EEUU: Technology provides new weapons in battling blight

 The disease, which can spread quickly and cause total crop failure if left untreated, is perhaps the most well-known potato pathogen in public perception, as it was responsible for the Irish potato famine of the late 1840s.

Early blight, common, potentially costly and caused by the fungus Alternaria solani, typically infects older leaves in warm, humid conditions. Both late and early blight — demarcated by the relative time of appearance in the field — can be chemically treated, and each can require intensive intervention, depending on region, climate and other factors.

Early blight, common, potentially costly and caused by the fungus Alternaria solani, typically infects older leaves in warm, humid conditions. Both late and early blight — demarcated by the relative time of appearance in the field — can be chemically treated, and each can require intensive intervention, depending on region, climate and other factors.

“We’re blessed to have abundant water as a resource here, but coming along with that is higher pressure in diseases,” said Amanda Gevens, Plant Pathology chair, professor and Extension specialist at the University of Wisconsin-Madison and administrative director of the Wisconsin Seed Potato Certification Program. “We do have a bit more management to tend to when we’re trying to get a healthy potato crop out of the ground.

“These two diseases at some point dovetail in terms of how we’re managing that.”

EVOLVING ENVIRONMENTAL LANDSCAPE

An ongoing consideration affecting prevention and treatment for both diseases is an EPA-proposed label change for chlorothalonil (trade name Bravo). The Proposed Interim Decision includes mitigations related to annual maximum application rate reduction on vulnerable and non-vulnerable soils, spray drift mitigations and probation of applications during inversions, probation of applications on saturated soils, and the creation of buffers to all aquatic areas.

“At this point, it’s unlikely that there will be changes to this. I just don’t know when their new plan will take effect,” Gevens said. “There are different quantities of chlorothalonil permitted depending upon the soil quality. If it is considered a soil of concern, then it further reduces the allowable usage of chlorothalonil on that land.

“For most of Wisconsin, actually most of the upper Midwestern potato production states, this would affect everyone. It is something that will be impactful. It will reduce our allowable usage by roughly half.”

The National Potato Council weighed in on the proposed change in comments to the EPA in late January.

“We believe it is essential for the agency to ensure the future availability of chlorothalonil remains an economical tool in potato production in the United States,” NPC wrote. “We believe the real-world information from the environmentally conscious producers on the actual use of chlorothalonil and the other fungicides used to protect the U.S. potato crop needs to be included in the agency’s decision to ensure that their decisions do not provide a negative economic impact on producers or provide a pathway for the development of resistance to any of the pest management tools used by any of the pathogens that are detrimental to the industry.”

While the potential changes pose concern, growers are already taking steps to address them, while advances in fungicide technology also provide reason for optimism.

“Some growers are just taking out chlorothalonil and putting in mancozeb,” Gevens said. “Mancozeb doesn’t have one of those Proposed Interim Registration decisions. … They’re fairly comparable in early blight control in the field and serve that same role of providing resistance management in the spray program overall. But it’s only temporary. I would suspect that soon, the EPA will be reevaluating mancozeb in a similar fashion, and we’ll see reductions there.”

Another solution involves advances in fungicide development, including treatments with multiple active ingredients.

“These are to growers’ advantage because they help to manage fungicide resistance,” Gevens said. “It comes as a package of two active ingredients that are good at managing early blight by themselves, but in using them together, it also helps us to manage the development of resistance within those pathogens.”

For late blight, Syngenta’s Orondis fungicide line, approved for commercial use in 2015, has proven effective, Gevens said.

A potato crop treated with Orondis Opti. Photos courtesy of Syngenta.

“It gives really strong protection inside the plant, and there’s no recognized resistance in our current pathogens populations,” she said. “Growers want to find fungicides for late blight that have protectant activity as well as something that’s locally systemic — getting inside the tissue in some way to either kill an infection that occurred a few days back or to kill an infection that’s going to happen two days in the future.”

The Orondis line includes three products targeting potato: Orondis Ultra, Orondis Opti and Orondis Gold. Orondis Ultra (active ingredients oxathiapiprolin and mandipropamid) and Orondis Opti (oxathiapiprolin and chlorothalonil), are both premixed foliar products, while Orondis Gold, which contains oxathiapiprolin and mefenoxam, is a soil-applied product.

“By mixing different modes of action, we can minimize the risk of resistance developments to these modes of action,” said Marine Mares, fungicide product lead for Syngenta. “We call it built-in resistance management.”

A barren potato field stricken with blight

An untreated potato crop.

Changes to the regulatory environment are part of the innovation process that is always at play in developing Syngenta products, said Jess Holcomb, technical product lead.

“We are a science-based company, so we are constantly looking for new active ingredients throughout multiple different pathosystems, including oomycetes, which include late blight,” Holcomb said. “When we look at the products available in our pipeline, current regulations will be something we look at to determine which products to bring forward.”

SCHEDULE OF PROTECTION

Specialty fungicides are part of the broad-spectrum treatment growers apply to both early and late blight, Gevens said.

For early blight, Wisconsin growers typically spray weekly during production, alternating between chlorothalonil or mancozeb alone and a treatment including a specialty fungicide.

“While late blight gets a lot of press and it’s a very destructive disease and an interesting pathogen, it really is early blight that costs our growers the most on an annual basis, because of the routine weekly treatments that management requires,” she said.

Fungicide application to control early blight usually begins when the plant canopy closes in early July, she said. Growers use a growth model to help determine the spraying schedule for foliar fungicides.

“Then we have many choices. It’s a good problem to have,” she said. “We have many fungicides with good activity in limiting early blight in potato. “Over the last few years, we’ve had a lot of new registrations in this space of potato early blight fungicides. It’s just a matter of timing where they fit into a weekly spray program.”

Gevens described early blight foliar treatment programs as “a very active and hot space” for agrichemical companies and growers alike.

“We’re always sort of in this race of resistance. It’s a resistance management treadmill,” she said. “You use a product, it works really well for 5-10 years, and then it’s not as good as it once was and we’re looking for the next thing that’s a little bit better.”

While some older products are producing improved resistance control, “we’re also seeing a lot of activity in adding or blending biologicals or biopesticides or natural products with some of the conventional fungicides. That’s really taking root now,” Gevens said. “In my career of almost 20 years, biologicals are often explored, but they don’t work as well as the conventionals, and people tend to walk away from them. But I think we’re at a time now where we have to take a fresh look at them.

“They may not work as well as another conventional treatment, but when that conventional treatment goes away or we’re limited in the amount of use, we start to look at these other materials again with a fresh set of eyes.”

Late blight, more sporadic in Wisconsin, is fought with the help of a disease forecasting tool based on rainfall, relative humidity and temperature. Blitecast monitors for risk of late blight, typically highest in mid to late July, Gevens said.

“All commercial varieties can get late blight, but there are a few varieties that have very high levels of resistance,” she said. “Once Blitecast indicates risk, growers will select a fungicide.”

While base protectants work on both early and late blight, “most of (specialty treatments) are unique to late blight and they don’t do anything for early blight,” Gevens said. “It does complicate the spray program.”

Specialty treatments for late blight will continue in every spray if weather is favorable, Gevens said. If moisture decreases and temperatures rise, spraying will occur every other week.

Late blight can be transferred in seed potatoes and can appear in late season, Gevens said. If that happens, there are seed-applied fungicides that can limit piece-to-piece spread.

While Gevens doesn’t foresee any potential regulation changes to such specialty fungicides in the foreseeable future, “ultimately, all pesticides will have a review,” she said. “But these newer, reduced-risk fungicides, their toxicity levels

are so low, especially for fungicides. They have very targeted effects (and) few unintended effects, which makes them great for environmental placement but also makes them at risk for resistance development.”

Gevens has also been working, both in her lab and with growers, on disease prevention technologies such as sprayable gene silencing and hyperspectral imaging to prevent potential pathogens before treatment becomes necessary.

“The earlier we can get at that, the healthier the plant can remain across the season, without so much chemical dependence,” she said.