Soybeans across Iowa are beginning to flower. The growing season thus far in 2010 has been wet, or rather very wet. Ponds have sprung up in fields across the state, and Iowa resembles Minnesota!
Diseases prevalent in soybean fields include brown spot in the lower canopy and bacterial blight in the upper canopy. "It is common to see both diseases on soybeans at this stage of the growing season, particularly with the weather we have been having," says Alison Robertson, Iowa State University Extension plant pathologist. "Brown spot severity increases with increasing periods of leaf wetness. Infection by the bacterial blight pathogen is favored by heavy rains."
Do you need to do anything to manage these diseases?
"No, not yet," she says. "As we get into summer and temperatures rise, bacterial blight should become less prevalent since it is more of a cool season disease. If frequent rains continue, however, brown spot could become an issue. When brown spot occurs predominantly in the lower canopy, it does not affect yield; however, when it moves into mid-canopy before the end of pod fill, it can reduce yield. Under such conditions, an application of fungicide may be warranted."
If Iowa returns to "normal" rainfall, deciding whether to spray a fungicide will be trickier. Robertson and her colleagues at ISU recently summarized data collected from more than 800 fungicide trials (small plot research from ISU and on-farm research from the Iowa Soybean Association's On-Farm Research Network and the ISU Corn and Soybean Initiative) conducted in Iowa between 2006 and 2009.
The mean yield increase across all trials was 2.2 bushels per acre. Where disease data was collected (225 treatments), yield response was greater when disease severity (brown spot, frog eye leaf spot, Cercospora leaf blight or downy mildew) was greater than 5% at R5. Higher yield response occurred with applications at growth stage R3, followed by applications at R2, then at R1.
Watch fields where you've had foliar disease problems previously
Based on this summary of the field test results, what advice can Robertson offer? "First, I think it is important to scout," she says. "Target those soybean fields where you have had foliar disease problems in the past. Talk with seed dealers about the soybean varieties you are growing to determine if they may be susceptible to one or more foliar diseases. Also, you need to consider the cost of fungicide plus application, and the price of grain."
Some soybean growers have great success with using foliar fungicides in Iowa. Others are disappointed. "I guess this should not be a surprise," says Robertson. "Specialty crop growers (apples, grapes, etc) may make up to 10 fungicide applications in one season; yet they continue to tinker with their fungicide regimes. If you are applying a fungicide, our advice is to pay close attention to details to begin to determine when fungicides will work on your farm."
Details such as disease presence, weather conditions during certain times of the year, other pests present, date of canopy closure, etc., can be the beginning of developing your own personal database. "Over a period of time," she notes, "you can develop and use your own data to help make fungicide decisions for your fields and your growing conditions."