Vegetable Crop Updates, Late Blight

Amanda Gevens, Professor & Extension Specialist, Plant Pathology Department, University of Wisconsin-Madison

July 29, 2021. Potato late blight has been confirmed in a commercial production field in Portage County, Wisconsin, just north of Bancroft.  The disease was present on lower canopy foliage in an isolated section of the field and the pathogen was sporulating.  The grower has been managing this field with appropriate fungicides, including the phenylamide fungicide mefenoxam (ie.  Ridomil).  An antisporulant fungicide, likely Previcur Flex, will be applied to the field yet today to curb sporulation and potential for spread.  The disease was identified in the field by an observant crop scout and a sample was submitted to my UW-Madison Potato & Vegetable Pathology Research and Extension Lab where we confirmed that it was Phytophthora infestans through multiple approaches, and we identified the clonal lineage (or strain) type as US-23.  Thanks to John Hammel and Monica Yu Chen from my lab for quickly addressing this sample.

US-23 has been the predominant clonal lineage in the US in recent years and most of the pathogen population in this lineage has been sensitive to the phenylamide fungicides including mefenoxam (ie:  Ridomil) and metalaxyl (ie:  Metastar).  However, there have been isolates with insensitivity or resistance which means that these fungicides are ineffective in managing late blight.  We will follow up with a fungicide resistance screen for this pathogen population in the lab.  We presume that this late blight could be controlled with mefenoxam and metalaxyl, and that this disease had more to do with coverage and highly conducive weather than pathogen fungicide resistance.

With last night’s storms and those anticipated for this evening, it is critical that potatoes are protected.  Fungicide selections with systemic and antisporulant activity are recommended at this time.  Registered fungicides for late blight control in Wisconsin potato (with modes of activity):

Information on the late blight pathogen and disease forecasting in Wisconsin:

Late blight testing in my lab or with the UW Plant Disease Diagnostic Clinic ( is free of charge.  Please contact me at or 608-575-3029 or the Clinic at or 608-262-2863 as needed.

UW-Madison Division of Extension Vegetable Crop Update 16

UW-Madison Division of Extension Vegetable Crop Update 15

UW-Madison Division of Extension Vegetable Crop Update 14

UW-Madison Division of Extension Vegetable Crop Update 13

Wisconsin Vegetable Crop Update blog site where you’ll find updates throughout the year.