If it is hot and humid leading up to the R3 stage of soybean growth, then frogeye leaf spot could become a major problem in soybean fields. The severity of the disease and the area of infection seems to be increasing, and resistance to strobilurin fungicides is becoming a factor, too.
The use of fungicides and planting disease resistant varieties of soybeans has almost become a necessity for growers looking to achieve full yield potential from their soybeans in the Midsouth.
“The best line of defense is to plant frogeye leaf spot resistant varieties of soybeans, but if a highly resistant variety isn’t available or the disease appears that it might overcome the plant’s defenses, then the option is to spray a triazole fungicide that is good against frogeye leaf spot,” said Jason Hamlin, owner of North Delta Crop Consulting at Dyersburg, Tenn.
Hamlin sees the need for a triazole fungicide because he has seen fields where the frogeye leaf spot has become resistant to the commonly used strobilurin fungicides, but he said, “the strobilurins bring a lot to the table,” and, therefore, he is recommending a mixture of an effective triazole and proven strobilurin.
“We always have some frogeye leaf spot, but what is happening now is that some of it is becoming resistant to the strobilurin fungicides. The frogeye leaf spot seems to be evolving,” he said.
“Those triazoles that are good against this disease are few and far between at the moment. There are several triazoles but not many of them are very effective against frogeye leaf spot,” he said.
The problem with seed variety selection against frogeye leaf spot is that the variety might not be resistant to other diseases that a strobilurin fungicide easily controls. That is why Hamlin and other agronomists want to keep the strobilurin fungicides useable.
He said, “I don’t recommend using straight strobilurins anymore but rather using a mixture of two modes of action fungicides that includes the right mixture of a triazole that is effective against frogeye leaf spot. The triazole in the mixture definitely needs to be labeled for frogeye control.”
Stratego YLD fills the bill for Hamlin because of its two active ingredients and mainly because its specific triazole, prothioconazole, the same one as in Proline, is labeled for control of frogeye leaf spot.
Mark Waddington, Bayer CropScience technical service representative in much of the northern Midsouth, says Stratego YLD has “two modes of action that really complement each other” when the fungicide is applied at the R3 or smaller stage of soybean growth.
Farmers are aware that frogeye leaf spot has increased, even if they are not aware of how strobilurin resistance is rapidly evolving since the first resistant strain of the disease was detected in Tennessee in 2010, Waddington said. Resistance is now in states on both sides of the Ohio River and Mississippi River.
Resistant or not, the disease is a problem. “Whether it is a more conducive environment, a more viral pathogen, cultural practices or all three, frogeye leaf spot is increasing,” he noted.
The disease likes hot and humid weather, and Hamlin explained that even during a drought hot and humid is still typical with heavy morning dews being enough to encourage disease development.
“A 25 percent average yield loss would be a conservative estimate, especially in our higher productive and higher risk areas. A 10 percent to 40 percent yield loss in different fields would not be unlikely across the Midsouth and into southern Illinois and Indiana,” Waddington said.
How wide spread evolving pathogens of frogeye leaf spot will spread is unknown. For more information Waddington can be reached by e-mail at firstname.lastname@example.org.