As spring approaches, farmers across the state prepare for potato season — the planting process begins after the last hard frost in April and May, according to Gather Wisconsin. A part of this preparation relates to the prevention of disease, specifically for late blight — also referred to as “blight.”
Affecting tomatoes and potatoes, this disease is caused by a fungus-like water mold called Phytophthora infestans, according to Wisconsin Horticulture. If the conditions are right, blight may kill a plant within ten days, effectively threatening the vitality of Wisconsin farms. To contextualize blight’s potential impact, this disease is historically known for its destructive role in the Irish Potato Famine.
For Wisconsin farmers working at an industry scale, blight is dealt with through measures like practicing crop rotation, maintaining soil hygiene and selecting high-quality seed potatoes, according to The James Hutton Institute. Lastly, farmers may prevent blight by spraying their fields with fungicides, but if overdone, this practice has its drawbacks, University of Wisconsin professor of plant pathology Andrew Bent said. To prevent blight and overspraying, professor and Department of Plant Pathology Chair Amanda Gevens uses a tool called Blitecast to communicate to farmers the appropriate time to spray fungicides.
“Almost all of the commercial potato producers use Blitecast in Wisconsin, whether it’s a direct connection to me [or not],” Gevens said.
Blitecast is a model that calculates disease severity values — which are risk values — based on the temperature and the amount of moisture in the air, Gevens said. The combination of optimal temperatures and over 90% humidity results in a certain number of DSVs per day. These daily DSVs are accumulated over time. If this sum reaches the threshold of 18, then there is a pressing risk for blight. Based on past research, the DSV of 18 — a value that is specific to Wisconsin’s environmental region — indicates farmers should take preventative action against blight.
Blitecast plays a crucial role in understanding the weather aspect of the disease triangle, which is a plant pathology model, Bent said. This model helps farmers determine when their crops may be susceptible to pathogens. It consists of three components — a susceptible plant, a pathogen and a favorable environment conditions for infection, according to Wisconsin Horticulture. Unless all three of these conditions are met, the risk for infection is low, Bent said. For most pathogens of plants — blight included — cooler and wetter conditions are favorable, hence why Blitecast monitors environmental determinants.
“In plant pathology, we learn more about the way that pathogens work in plants, so we can sort of outguess them — we can be smarter than the pathogen,” Gevens said. “Then, we can use that information to improve our management of the disease.”
Considering Wisconsin was the third biggest potato producer in the county in 2023, ensuring these farms are safe from disease is important to the state’s economy, according to the 2024 Wisconsin Agricultural Statistics report. The state’s large-scale cultivation of potatoes represents a valuable agricultural commodity — the 2023 potato yield was estimated to value over $400 million, according to the Wisconsin Potato and Vegetable Growers Association.
Due to the size of this industry, farmers grow potatoes in line with commercial expectations, Gevens said. For instance, a farmer may grow varieties — such as red and yellow types or russets — that are desired by the fresh markets. But, these types typically do not have substantial resistance to blight. Furthermore, potatoes in general are more susceptible to pathogens than other vegetables due to the difficulty of potato breeding, Bent said.
In response to the potato’s susceptibility, farmers spray fungicides. But, farmers must be cautious not to overspray their crops, Gevens said. Overspraying should be avoided because it requires an investment in labor, fuel and the cost of the fungicide itself. In this way, Blitecast helps farmers not only protect their crops from disease but also ensures farmers aren’t taking on extra costs.
Considering Blitecast uses data from weather stations all over the state, the information provided to farmers is well-informed and creates accurate readings. Essentially, Blitecast guides Gevens, so she only advises for the spraying of fungicide if it is absolutely necessary.
“[Once] the disease triangle is met … now start using the fungicides,” Bent said. “Until then, there’s no point in spraying all that fungicide … but pesticides are essential to our ability to be feeding humans on this planet.”
Looking forward, one issue potato farmers face relates to the state’s changing climate. In Wisconsin, wetter conditions are expected to become more frequent, according to the Wisconsin Initiative on Climate Change Impacts. This ultimately increases the likelihood for the spread of pathogens. The impact of these weather events may affect potatoes more severely because potatoes lack the genetic resistance to pathogens other crops have, Gevens said.
But, Gevens remains watchful of these changes in weather patterns.
“As we see changes in weather, we are continuing to evaluate our disease forecasting tools and models to make sure they are still effective for managing these critical diseases,” Gevens said. “Longer term, we hope to have resistances within potatoes and vegetables that will give us more durable resistance without the use of fungicides.”
To learn more about Blitecast, visit UW Vegetable Pathology or access Geven’s biweekly newsletter, “Vegetable Crop Update,” that summarizes findings from Blitecast for farmers to read.
