Disease: Watch Out for Yield Effects of Foliar Diseases

Keeping a close eye on yield-robbers is critical to maximizing on-farm profits and yields. Even in high production years like 2014, foliar diseases can take a bite out of soybean productivity. Add to that the emerging issues of fungicide resistance in the frogeye leaf spot pathogen (Cercospora sojina), and there are a number of issues worth watching.

“We saw less of a problem with frogeye leaf spot this year compared to some recent years,” says Carl Bradley, Ph.D., associate professor of plant pathology at the University of Illinois. “While it’s not a major disease in Illinois, we do find it at some level somewhere in the state every year, but it is usually more of an issue further south.”

In Southern Illinois, growers can see yield losses due to frogeye leaf spot, especially if they’re growing the wrong variety. If a grower in that area planted a susceptible variety, frogeye leaf spot probably did show up in 2014, explains Bradley.

Bradley’s research plots, where he studies ways to control the fungicide-resistant variant of the pathogen that causes frogeye leaf spot, are located in extreme Southeastern Illinois at the University of Illinois’ research farm near Dixon Springs. He said 2014 brought cooler weather in July, with low temperatures about 5 degrees below the 30-year average low. That unseasonably cool weather helped, as temperatures dropped at a critical time, slowing the spread of the disease.

“From looking at our research plots, the disease didn’t develop as quickly as it has in the last few years, and we think that was because of temperature. Our cooler July weather helped push back the development of symptoms. We ended up with a significant amount of the disease, but it showed up later and had less time to affect yield.”

Bradley says that researchers are continuing to track frogeye leaf spot resistance. Resistant strains have been documented in 10 states and while no new states were added to the list in 2014, researchers did identify additional areas with documented fungicide-resistant strains.

“Fungicide resistance is definitely out there and it will continue to be a problem,” explains Bradley. “We are seeing an increase in the prevalence of this disease in some areas, and in some cases, growers are having trouble controlling it.”

Management Key to Reducing Resistance

He adds that the key to slowing down the development and spread of fungicide-resistant strains is to apply fungicides that use different modes of action. Careful attention to selecting varieties can also help. “If frogeye leaf spot is a problem, growers need to find a variety with a higher level of resistance. Don’t just rely on fungicides to manage the problem.”

He adds that crop rotation is also important to keep disease pressure and fungicide resistance at bay. Most diseases are manageable with rotation, he explains. “The risk of higher levels of disease definitely increases with continuous soybean.”

Regardless of the management practices used, Bradley says that environment will still play the largest role in determining the extent of disease pressure. “Whatever weather conditions we have, we have to keep in mind that we have the full spectrum of soybean fungal pathogens present in most fields,” he explains.

“All of the fungi that cause these diseases are out there already, and harsh winter weather won’t help. If it can overwinter in Illinois, it will. Fungi can survive pretty well, even in a cold winter,” he adds.

Other Disease Impacts on 2014 Yields

Other diseases prevalent during the 2014 growing season included Sudden Death Syndrome (SDS) and Sclerotinia white mold. “The most important disease that we saw, in terms of the high level of incidence, was SDS.”

Bradley adds that both diseases are likely to show up in a high-yield environment like 2014. “Some growers may feel like a little bit of SDS means you’re likely to have good yields,” Bradley explains. “But if SDS shows up early in the season, it can create major yield losses.”

Bradley noticed one unusual occurrence in 2014 in the Southwestern part of the state, near Belleville, Ill.  He said that SDS showed up much earlier in some area fields than normal. “We don’t usually see it hit until soybeans reach the reproductive phase, typically at the early stages of podding,” he says. “However, there were several instances where it appeared when plants were still in the vegetative stages.”

He adds that while some plants did seem to grow out of it, many didn’t branch as much as they normally would and had fewer pods, resulting in big yield losses in some areas.

While white mold (Sclerotinia) did appear in parts of Central and Northern Illinois, its overall appearance was not as bad as it was in 2009. Bradley added that cooler summers and higher moisture levels work together to create a good environment for white mold, to the extent that it did cause some yield losses to growers.

In addition, SCN was also present in 2014. “It’s easy to forget about SCN in years like 2014 when we have good yields and the symptoms of SCN weren’t as visible,” says Bradley. “But growers need to keep in mind that without proper management, SCN always will be causing yield losses.”

More information about fungicide resistance is available in Plant Management Network’s “Principles of Fungicide Resistance: Focusing on Soybean and Corn Production” free webinar.