The US-23 genotype of Phytophthora infestans (the causal agent of late blight) was detected on commercial potatoes in Adams and Marquette Counties in Wisconsin and in Illinois, confirmed by University of Wisconsin – Madison researchers Dr. Yu Chen and Mr. Shane Hansen. Dr. Amanda Gevens (UW-Madison) reports that detected lesions were classic with brown, water-soaked discoloration on stems and leaves (Figure 1) with sporulation on leaf undersides (Figure 2).
The US-23 genotype is generally still sensitive to mefenoxam/metalaxyl, further testing is being conducted on these recent isolates. Risk-based fungicide recommendations are available for Michigan through the MSU Potato Disease website, and it may also be useful to reference the updated list of fungicides registered for late blight control in Wisconsin.
Dr. Gevens also reports that P. nicotianae has been detected this season, so not to immediately assume late blight, but check for sporulation and consult a specialist or diagnostic clinic.
Figure 1. Potato late blight, water-soaked lesion on top surface of leaf (left). Figure 2. Potato late blight, sporulation on reverse side of affected leaf (right). Photos by Dr. Amanda Gevens, UW-Madison.