If you’ve ever thought about spraying a fungicide on soybeans, this would be the year to do it, says Ed Sikora, Auburn University Extension plant pathologist.
“All fields in Alabama, for the most part, have now been exposed to the soybean rust pathogen. We had a lot of soybeans planted late this year, and double-cropped beans were delayed due to wet conditions. These beans are at extreme risk. If you’ve ever thought about spraying a fungicide, this is the year to do it,” said Sikora at the recent East Alabama Crops Tour.
Fields in south Alabama that have not reached the R6 (full pod) growth stage should be treated with a triazole fungicide if a fungicide has not been applied previously, he advises. A second fungicide application may be warranted if it has been more than three weeks since the last application and the R6 growth stage has not been reached.
“Based on the aggressiveness of soybean rust this year, I now think an application during the bloom stage or earlier may be warranted in high-value crop situations. Rust will cause yield losses in unprotected fields in Alabama this year,” he says.
Sikora also recommends that fields in north Alabama be treated with a triazole fungicide if a fungicide has not been applied previously and the crop has not reached R6.
“Based on what we observed at Sand Mountain, I am concerned that full-season soybeans are acting as inoculum reservoirs for double-cropped or soybean fields planted late in the season. Of course, as in all cases, growers need to determine the yield potential of these late-planted soybeans and the cost of a fungicide application.
"However, it does appear soybean rust has a high likelihood of causing yield losses throughout the state.”
The goal, says Sikora, is to get soybeans into the R6 stage. “We try to get soybeans into the R6 stage where the pods are completely full. If you can get them to that point, you no longer need to spray. If beans are in the R5 stage with rust in the low to mid-canopy, you’ll probably see some yield loss. Going in with a late fungicide application — probably some triazole — probably would be good, with some curative activity.”
Soybean rust was detected in a research trial at the Sand Mountain Research station in Crossville in DeKalb County this past week, says Sikora.
“The plot was completely defoliated. I thought it was a joke at first, but the few leaves remaining on top were covered with soybean rust pustules. Lab tests confirmed the severe rust outbreak. The variety is defoliating about two weeks ahead of schedule. Pods are still trying to fill in some plots, but no leaves are present. No fungicide was applied to this test. The variety is Asgrow 4933 RR and was planted on April 26.”
Adjacent to this plot was another test where the soybeans were only at full bloom, but rust was already in the mid canopy, he says.
“The disease is progressing much, much earlier then we have ever seen in Alabama, or in the United States.”
Sikora also visited trials at research stations in Brewton and Fairhope near the Gulf Coast. “Soybean rust is building up rapidly at these sites in south Alabama, at least two weeks ahead of schedule from previous years. I have also observed very high levels of the disease in the southeast region of the state on kudzu and in commercial fields.”