Carl Bradley talks about University of Illinois fungicide research trials during a presentation at Agronomy Day held at the Northern Illinois Agronomy Research Center. He also discussed two soybean diseases — septoria brown spot and frogeye leaf spot. He stressed the importance of managing fungicide applications to prevent the development of resistance to products.
Carl Bradley talks about University of Illinois fungicide research trials during a presentation at Agronomy Day held at the Northern Illinois Agronomy Research Center. He also discussed two soybean diseases — septoria brown spot and frogeye leaf spot. He stressed the importance of managing fungicide applications to prevent the development of resistance to products.
SHABBONA, Ill. — Septoria brown spot is a disease that can be easily found in soybean fields.

“I can go out to any of your soybean fields and find this disease,” said Carl Bradley, University of Illinois Extension plant pathology specialist and associate professor. “But that doesn’t mean it causes yield loss every year.”

Septoria brown spot starts in the lower canopy of the plant.

“Look for leaves starting to turn yellow and black dots all over,” advised Bradley during a presentation at Agronomy Day held at the University of Illinois Northern Illinois Agronomy Research Center. “Rain splashes spores from the soil onto the lower leaves, and when you get more rain, that splashes the spores to the next set of leaves and so on.”

Therefore, weather during the growing season will impact the amount of yield loss from this disease.

“If we get premature defoliation in the upper canopy, that’s when we can have some yield loss,” Bradley noted.

Last year, he said, researchers evaluated 93 isolates.

“We did this on a whim, and we determined the EC50, which is the effective amount of fungicide that will reduce the fungus growth by 50 percent,” he explained.

The results showed it took less than 0.5 parts per million of strobilurin fungicide to inhibit the fungus on 91 of the isolates.

“But for two of the isolates, it took over 2 ppm of the fungicide to inhibit the fungus,” Bradley reported. “That’s over 100-fold more fungicide, and those two isolates that are resistant to the fungicide came from DeKalb County.”

Since this was a small sample, Bradley said, he was a little startled by the results.

“Normally finding fungicide resistance is like finding a needle in a haystack,” he said. “We only had 93 isolates, and we found resistance in two of them — I was surprised by that.”

However, the professor said, the good news is about 2 percent of the population was resistant, so that means the fungicide worked last year.

“If you had septoria brown spot in the field and you killed 98 percent of the population, it will look like you controlled the disease,” he added.

Fungicide Guidelines

The same management principles that are important to preventing herbicide resistance apply to fungicide resistance, Bradley noted.

“The bottom line is we don’t want to be using a single active ingredient, and most companies are marketing products with more than one active ingredient,” he said. “We don’t want to spray a single strobilurin active ingredient knowing that in DeKalb County we have some isolates resistant to strobilurin fungicides.”

Bradley stressed that farmers should not rely on fungicides as a silver bullet.

“Start with a good genetic base — pick varieties and hybrids that have good levels of resistance,” he stressed.

Farmers should decide to use a fungicide based on the disease level of the field.

“Base your decision on scouting, the susceptibility of your hybrid or variety and the weather conditions,” Bradley advised.

Although frogeye leaf spot is not a big problem in northern Illinois, Bradley said, in 2010 researchers began to find isolates of the fungus resistant to strobilurin fungicides.

In a fungicide trial at the Dixon Springs research center, the untreated check of soybeans had 40 percent severity in the upper one-third of the canopy or 40 percent of the leaf area was covered with frogeye leaf spot lesions.

“The products with strobilurin only did not have great control. It was similar to the untreated check,” Bradley reported. “The products that contained strobilurin and triazole had better levels of control, but not all of the products are the same. They have different rates of active ingredients.”

The soybeans in the untreated check yielded about 50 bushels per acre.

“The plots that received the fungicides with only strobilurin did not provide positive yield response,” he said. “The fungicides with more than one chemistry resulted in increased yield responses.”

Corn Studies

U of I researchers have conducted fungicide research trials on corn since 2008 and have now tested products in 40 different environments.

“Over these 40 environments, we saw an average of 5.2 bushels per acre yield response from the fungicide applications,” Bradley said.

“However, as corn prices and fungicide prices fluctuate, the yield response you need to be profitable will bounce around,” he said. “With corn at $4 per bushel, you need between five- to eight-bushel yield responses to be profitable with foliar fungicides.”

Across these 50 environments, 50 percent had at least a five-bushel average yield response and 43 percent of the time at least an eight-bushel yield response.

In 26 of the 40 environments, there was relatively low disease pressure where the untreated checks had less than 10 percent of the leaf area affected by disease.

“The average yield response in this group was 2.4 bushels per acre for the fungicide,” Bradley reported. “And 31 percent of the time, we achieved a five- or eight-bushel yield response.”

With a moderate to high disease pressure where at least 10 percent of the leaf are affected by foliar disease, he said, the average yield response increases to 10.4 bushels per acre response and the frequency of achieving a five- or eight-bushel yield response went up to 86 percent for five bushels and 64 percent for eight bushels per acre.

“We see a lot more consistency with yield response when we’re controlling diseases,” Bradley said. “That’s not to say you won’t see yield responses with low disease pressure — we still did, but not as often, around 35 percent of the time.”