Content by Amanda Gevens

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 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 group 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.

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