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Field Notes Talks Disease and Weed Management in a Very Wet Year

By Phyllis Bongard and Alison Robertson

After the rainiest April through June period recorded at many Minnesota weather stations, growers are facing increased challenges in weed and disease management in corn and soybeans. To address these issues and discuss management strategies, Dr. Alison Robertson, Field crops pathologist at Iowa State University, and Dr. Tom Peters, Extension weed scientist at University of Minnesota Extension, joined moderator Liz Stahl, Extension educator-crops, in the July 3 session of Strategic Farming: Field Notes.

Get out and scout!

In this unusual year, crop development can vary widely in a single field, complicating management decisions. Getting into the field to assess both crop health and weed management practices is more important than ever.

Got water? Watch for these soybean diseases . . .

Excessive moisture and flooding favors different pathogens, like the water-loving oomycetes Pythium and PhytophthoraPythium can cause damping off in soybean (and corn) in saturated soils and thus may be more common in later planted fields this year. Phytophthora root rot (PRR) is favored by warmer soil temperatures than Pythium. If PRR infection has occurred, dying soybean plants with the characteristic chocolate brown stem lesions may be noticed later this season. While there are no rescue treatment options for either of these diseases, soybean varieties with resistance to specific races (now called pathotypes) of Phytophthora sojae are available. However, populations of P. sojae are evolving and overcoming a lot of the resistance genes that are currently used in commercial soybeans. If PRR continues to be a problem, seed treatments may provide some protection.

What about other soybean diseases? Dr. Robertson’s colleague, Dr. Darren Mueller, recently published on article on the risk for white mold and sudden death syndrome (SDS) this season. While the wet weather has increased the chances of disease, conditions will need to remain favorable through July and early August for these diseases to develop.

For those hoping that flooding will drown out the soybean cyst nematode (SCN), Dr. Greg Tykla, Iowa State University, has mixed news. Unfortunately, nematodes can survive flooded conditions for several months; however, where soybean planting has been delayed, the number of SCN generations will likely be reduced, so the overall population increase may be less.

And these corn diseases

The corn disease, crazy top, is more common after flooding, usually affecting scattered plants in the field. Infected plants will be barren, so any yield reductions will depend on the number of plants involved.

Physoderma maydis causes two diseases that are more common under wet conditions especially when they occur at V4 through V8, Physoderma brown spot – a foliar disease – and Physoderma node rot. Although some hybrids are more susceptible to the node rot, these diseases rarely cause yield reductions.

Heavy rains can increase the incidence of Goss’s wilt. By bruising the leaves, heavy rain and/or hail can create entry points for the bacteria that causes the disease.

Stalk diseases are associated with plant stress. A root infection, for example, could develop into stalk rot later in the season. Early stalk diseases include Pythium or bacterial stalk rot, while AnthracnoseGibberella, and Fusarium stalk rots show up later. Gibberella could be problem this year; evaluate corn stalks at the R5 to R6 stages.

Moving forward with high moisture conditions

If stem and root rots, Crazy top, Physoderma, or Goss’s wilt are present, the most important thing to do is scout. Dr. Robertson urges growers to get out of the truck and into the fields past the end rows to note the diseases that are present. That information will help in selecting varieties and hybrids with the right disease resistance for the following years. If you’d like assistance identifying disease in Minnesota, send photos to the Digital Crop Doc and a UMN expert will help. Remember that every time a disease is present, inoculum that survives in soil or on residue builds. If the environment is right and a susceptible host is present, the risk for disease increases.

Certain foliar diseases, such as frogeye leaf spot in soybean, northern corn leaf blight, and tar spot in corn, can be managed with fungicides. If you see tar spot in the field, check the resistance level of the hybrid. Many hybrids have good resistance and a fungicide application may not be needed. However, if the hybrid is more susceptible and tar spot is present, applying fungicide is an option. Crops that were planted later are at higher risk, due to more favorable temperature and humidity conditions for disease development during the grain filling period.

Would a fungicide application pay this year? The potential for a positive ROI is complicated by this year’s conditions. Late planting, potential nitrogen loss, and other factors impact yield potential, which in turn affects the ROI. Keep in mind that fungicides protect the crop from disease, but they can’t overcome other issues. The Crop Protection Network ( is planning to release a tool to help calculate ROIs for fungicide applications soon.

The benefits of prophylactic applications are hard to define. Some physiological benefits from QoIs (strobilurons) have been noted, but they are affected by genetics and hard to justify. In contrast, fungicides applied to manage disease should see a positive ROI.

Strategies for weed management

Dr. Peters has been waiting 35 years for the perfect growing season. 2024 is not it. Weeds are bigger than they should be, herbicides are not working as well, and there are drowned-out areas in fields to deal with.

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

Other weeds that are usually less common are more widespread and thriving in the wet, cool conditions. Nightshade spp., smartweed, barnyardgrass, yellow nutsedge, and even bur cucumber have taken advantage of the unique weather conditions.

Postemergence herbicides have not been performing as well this year. During 2023, Dr. Peters received frequent calls about weeds not dying. During the drought, weeds developed thicker cuticles which reduced herbicide uptake. This growing season presents a different set of issues. While there are no issues with thick cuticles, herbicide uptake has been reduced because weeds are growing slower than normal. Consequently, there are weed escapes this year even from reliable programs such as common lambsquarters control with glyhphosate.

Dealing with larger weeds

There have also been limited opportunities to apply herbicides in a timely manner this year, so weeds are larger. Peters suggests looking at tank mix partners – depending on herbicide traits - with the primary herbicide to get better uptake. However, be careful. If crops aren’t growing, they’re not metabolizing herbicides as efficiently and mixtures might cause more injury symptoms than in other years.

Soil residual herbicides such as chloroacetamides, triazines, or even a PPO inhibitor, may offer additional control options. Of these three groups, the Group 15 chloroacetamide herbicides (Dual Magnum, Outlook, Warrant, etc.) are still in play, particularly if waterhemp is a primary production challenge. However, be careful with the triazines due to carryover. Last year, carryover was a concern because microbes lacked moisture to break down herbicides. This year, the opposite cold and wet soil conditions have had the same result. Microbes require moisture and warm temperatures to degrade chemicals, but they can’t swim so their activity has been reduced. Be very careful with products applied late in the season – including rescue treatments or products applied to drown out areas - and follow rotation restrictions on the label.

Following crop stage application restrictions is also a challenge this year due to the wide variation in growth stages in the same field. Comprehensive lists of corn and soybean herbicides and their growth stage cutoffs were recently published on Crop News for your reference. Cutoffs for applying herbicides with drop nozzles are also included.

As for non-chemical weed control methods, it’s too wet and weeds are too large for inter-row cultivating. However, if fields start to dry out, it might be the perfect environment to try the WeedZapper (electrocution). The WeedZapper works better when weeds are turgid. Second, the WeedZapper is effective at reducing seed production from weeds..

Too late to plant or replant – Now what?

If no crop will be planted, playing the long game with weed management is critical. If drowned out areas and field edges are ignored, weeds will produce seed and the clock on weed control will be reset which then will require another four to six years to get the seed bank under control. Decide how to manage field edges and fencelines. Mixtures with Valor, Sharpen, 2,4-D, Clarity, Roundup, etc. can be applied in drowned-out areas to control weeds and have crop rotation restrictions that align with crops Minnesota growers typically plant.

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