Integrated Crop Management

Update: Soybean rust and other foliar diseases

Another growing season is passing and soybean rust remains confined to the southeastern United States. With drier-than-normal conditions throughout much of the southeastern United States early in the spring and well into summer, soybean rust has not been able to spread too far from the overwintering sites.

To date for 2006, there are 28 counties in six states with soybean rust. In comparison to 2005, there were 21 counties with soybean rust in early August (see maps).

Soybean rust map of reports, 2006 [1]
In 2006 (top map), 28 counties in six states reported soybean rust as of August 7. Red areas show where rust was found; green counties where rust was found earlier this year and is no longer there. In 2005 (bottom map), 21 counties reported soybean rust in early August.

Late season management options

When do Iowa growers need to stop worrying about soybean rust for this season? The soybean crop, on average, is currently at growth stage R4 to R5 in Iowa. In the next few weeks, most soybean acreage will be at R5 or later. The chances of soybean rust spreading from the southeastern United States to Iowa in the next few weeks are very, very low. The Iowa State University Soybean Rust Team recommends not spraying soybean with fungicide later than R5 as fungicide sprays are either not legal or not economically profitable. For more detailed information on applying fungicides for management of soybean rust, read the bulletin entitled Asian Soybean Rust Management--2006 (PM 2028) [2]. Copies of the publication may be ordered on the Web through the Iowa State University Extension Distribution Center [3].

Other foliar diseases

What has been seen in Iowa soybean fields this season? Sentinel plots have been intensively scouted for soybean rust and other diseases weekly throughout the growing season. Because of the hot, dry conditions throughout most of Iowa, very little foliar disease has been reported. Disease reports from the sentinel plots have included Septoria brown spot, downy mildew, frogeye leaf spot, and bacterial blight but none at high levels.

Downy mildew [4]
Downy mildew, but not rust, has been reported at sentinel plots this year. (Daren Mueller)

This article originally appeared on pages 221-222 of the IC-496(23) -- August 7, 2006 issue.

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