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
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
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
“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
The same management principles that are important to
preventing herbicide resistance apply to fungicide resistance, Bradley
“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
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
“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
The soybeans in the untreated check yielded about 50 bushels
“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.”
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.”