Current status of chlorothalonil labels impacting Michigan potato production

Jaime Willbur<willbur1@msu.edu>, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences - March 03, 2026

Updates for the 2026 season regarding chlorothalonil product labels following the EPA Interim Decision.

Composite image showing symptoms of potato late blight. The left shows water-soaked lesions with light green borders on the upper leaf surface. The right shows fuzzy, gray-white growth along lesion margins on the underside of a leaf. — Figure 1. Examples of potato late blight with typical water-soaked lesions on forward (left) and reverse (right) sides of leaves. Light green border sometimes present around lesions. Faint gray-white, fuzzy growth observed at margins of lesions on the underside of leaves, apparent in humid conditions. Photo by Jaime Willbur, MSU.

In Michigan potatoes, chlorothalonil-containing products are a critical backbone in preventative early and late blight fungicide programs. The EPA Interim Decision (December 2024) identified maximum annual rate reductions across many crops and agricultural use sites. In potato, new seasonal use limits have been reduced to 8 pounds of active ingredients per acre per year in “non-vulnerable” soils. However, much of potato production is conducted on sandy soils, therefore this rate will further be reduced to 6.5 pounds of active ingredients per acre per year in “vulnerable” soils. These label changes have been submitted by product registrants for review by the EPA.

Current status of chlorothalonil labels from EPA Office of Pesticide Programs

According to a representative from the Environmental Protection Agency (EPA) Office of Pesticide Programs, “EPA is currently in the process of reviewing chlorothalonil label amendments that incorporate the Interim Decision mitigation measures. Currently, EPA does not have a timeframe for when those labels will be stamped and approved. However, once those labels are approved and per the registration review label amendment approval letters, registrants are able to ‘distribute or sell [the] product under the previously approved labeling for 12 months from the date of [the] letter,’ … once EPA stamps and approves a chlorothalonil label amendment, registrants are able to sell/distribute their previous chlorothalonil product for up to 12 months from that date. Given this information, growers are able to purchase any product currently available to them and use them according to the label instructions on the product.”

Current maximum use rates for these products are 11.25 pounds of active ingredients per acre per year. For the 2026 growing season, Michigan potato growers can expect to use chlorothalonil according to the rates and seasonal limits listed on purchased product labels.

For additional details and information, questions can be directed to Kevin Kern, Michigan Department of Agriculture and Rural Development (MDARD) pesticide registration specialist, at kernk@michigan.gov or 989-239-7637.

Additional background information

After a mandatory 15-year registration review process that began in March 2012, “EPA identified dietary risks of concern and ecological risks of concern for chlorothalonil and identified necessary rate reductions to reduce the amount of chlorothalonil entering drinking water, thus reducing dietary exposure and risk in vulnerable soils.”

“Vulnerable soils” are:

Sand, loamy sand or sandy loam soil, as defined by the U.S. Department of Agriculture’s soil classification system without a restrictive layer that impedes the movement of water through soil.
Less than 2% organic matter content.
Occur where the water table is 30 feet or less from the surface.

Soils that do not meet all three of these criteria are considered “non-vulnerable.”

A horizontal flowchart illustrating a registration review process. Blue boxes connected by arrows read: “Open Docket (Preliminary Work Plan),” “Final Work Plan,” “Issue Data Call-In,” “Draft Risk Assessment,” “Proposed Interim Decision,” “Interim/Final Decision,” and “Risk Mitigation & Label Changes.” Yellow boxes labeled “Comment Period” appear above the Draft Risk Assessment and Proposed Interim Decision steps. — Figure 2. Diagram of the EPA registration review process (Diagram credit: U.S. EPA website). The chlorothalonil review (Case #: 97, Docket #: EPA-HQ-OPP-2011-0840) is at the “Risk Mitigation & Label Changes” phase. More information on this review process can be found https://www.epa.gov/pesticide-reevaluation/registration-review-process.

Information and locations for positive detections of potato and tomato late blight can be found at the PlantAid website (previously USAblight.org) and on the PlantAid Plant Health Map - Late Blight. Fungicide sensitivity varies with Phytophthora infestans genotype so suspect samples should be submitted to help advise management practices. Please refer to the Michigan State University Plant and Pest Diagnostics or PlantAid Sampling Instructions websites for more information on their services and instructions for proper sample submission.

Please visit the Michigan State University Potato and Sugar Beet Pathology website for more information.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).