Oct 17, 2019
UW-Madison pathologist Gevens zeroed in on both early and late blight

In the Dairy State, it’s easy for a humble vegetable like the potato to get lost amid all the talk of milk and cheese. Consequently, it’s a little-known fact that Wisconsin is the third-largest tuber producer in the country, just behind Idaho and Washington. The state’s growers plant around 70,000 acres of spuds each year, part of a $340.8 million industry, so they stand to lose a great deal should anything go wrong with the crop. And plenty could go wrong.

Plant diseases such as late blight present a serious threat to potatoes, and a timely response to an outbreak can sometimes mean the difference between boom and bust. Fortunately for potato farmers, Amanda Gevens has their back.

As associate professor and chair of plant pathology, Gevens spends much of her time studying and tracking potentially devastating diseases in potatoes and other vegetable crops. She’s also a potato and vegetable pathologist with the UW-Madison Division of Extension, so she’s in constant communication with growers throughout the state, keeping them apprised of the presence of diseases and helping them make the best decisions in protecting their crops.

UW-Madison’s College of Agricultural & Life Sciences (CALS) asked Gevens about some of the pathogens she studies, how her research informs management recommendations for growers, and the methods she uses to get sound advice in the hands of those who need it most.

What is the main pathogen you study, and how does it affect potatoes and vegetable crops?

Phytophthora is a genus of a group of highly aggressive plant pathogens that are referred to as water molds, or oomycetes. Most plant pathogens, particularly those that affect agricultural crops, are fungi. But we have a few unusual (nonfungal) pathogens that affect crops, and this group of water molds creates substantial damage. The Greek underpinning of the name is “plant destroyer.” One of the most infamous Phytophthora pathogens causes late blight, which played a big role in the Irish potato famine.

How great is the damage it can cause?

Globally, it’s $5 billion each year, and it poses a significant threat to food security. Here in Wisconsin, Phytophthora costs us about $16 million a year. That’s sort of a five-year rolling average that includes crop losses in both potato and tomato as well as additional fungicides required to limit the disease.

Through your research, what have you discovered about Phytophthora’s resistance to fungicides? 

We monitor a particular water mold called Phytophthora infestans in this state, both in potatoes and tomatoes, and we essentially build a culture collection. With all of these samples, which we term “isolates,” we’re closely examining the population. One of the things we do is assess the isolates for resistance to one of the most potentially effective fungicides, which is called mefenoxam or, its trade name, Ridomil. This fungicide is a tried-and-true, strong manager of the disease when it works. But it’s important to know when resistance is present, because when it is, it’s just like applying expensive water to the crop. Fortunately, there are several other fungicides that can be recommended when resistance is identified.

I also partner with the UW Plant Disease Diagnostics Clinic at CALS to offer free late blight diagnostics to all gardeners and growers in the state. Beyond the diagnostics, my lab conducts “strain typing,” which tells us what types of the late blight pathogen, or pathotypes, we’re dealing with. We also offer this service on a national scale. So the recommendations are state-specific, but the information that comes out is used regionally and nationally as we learn about the pathotypes that are here. All of this informs our recommendations for disease management both in the near and long term.

What are some of these recommendations?

The first recommendation is always start with clean stock. Late blight is monitored and tracked by the Wisconsin Seed Potato Certification Program, which is a part of my department. The program is headquartered in Antigo, with additional offices in Rhinelander and Madison. It’s important to note that certified seed potatoes meet the bar of containing no late blight pathogen. Starting with these seeds is a critical first step.

For tomatoes, it means generating transplants locally, where the pathogen is not known to overwinter and persist. Tomato seed doesn’t harbor the late blight pathogen, so typically it’s coming in on transplants from other places.

For both potato and tomato growers, we offer a disease forecasting tool through our real-time interactions with growers. It uses an algorithm based on the past four decades of research on environmental parameters — temperature, precipitation, etc. — to provide a risk rating for late blight on a daily basis. This is called a disease severity value, or DSV. Each day, you get up to four DSVs. When these accumulate up to a certain number, that’s when growers know they have to start utilizing preventative fungicides to limit late blight in potatoes because most commercial varieties do not have substantial resistance to late blight. The tool is based on weather conditions within production potato plant canopies in multiple regions of the state.

We’re always thinking about what we would term “pathogen ecology” — how these pathogens move, persist, and act within their environment. That’s all part of what we’ve done to help build better management recommendations for growers.

What happens if you have a crop that is experiencing a late blight and the pathogen has fungicide resistance?

When there is a first detection of late blight made in the state, within a day — usually within hours — we will identify what the pathotype is and whether or not it’s resistant to mefenoxam. But that’s not the only fungicide, right? We have about a dozen others that work well against late blight, so there are other options. That information helps growers make better prescriptive decisions.

Typically, if there’s a hot spot for late blight identified in a field, they’ll kill the plants there and then very aggressively manage the disease in the rest of that field, and all the surrounding fields will get placed on a more stringent fungicide program. So they’ll be spraying more frequently, and they’ll be using fungicide selections that have been more active against late blight if infection has occurred.

How do you ensure your recommendations get in the hands of those who need them?

We have multiple mechanisms. One of the most effective is the phone. When there is a first detection, I often spend several hours, if not days, on the phone talking to growers, crop consultants, and industry partners that are affected by the disease finding.

I also manage and write the weekly Vegetable Crop Update newsletter for the UW Division of Extension. Through this mechanism, growers can see those DSVs accumulate and know of risk that’s coming. But if a disease is detected between issues, I also put out supplements and provide updates once we know the identity of the pathogen. We also have a Facebook page and a Twitter account for program presence, but in terms of real-time information sharing for decision-making, it’s by phone, text, or the newsletter.

Yours is primarily a research and extension appointment, but what about teaching?

In all the work that I do in support of potato and vegetable production in Wisconsin and beyond, I aim to use the research questions as real-world case scenarios for graduate student training. Mentoring students and staff is tremendously rewarding and furthers good, useful studies while passing on critical research and outreach skills to the next generation of highly capable and much-needed agricultural scientists and practitioners. This is what ensures food security and economic stability.

What other diseases do you spend time tracking?

Another pathogen that I’m very interested in is Alternaria solani; and unlike Phytophthora, it’s a true fungus. Almost every plant has an Alternaria species that affects it. Alternaria solani creates a disease called early blight on potato and tomato, which is something we see every year.

Late blight, even though it often gets more press and it can create much bigger losses economically, it’s what we would term sporadic. Not every field in Wisconsin is going to see late blight every year. In fact, the number of fields that have actually gotten it could be in the handful. But it’s the economic risk that is so great.

With early blight, nearly every potato field sees it every year in Wisconsin. It starts up about the first of July in the potato crops, and it continues until the potato crops senesce, or break down at the end of the season, either naturally or with use of senescence treatments referred to as desiccants.

Have you studied fungicide resistance in Alternaria as well?

Yes. For Alternaria, there are over 40 fungicides registered for use in Wisconsin potato and vegetable crops. To date, we’ve focused our studies on just one group of fungicides, the Quinone outside inhibitors. Within this group, the fungicide azoxystrobin is most common and has been relied upon for over two decades for control of potato diseases.  When we know that the population in a certain area or a specific field has resistance, then we tell that grower, and they’ll change usage of that fungicide to something else with greater activity, just as we do with Phytophthora. This work has led to other discoveries such as improved understanding of pathogen movement and introduction.

Learn more about the Department of Plant Pathology’s disease forecasting tool and disease severity values.

— Article by Nik Hawkins, UW-Madison CALS

— Top photo by Michael P. King, UW-Madison CALS






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