Strategic Farming: Field Notes talked disease and weed management in a wet year

After the rainiest April through June period recorded at many Minnesota weather stations, corn and soybean growers are facing increased weed and disease management challenges. Assessing both crop health and weed control practices is more more important than ever.

Dr. Alison Robertson, field pathologist at Iowa State University, tells growers to “get out of the truck and into the fields to see what diseases are present.” Armed with that information, growers can match varieties and hybrids with the disease resistance traits.

What might growers see this year? Excessive moisture and flooding favors different pathogens, like Pythium that causes damping off and Phytophthora root rot in soybean. While there are no rescue treatment options for either of these diseases, soybean varieties with resistance to Phytophthora are available.

Other soybean diseases, such as white mold and sudden death syndrome, are also favored by wet weather, but conditions will have to be right through July and early August for these diseases to develop. For those hoping that flooding would drown out the soybean cyst nematode, unfortunately, the nematode can survive flooded conditions for several months; however, where soybean planting was delayed, the number of SCN generations will likely be reduced, so the overall population increase may be less.

In corn, wet conditions favor less common diseases like Crazy top and Physoderma, diseases that rarely cause yield reductions. However, heavy rains and hail can increase the incidence of Goss’s wilt, a disease that can be economically important.

Every time a disease is present, an inoculum that survives in soil or on residue builds. If the the environment is right and a susceptible host is present, the risk for disease increases.

Some foliar diseases, such as frogeye leaf spot in soybean and tar spot in corn, can be managed with fungicides, unlike the corn and soybean diseases mentioned above. However, the potential for a positive return on investment (ROI) is complicated by the conditions this year. Late planting, potential fertilizer nutrient loss, and other factors impact yield potential, which in turn affects the ROI. Fungicides can protect the crop from disease, but they cannot overcome other issues.

Dr. Tom Peters, Extension weed scientist at University of Minnesota Extension, has been waiting 35 years for the perfect growing season. “2024 is not it,” he states. “Weeds are bigger than they should be, postemergence herbicides are not working as well primarily because of cool and wet conditions, and there are drowned-out areas in fields to deal with.”

Weed characteristics demand respect this year. Wet conditions with cooler temperatures have extended weed germination and emergence periods. Early season weeds like kochia, common lambsquarters and ragweed, that should be done emerging, are still germinating.

Other weeds that are usually less common in our area are more widespread in 2024 and are thriving in the wet, cool conditions. For example, nightshade, smartweed, barnyardgrass, yellow nutsedge and even bur cucumber have taken advantage of prolonged cool and wet conditions.

Postemergence herbicides have not been performing as well this year. During 2023, Peters received frequent calls about weeds not dying. Weeds had developed thicker cuticles during the drought which reduced herbicide uptake. During 2024, herbicide uptake has been reduced because weeds are not actively growing.

There have also been limited opportunities to apply herbicides in a timely manner, so weeds are larger. Peters suggests adding tank mix partners – depending on herbicide traits - with the primary herbicide to get better uptake and control. However, like weeds, crops are not growing so we need to be careful with crop injury since crops are not metabolizing herbicides as efficiently and mixtures might cause more injury symptoms than in other years.

Soil residual herbicides may offer additional control options. Group 15 (chloroacetamide) herbicides are still in play, particularly if waterhemp is an issue. However, be careful with the triazines and other products applied late in the season due to carryover concerns. Be sure to follow rotation restrictions and all label instructions.

Weed management in drowned-out areas is critical. If these areas are ignored, weeds will produce seed and the weed control clock will be reset requiring another four to six years to get the seed bank under control.

For more information from University of Minnesota Extension, visit extension.umn.edu/crop- production.

Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.

For more news from U of M Extension, visit https://www.extension.umn.edu/news or contact Extension Communications at extdigest@umn.edu. University of Minnesota Extension is an equal opportunity educator and employer.