A Newsletter for Commercial Potato and Vegetable Growers Prepared by the University of Wisconsin Madison Vegetable Research and Extension Specialists
Hello,
Happy New Year! This newsletter focuses on the recent discontinuation of Blocker production and implications and strategies for disease management in potato. Please read the complete article for details and disease decision making support.
In mid-December 2025, AMVAC Chemical Corporation, the manufacturers of the fungicide Blocker (pentachloronitrobenzene, PCNB), decided to stop producing Blocker 4F and 10G. In effect, this concludes production of Blocker for the future. The Blocker that is on the shelves at this time is all that will be available for future use. It is estimated that this will leave potato growers with a supply that may satisfy only 25-33% of the potato use demand in 2026.
Potato growers in Wisconsin have relied upon Blocker for strong common scab and Rhizoctonia control and will face challenges in the 2026 growing season in planning for disease management. Additional vegetable crop production uses of Blocker include crucifer (cabbage, broccoli) Rhizoctonia and Clubroot management.
While the US Environmental Protection Agency has, for several years, proposed canceling registrations of Blocker due to ecological and human health risk concerns, this stopping of manufacturing of Blocker was a decision of AMVAC. There is no change to the registration of Blocker at this time, but access to this fungicide will be extremely limited in 2026 and non-existent beyond this year.
I hope to see many of you at the upcoming education conferences.
WPVGA & UWEX Grower Education Conference:
https://wisconsinpotatoes.com/events/2026-grower-education-conference-industry-show/
Wisconsin Fresh Market Fruit and Vegetable Growers Conference:
https://www.freshfruitvegetable.org/
Best regards,
Amanda
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Amanda Gevens
Associate Chair, Professor & Extension Specialist
Administrative Director of the Wisconsin Seed Potato Certification Program
Fritz Friday Chair for Vegetable Production Research
Plant Pathology Department
1630 Linden Drive Room
University of Wisconsin-Madison
Madison, WI 53706
gevens@wisc.edu/608-575-3029
Potato & Vegetable Pathology Research and Extension
https://vegpath.plantpath.wisc.edu/
