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How to manage herbicide resistance, pests and disease on the farm

Grace Connatser
Wisconsin State Farmer
Growers of major vegetable crops in Wisconsin applied a great variety of fungicides, herbicides, and insecticides to protect those crops in 2016.

Experts from University of Wisconsin-Madison are giving tips on how to best manage herbicide resistance, pest control and disease in the field.

Herbicides and weed prevention

Rodrigo Werle, an assistant professor and extension weed scientist in the Department of Agronomy, says that overall Wisconsin farmers are losing the battle against herbicide resistance – but there are still ways to get around the issue.

Werle says you should first consider if your varieties have herbicide-resistant traits that help them tolerate stronger herbicides that have an easier time killing weeds, like glyphosate. It also helps to clean the combine head between fields because weed seeds can attach themselves to the combine and then release in another field, which can spread weeds out of your control. Werle urges farmers to plan and strategize as we go into the harvest season.

"We're urging our farmers to be strategic with how they go about harvest," Werle said. "Start harvesting the weed-free fields and then move to the weedy fields last. Try to remove those weeds before you go through and combine."

Rodrigo Werle

As we begin to purchase seeds for next year between October and January, Werle also said pay attention to the benefits of each variety and their traits. You should also get your weeds tested for resistance to certain herbicides, which will help you better plan and manage your weed-fighting strategy. Werle said his department conducted a study of waterhemp samples in 2018, which found that 95% of the samples were resistant to the standard 1x dose of glyphosate (Roundup is the major brand of glyphosate-based herbicide). These resistant weeds were most common in southwestern Wisconsin.

In southeastern Wisconsin, the same study found that many weeds were resistant to protoporphyrinogen oxidase (PPO) inhibitors. Werle said that in general, PPO, glyphosate and acetolactate synthase (ALS) inhibitors are not doing enough anymore and that farmers should seek alternatives. Werle also suggested spraying a pre-emergent herbicide to not only contain existing weeds, but to also prevent them from growing at all.

"An effective pre-emergent herbicide program is going to be the foundation," Werle said. "The better your pre-emergent herbicide program is, the more likely you will be to be successful with post-emergence ... control."

Werle said farmers should ask themselves some questions: What is the yield potential with my herbicides and crop traits? What is the disease tolerance? How much and what kind of seed is available? Who will spray herbicides? And what herbicides will be available in 2021?

Werle noted that XtendFlex soybeans, known for their high tolerance for glufosinate herbicide, were recently approved for commercial use in the US for 2021, while dicamba was ruled illegal by a US court, although the Environmental Protection Agency is fighting the ruling. While dicamba's effectiveness was not very evident in UW studies, Werle said the efficacy of 2,4-D choline was more prevalent.

Farmers should also ensure that they don't do a pre-emergent spray too early, or crops will suffer late in the growing season because the sprays are usually only effective for 30 days. 

The 27 trials conducted in 2016 on control of perennial weeds in alfalfa explored such variables as timing, product choice, and seeding rates, Extension Service weed scientist Mark Renz told attendees at the annual series of pest management updates.

Pest management

Bryan Jensen, a professor in the UW-Madison Department of Entomology, said considering your crops' traits is also very important for pest management, and while there are many ways to save money in pest control, field scouting remains one of the most effective ways to determine your field health.

Whether you do it yourself or you hire someone to do it, Jensen said field scouting is one of the most important steps in a pest management plan because you can't know the true state of your fields and what you need to do for them without seeing it for yourself. 

"That's an integral part of insect pest management," Jensen said. "A lot of what we do in insect pest management is rescue by nature. We scout what's in the field and then we react accordingly."

Jensen said farmers may consider buying seeds with above-ground traits, but he advised against it because they are not an integrated pest management practice. He also said it's difficult to predict what the summer's corn and stalk borer season will look like, especially since recent years have seen historic lows of the pests. Further, above-ground traits won't provide protection for pests like black cutworms and armyworms, which could be a more pressing concern. He recommended corn farmers check the Handy Bt Trait Table for US Corn Production to determine their best plans.

Bryan Jensen, a professor in the UW-Madison Department of Entomology, said considering your crops' traits is also very important for pest management.

While above-ground traits are not IPM practice, Jensen says below-ground traits are and recommended them in dealing with corn rootworm beetles, which tend to cause problems in late summer. The below-ground seed traits can save farmers money as well as increase resistance to problem pests, Jensen said. 

"Beetle scouting is going to be extremely important for us, managing root worms for resistance management, but also for economical management," Jensen said.

Corn farmers have many options when dealing with CRW beetles, including crop rotations and seed treatments, or even doing nothing if population counts are low enough – but all farmers should diversify their pest management strategies and tailor them to individual fields, Jensen said. He said the biggest mistakes farmers make are cutting corners and misplacing yellow sticky traps. Jensen said farmers should scout no less than 50 plants per field and should scout multiple times, and make sure to put the traps throughout the field and not just on the edges.

Jensen added that soybean farmers should also scout regularly for aphids so they can stay on top of aphid management plans by knowing how quickly populations grow. He also emphasized that farmers don't need to worry about yield loss if SBA count is at or below 250 aphids per plant, especially if other issues like disease are at stake. He also said farmers should not spray at that level because there is almost no return on investment for the herbicide used and it could cause flare-ups of two-spotted spider mites.

Among the 2017 corn diseases, southern rust relatively new one becoming more frequent in Wisconsin.

Controlling crop diseases

Damon Smith, an associate professor and field crops extension pathologist with the Department of Plant Pathology, also offered tips on how to identify and control crop diseases in 2021.

Corn and soybeans will be subject to several spreading diseases next year, with soybeans suffering from white mold, sudden death syndrome, soybean cyst nematode, phytophthora root and stem rot and frogeye leaf spot. Farmers will likely also see tar spot, gray leaf spot, northern corn leaf blight, gibberella ear rot and southern rust on corn crops.

"We've had southern rust move into the state now. There were some concerns of southern rust moving early, given that we had the remnants of a hurricane move up from the Gulf Coast," Smith said. "It didn't move in early, but it did definitely move in on time, I would say."

Smith said many crops are genetically engineered at the baseline level to have resistance to some of these diseases, although it is only a small amount of security. Smith said next year's soybeans and corn will probably fall into the "moderately susceptible" category of various diseases, which means farmers will need to do some of their own homework on what strategies for disease prevention work best for them.

Ribosomal protein small subunit (RPS) gene family works well in disease prevention and management, Smith said, especially the RPS1k gene, which a study showed was 99% effective on fields in disease prevention. However, Smith said finding RPS1k genes in crop varieties is rare now, so farmers should opt for other RPS genes if they can, like RPS1c, which was 75% effective in the same study.

Smith emphasized Werle's point that once seed treatments run past their 30-day growth mark, crops can no longer rely on outside help – it's only up to genetic resistance at that point. It's important for farmers to consider genetic resistance and treatment resistance and balance them carefully, rather than fully rely on seed treatments for disease prevention, Smith said.

"Once you start seeing some of the damage from the late season or mid-season moving in, those seed treatments aren't protecting at that point," Smith said. "You're relying basically on the inherent genetic resistance at that point to keep some of these diseases ... at bay."

Frogeye leaf spot is becoming an increasing concern among farmers, Smith said. He said it's unclear why it is springing up, but the growth is linked to a lack of genetic resistance in many crop varieties, and short crop rotations may also play a factor. In general, Smith said crops are beginning to lose their diversity in traits and resistance levels due to the conglomeration of agribusiness and seed dealers. He said frogeye leaf spot has caused an average $1.15 loss per acre on southern Wisconsin farms in the past decade, when in the late 90's and early 2000s it was only four cents an acre.