Late blight has been confirmed in a potato field near Alliston, Ontario, just days after the US23 strain was identified in Michigan state. To some extent, the finding was a surprise since spore traps in the Alliston area did not catch spores.

“A couple of days ago, on August 14, I collected a sample from a potato field near Alliston that looked suspiciously like late blight, but there was negligible white, fuzzy growth on the underside of the leaves and almost no stem lesions,” says Eugenia Banks, consultant to the Ontario Potato Board.  “I thought that this might be the ‘new’ blight Phytophthora nicotianae that does not sporulate very much, but the lab results are back and it is late blight.”

Banks speculates that the spate of hot weather probably restricted sporulation. And that’s why the spore traps did not detect the disease. A sample has been sent to Dr. Larry Kawchuk, Agriculture and Agri-Food Canada, Lethbridge, Alberta to identify the late blight strain. 

Until results are known, growers should note that the US 23 strain of potato blight is still susceptible to the fungicide Ridomil.  This crop protection product plus Kocide is a good tank mix for fields that will not be topkilled soon.