Diseases of Potatoes: Late Blight

Late blight of potatoes can be an extremely destructive disease of this crop causing severe yield reductions and considerable rotting of the affected crop in storage.

In Alberta, this disease is of sporadic occurrence, generally in the presence of hot or humid summers that occur every ten to fifty years. The only recent extensive province wide outbreak occurred in 1993 in humid areas of the United States. In wetter areas of Canada on the east and west coast late blight disease outbreaks occur annually. The fungus, previously limited in the number of races that decay the potato crop, now occurs in a multitude of strains. Races 8 and 11 are prevalent in Canada, both of which readily attack potato and tomato crops under suitable weather conditions.

Disease control is achieved by one of several effective fungicides and by importing seed potatoes from areas where disease outbreaks areas uncommon, such as central and northern Alberta. Resistance by this fungus to fungicides occurs frequently.

Photo: Evans	Picture description A few brown spots can turn the whole leaf brown overnight	Photo: Evans	Picture description Early leaflet infection
Photo: Evans	Picture description Earliest infection	Photo: Evans	Picture description Late blight can progress extremely fast under warm humid conditions
Photo: Evans	Picture description Underside of leaves show a very "fragile" weft-like fungus visible in early mornings	Photo: Evans	Picture description General fungicides give effective control if applied as soon as the first signs of disease appear
Photo: Evans	Picture description Late blight can penetrate into tubers which when stored carry the disease over from year to year	Photo: Evans	Picture description Light blight infection of tubers. Infected tubers do not overwinter outdoors in Alberta as they do in eastern Canada and B.C.