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Strategic Farming: Field Notes session tackles corn rootworm and corn and soybean disease and fungicide recommendations

This article was written by UMN Extension crops educator, Angie Peltier; Extension IPM specialist, Bruce Potter; and Extension corn and soybean plant pathologist, Dean Malvick.

On the June 29, 2022 Strategic Farming: Field Notes episode, Bruce Potter, Extension Integrated Pest Management Specialist, Andy Nesseth, part owner, agronomist and crop consultant from Extended Ag Services of Lakefield, MN and Dean Malvick, Extension corn and soybean plant pathologist joined moderators and Extension educators David Nicolai and Ryan Miller to discuss corn rootworms and crop diseases.

To listen to a recording of this episode subscribe to Strategic Farming: Field Notes on your favorite podcasting platform or visit this website:

Western corn rootworm beetles. Photo: Angie Peltier

Corn Rootworm

Sticky trap survey

The feeding of corn rootworm larvae prunes corn roots. This injury can reduce yield by affecting water and nutrient uptake and reduce harvestability when affected plants lodge. Determining potential rootworm risk should be based in part on the adult populations in a field. The period over which rootworm beetles are laying eggs is critical for understanding potential rootworm pressure in the following growing season.

To help corn producers, their advisors, and corn entomologists better understand what is happening with rootworm populations in Minnesota farm fields, in 2018 Extension corn entomologist Dr. Ken Ostlie started to explore the potential for a network of corn rootworm cooperators. This Minnesota Corn Research & Promotion Council-sponsored trapping program uses sticky traps to capture corn rootworm beetles as they move within corn fields. Participants place and monitor traps, counting beetles weekly and reporting trap catches to UMN Extension. Some corn rootworm populations have proven themselves capable of overcoming multiple management tactics. When combined with evaluations of root injury, the information gathered through this trapping network provides information regarding how well (or not so well) hybrids expressing Bt proteins are managing rootworms.
Concerning sticky trap catches

Discussion also focused on some of the most recent results from the network that have begun to concern entomologists. In some livestock-heavy regions of Minnesota where continuous corn is common, rootworm population densities in fields planted to non-Bt hybrids and in Bt hybrids have recently become indistinguishable, revealing resistance-management concerns need to be addressed to maintain long-term crop productivity.

With the understanding that relying on Bt traits and in-furrow insecticides alone to manage rootworms is no longer an option in southwestern Minnesota, crop advisors like Andy Nesseth work with producers to develop multi-year management strategies that include crop rotation and strategic deployment of Bt and non-Bt hybrids. Working in areas in which continuous corn production is desired to feed livestock, planting a crop such as soybean at minimum every 4-years is also highly encouraged. It helps to have field-specific beetle data to inform conversations and decisions.
Joining the sticky trap network

According to degree-day models, corn rootworm egg hatch is currently peaking in southwest Minnesota. This is therefore the time of year that Bruce Potter begins to send out yellow sticky traps to those in his on-farm trapping network. Those with an interest in monitoring sticky traps should contact Bruce Potter.

Adult beetle management

A potential threat that rootworm beetles pose to the 2022 crop is clipping silks during pollination. Consider treating if there are more than 7 or 8 rootworm beetles feeding on silks early on during pollination and silks have been clipped to have less than ½ inch exposed. However, beetle densities do not often get high enough to impact kernel set. A potential management tactic for future corn crops is “beetle bombing” to reduce egg-laying adult populations. Effective adult management through foliar insecticides is also quite difficult to achieve. Applications need to be timed to when female beetles swollen with eggs can be observed in fields. One might unintentionally time an application too early, in which case they will be controlling the beetles that emerge first – mostly males. An application that comes too late can allow significant egg laying to occur. More than a single application can be required in some years due to this timing uncertainty. As a result, adult beetle control is not recommended as a general management practice.

Gathering data in 2022 for 2023 corn

At this point in the growing season, corn rootworm management for the 2022 crop has already taken place. Going forward, time dedicated to corn rootworm in 2022 will rather inform corn rootworm management in 2023. An alternative to the use of sticky traps to determine your 2023 rootworm risk, whole plant beetle to determine the number of beetles/plant can be effective. If there is an average of 0.75 (2nd year) to 1.0 (3 or more years) or more beetles per plant, then a 2023 corn crop planted in that same field will be at risk for significant injury. For those northern corn rootworm populations that lay their eggs in soybean fields (aka: extended diapause), 2024 crops are at risk at higher levels (4.5 /plant). These thresholds are based on work in non-Bt corn.

Addressed by the presenters was whether resistance to a Bt trait is permanent in a given field’s rootworm population. Resistance issues often aren’t simple…. Research has shown that resistance to one Cry3 Bt protein in effect provides resistance to other Bt proteins. For example, rootworms that are resistant to Cry3Bb1 (Yieldgard Rootworm) are also resistant to Bt traits mCry3A (Agrisure RW) and eCry3.1Ab (Agrisure Duricade). In addition, even if one moved away from planting hybrids with Bt proteins to which their rootworm populations are resistant, rootworm population shifts back toward sensitivity are unlikely to occur. There is no evidence that having the Bt-resistance trait negatively affects either rootworm productivity or survival and so even when the Bt trait is no longer deployed there is no “cost” to the population’s holding onto the Bt-resistance trait.

How late planting may affect rootworm pressure

When eggs are hatching in fields of late-planted corn, larvae encounter smaller root systems and injury symptoms can be much more significant than when larvae feed on larger sized root systems. As silks in earlier planted fields begin to degrade as a rootworm beetle food source, nearby later-planted fields may also attract these beetles.

Sudden death syndrome of soybean. Photo: Angie Peltier

Sudden death syndrome of soybean

Now that corn and soybean plants have emerged, they enter a period of time during the later vegetative growth stages in which few diseases are apparent, although exceptions occur! It is often during the reproductive phases that diseases develop further and symptoms and signs appear.

One of the diseases that can cause significant yield loss in soybean is sudden death syndrome (SDS). Although this disease is more prevalent in southern Minnesota, over time as the fungal pathogen has spread further north so has disease risk. There are regions of the state in which soybeans have experienced the early-season environmental conditions (wet soils after planting) that favor infection by the fungus that causes SDS. Whether or not we will see foliar symptoms and severe SDS will depend upon significant rainfall occurring during reproductive (R) growth stages in July and August.

SDS is likely established in plants throughout the state as infection typically takes place during the first 3 weeks of soybean growth. Early season symptoms have been observed in Dr. Malvick’s SDS research plots in Waseca and Rosemount, MN. The interveinal chlorosis (yellowing of the tissue between the leaf veins) observed in leaflets can only be observed on a small percentage of plants at this time in the growing season, whereas if sufficient rains fall in July and August, foliar disease incidence could reach 70% or more. It is recommended that people should pay close attention to a history of observing SDS in a particular field rather than trying to “find this needle in a haystack” by relying on early-season scouting to pick out the fewer than 1% of plants that might exhibit early-season symptoms.

Tar spot of corn

First observed in Minnesota in 2019 and now established in southeast Minnesota, tar spot in corn has spread within the state as far north and west to areas near St. Cloud, MN. Tar spot in Minnesota is new enough that although corn hybrids differ in their susceptibility, many seed companies do not yet have tar spot ratings in their seed catalogues.

At this point in the growing season, there is not much that can or should be done to manage tar spot beyond preparing to diligently scout the corn crop and making sure that a fungicide that has shown high levels of efficacy against this disease is available should you need it.

In the limited years that tar spot has been observed in Minnesota, symptoms haven’t been previously observed before around the third week in July. With late planting throughout much of Minnesota, there is a chance that initial infections might be observed on younger plants than they have in previous growing seasons.

Those areas of the state in which corn is the dominant crop in the rotation are likely at more risk for tar spot as fungal inoculum can build up, causing more spores to be released to cause the first infections. More spores can produce more of the raised, black fungal structures, each of which can produce more spores for additional cycles of infection.

Scouting for tar spot is important as the fungus that causes tar spot can produce multiple cycles of infection. Finding the raised, black fungal structures characteristic of tar spot infection toward the beginning of an epidemic can help producers to know where the risk of tar spot this season is greatest so that they can properly time a foliar fungicide application to meet their level of risk.

Fungicides appear most effective for tar spot when applied VT-R2, if conditions justify their use. They provide protection for 3 to 4 weeks after application and there will likely be instances when delaying an application just a bit, can mean that a single application is needed, rather that two. However, applying a fungicide too early can risk both not protecting the uppermost leaves that contribute the most toward grain yield and may necessitate an additional fungicide application. Applying one too late can run the risk of the crop sustaining more yield loss than is necessary.
White mold (Sclerotinia stem rot) of soybean. Photo: Angie Peltier

White mold of soybean

Fungicides differ in their efficacy in protecting soybeans from fungal diseases. Adequate carrier volume, full label rates of active ingredients and proper application timing can provide the best protection against white mold. If forecasted weather conditions are favorable for disease to occur and rows are closing, application should be timed to coincide with beginning flowering (R1), if rows haven’t yet closed wait until full flower (R2). Applications that occur at beginning pod (R3) tend to provide a less protection than earlier flowering timings.

Frogeye leaf spot in soybean and fungicide resistance

Similar to tar spot in corn, frogeye leaf spot (FLS) is caused by a fungus that with each newly developed lesion capable of producing spores causes multiple cycles of infection. Unlike tar spot in corn, isolates of the FLS pathogen in Minnesota have been confirmed to be resistant to the widely-used strobilurin (QoI, Group 11) class of fungicides. Producers may consider a beginning pod (R3) fungicide application if disease is already present in the lower canopy and warm and humid weather conditions favor FLS. In that instance, please pay close attention to the active ingredients listed on the label of your intended product. If a Group 11 fungicide is listed, your product will be relying on the other fungicide groups for FLS management. Sometimes the rates of individual non-QoI active ingredients in fungicide pre-mixes are below the rates that are effective for managing FLS. To both improve efficacy and reduce development of fungicide resistance, select a different formulation of your intended active ingredient if the rate of the remaining effective a.i. is lower than the label rate of formulations with that a.i. alone.

Foliar fungicides to manage disease.

Results from multi-year trials with fungicides on corn and soybean in Minnesota can be found here:

Thank you to the Minnesota Soybean Research & Promotion Council and the Minnesota Corn Research & Promotion Council for their generous support of this program!

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