Wisconsin potato growers have relied on Blocker for common scab and Rhizoctonia control
By Amanda Gevens, professor and Extension specialist, University of Wisconsin (UW)-Madison Department of Plant Pathology
In mid-December 2025, AMVAC Chemical Corporation, the manufacturer 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 U.S. Environmental Protection Agency has, for several years, proposed canceling registrations of Blocker due to ecological and human health risk concerns, AMVAC ultimately made the decision to stop manufacturing Blocker.
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.
Blocker provides multi-site interference with fungal metabolism and is an active protectant. The fungicide is a non-systemic, broad biocidal soil treatment and has minimal vapor action.
