Late blight (phytophthora infestans) was a major culprit of famines in Western Europe around the mid 1800’s
Infected potato tubers are the primary source of inoculum. That includes potatoes in storage, infected tubers missed during harvest that remain unfrozen during the winter (volunteers), seed tubers and infected waste heaps along with infections upon other host plants.
Blight is favoured by free moisture and cool to moderate temperatures. Night temperatures of 50 to 60 F and day temperatures of 60 to 70 F are most favourable for disease development. Free water from rain, dew and overhead irrigation all provide the water necessary for pathogen infection and development.
Spores develop in three to five days and require 12 hours of free moisture for infection to occur. Lesions on leaves and stems become visible as small flecks within a few days after infection.
So with the current heat wave one could easily be forgiven for thinking “no risk of blight this year!”
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