Wisconsin: Late Blight Risk Threshold Exceeded — Growers Advised to Apply Preventative Fungicides

United States, Wisconsin. Across most of Wisconsin’s potato-growing areas, accumulated Disease Severity Values (DSVs) have reached or passed the action threshold of 18. That means weather favourable to late blight has occurred since roughly 50% crop emergence, and fields now warrant preventative fungicide protection. The alert comes from the weekly Vegetable Crop Update newsletter published by the University of Wisconsin–Madison on 12 July 2026.

What the model tracks

The risk values are generated from the VDIFN platform (agweather.cals.wisc.edu) and published by the vegetable pathology lab led by Prof. Amanda Gevens. Two indicators drive the recommendations:

• DSV ≥ 18 — the late blight (Phytophthora infestans) action threshold, triggering a preventative fungicide application;
• P-Day ≥ 300 — the threshold for early blight (Alternaria solani).

Accumulation of both values starts from the date when roughly half the plants in a field have emerged.

Where the risk stands

As of 11 July, the 18-DSV threshold had been passed almost statewide — the Arlington area was the exception, still sitting at 12. Weekly gains ranged from 0 to 7 DSVs.

Location	DSVs (as of 11 Jul 2026)	P-Days (as of 11 Jul 2026)
Grand Marsh	26	306–470
Spring Green	23–25	332–499
Hancock	19–21	298–466
Plover	18–21	300–465
Antigo	12–21	181–340
Rhinelander	10–18	143–275
Arlington	12	321–493

The highest readings were recorded at Grand Marsh (26 DSVs across all planting dates) and Spring Green (up to 25 DSVs on early plantings). At Rhinelander only the earliest fields have reached the threshold so far.

What specialists recommend

Every field that has accumulated 18 or more DSVs should receive a preventative fungicide application against late blight. An updated list of products registered in Wisconsin for the 2026 season is available on the lab’s website.

At the same time, nearly all areas south of Antigo have passed the 300 P-Day threshold for early blight and require preventative treatment as well. Over the past week, regions of the state accumulated between 57 and 64 P-Days. The earliest plantings around Antigo (emergence in late May or early June) are now at or above threshold.

Weather background

The past week ran 4–8 °F above normal statewide, with heat indices topping 100 °F on some days. Rainfall was highly uneven: several south-western counties received 3–6 inches, while north-central and south-eastern districts stayed dry. The mid-to-late July outlook calls for above-normal temperatures and below-normal precipitation across the state, with minimal rain chances over the next seven days.

Hot, dry weather lowers the odds of new infections, but the risk already banked in the models keeps preventative sprays justified — especially in fields with a disease history, a dense canopy and frequent irrigation.

Also in this issue

The newsletter flags two further concerns for the state’s potato growers:

• Potato virus Y. Between 5 and 12 July, colonising aphids (green peach aphid and potato aphid) were again detected in central Wisconsin fields and in seed potato fields in the north of the state. Specialists advise starting insecticide applications once colonising aphids are confirmed established in the crop: a full-field foliar application, a repeat with the same active ingredient one week later, then rotation to a different mode of action.
• Hail. Several operations reported hail damage. Defoliation below 30% generally causes minimal yield loss; 30–70% brings a measurable reduction; above 70% is critical, particularly where stems are also injured. Tuber initiation and early bulking are the most vulnerable stages.

---

Source: Vegetable Crop Update, UW–Madison Vegetable Pathology, 12 July 2026 https://vegpath.plantpath.wisc.edu/2026/07/12/vegetable-crop-update-jul-5-2026/

Lead photo: Potato field heavily infected by P. infestans.JPG, Wikimedia Commons — https://commons.wikimedia.org/wiki/File:Potato_field_heavily_infected_by_P._infestans.JPG (check the licence line on the file page and credit accordingly)