainty is the role of kudzu; the weed is widespread in the eastern and southern United States. Tennessee and Georgia reported new growth of kudzu two weeks ago. Last week, USDA confirmed that several kudzu accessions from Louisiana were very susceptible to soybean rust. Since the disease has not been found there, we may say that the spores either have not been there or were present at very low levels, which make disease detection difficult.
Given these uncertainties, the risk to northern soybean production regions during this season still remains largely unpredictable. If the disease is found widespread in alternative hosts and volunteer soybeans in May in coast states including Louisiana, Mississippi, and Alabama, computer models indicate that spores of the fungus can reach northern production regions before July. The next few weeks are critical. Let's keep watching the USDA soybean rust website  for updates.
This article originally appeared on page 72 of the IC-494 (8) -- May 2, 2005 issue.