This year's hurricane season started early and the arrival of Tropical Storm Arlene has raised some concerns regarding the risk of soybean rust (SBR) this season. The early start of the hurricane season reflects some similarities between the soybean rust situation and the southern corn leaf blight epidemic in 1970. In that epidemic, one major condition was an unusual tropical storm that occurred in June, which facilitated the spread of the southern corn leaf blight (SCL) pathogen from southern Mississippi and Alabama to the northern Corn Belt.
Compared with SCL in 1970, soybean rust is still not advanced sufficiently to develop regional epidemics. SCL in the 1970s was at an epidemic proportion before May 20. For soybean rust, detections of the disease are still limited in Florida and southern Georgia, although Arlene could facilitate the spread of this disease. According to a study presented at a recent Iowa State University seminar, the Caribbean Islands and Mexico are not likely source regions early in the growing season because those regions are in a dry season before June. Latest surveys by the United States Department of Agriculture Agricultural Research Service from those regions also report no detection of SBR in 2005.
Computer modeling suggests there will be an increase in the movement of soybean rust spores in June. The models show an increase in spore movement into northern regions reaching as far as some north central states with light spore concentrations. However, the predicted weather for the next 30 days is not good for disease development. With low spore numbers and unfavorable conditions the next 30 days, the effects from Arlene do not change the overall risk assessment in the northern soybean production regions
for this season. The likelihood to see widespread soybean rust in Iowa and surrounding states (excluding Missouri and southern Illinois) before the end of July is not high. The first detection date may be further delayed as weather unfavorable to rust continues.
We cannot rule out the possibility that the disease may have reached areas beyond southern Georgia, although it has not been detected. Intensive scouting has occurred in the South, but the lack of experience with soybean rust in the United States does not allow us to say for sure that soybean rust is not present in areas beyond southern Georgia.
Computer simulation of historical weather data (1997-2003) for potential deposition areas of soybean rust spores from central Florida if a large amount of spores are produced there. The upper map is for the latter half of June and the lower map is for the first half of July. The calculated available index (CAI) represents a qualitative index (0-7) for possibility of spore deposition.
This article originally appeared on page Page 5 of the IC-494(14) -- June 13, 2005 issue.