By Mark Glady
In terms of unpredictability, 2019 was hard to beat. The above-average rainfall, late planting, cool temperatures and overcast skies that made Minnesota farms short on growing degree units all contributed to the onset of disease — far more for soybeans than for corn.
Here’s a look back at the disease pressure we saw in Minnesota in 2019.
Little disease in corn
Except for small areas of eyespot and common rust, corn disease in my area of west-central Minnesota was virtually nonexistent in 2019. This is ironic, considering our wet, cool, cloudy weather. The answer may lie in the disease triangle, which is made up of three parts: the host, the pathogen and the environment.
We had the host — our corn crops. As far as the pathogen, the previous winter was tough. It was bitterly cold, the ground was frozen by early November and spring was late. A large number of fungal spores may have been unable to overwinter in those harsh conditions or survive our cool, wet spring.
As for the environment, most of the time temperatures were moderate: 70 to 80 or 85 degrees F. We only had a couple of days in the 90s. We also had plenty of summer rainfall. These factors were conducive to disease development. I would lean toward spores not surviving the harsh winter and cool spring as the reason for lack of corn disease this year.
Widespread disease in soybeans, especially white mold
This is the second year in a row that I’ve seen frogeye leaf spot in soybeans. While the yield impact from the disease was not severe, it was prevalent throughout Minnesota.
However, the big soybean disease this year was soybean white mold (SWM), which absolutely blew up in fields this year, with yields suffering as a result. It was more severe in the northern and western portions of my geography, and less so toward the south.
SWM came in late — during the second half of August or even later in some areas. Our cool, cloudy, foggy, dewy, wet weather was very hospitable to SWM development. Usually, the later SWM appears, the less of a toll it takes on soybean crops. Because many soybean fields were planted late this year, maturity was behind, leaving plants more vulnerable. This disease is known for causing substantial yield losses. SWM was by far the most severe disease in my geography this year and the primary disease we have to figure out how to control.
What to do about SWM and other diseases in 2020
The No. 1 defense against SWM is planting a variety that’s resistant to the disease. No. 2 is crop rotation. It can help to rotate fields out of soybeans for several years, though it’s difficult to do.
Another option is to apply a foliar fungicide that’s labeled for the disease you want to manage. With SWM, for example, there are several fungicides on the market that “suppress” it, but very few products that are labeled to “control” it. Work with your agronomist to choose the best options for you.
It’s hard to know what types of disease pressures 2020 will bring because we can’t predict the environment. What we can do is destroy the disease inoculum from this year and bury infected residues with tillage. This can help decompose some fungal spores — not so much for SWM, but for other diseases. The more you can bury residue and prevent it from staying on the surface of the soil where next year’s crop will be growing, the better off your crops will be.
Glady is a regional agronomist with WinField United in west-central Minnesota. Contact him at firstname.lastname@example.org.