Late blight a concern again this year

There were multiple outbreaks of aggressive and destructive pathogen in fields and market gardens last year

Over the last few years, there has been a great deal of concern in Alberta surrounding late blight, a serious disease that affects mainly potatoes and tomatoes. This year appears to be no different.

“This disease is caused by a fungal pathogen called Phytophthora infestans,” said Robert Spencer, commercial horticulture specialist at the Alberta Ag-Info Centre in Stettler.

“The favourable conditions for disease development, combined with the presence of the pathogen, have resulted in multiple outbreaks of late blight in commercial, market garden and urban potato and tomato crops throughout parts of Alberta. For 2014, this disease continues to be a risk for all solanaceous crops (potato/tomato family) grown in Alberta.”

Late blight is found in most potato- and vegetable-growing areas of Canada, although does not occur every year on the Prairies. It can also affect eggplants, peppers, petunias and some related solanaceous weeds, such as nightshade and wild tomato. Late blight is an aggressive disease that, if left unchecked, can cause significant and rapid crop losses in fields, greenhouses, and potato bins.

The risk of introduction comes from either infected transplant material (tomatoes or other host crops) or infected seed potato stock (either imported or carried over). During the season, if spore loads build up, there is a risk of introduction of the pathogen via windblown transfer.

“Initial symptoms of late blight are typically noted on older leaves, appearing as dark, water-soaked areas (lesions), sometimes with yellow edges, that move in from leaf tips/margins, becoming brown and brittle within a couple days,” said Spencer. “Late blight lesions are not contained by the leaf veins. Lesions may also develop on plant stems and on potato tubers and tomato fruit. Late blight develops most quickly in warm, wet/humid conditions and can spread very rapidly through plantings. Plants may be rapidly defoliated, die and yields can be significantly reduced.”

Potato tubers may be infected by spores produced on the foliage which are subsequently washed into the soil. Infected tubers may have irregular, sunken lesions (often first found around the eyes) and causes rot and a reddish-brown discolouration of the internal tissues.

On the Prairies, late blight does not form an overwintering spore, but overwinters on living tissues, such as infected seed potatoes, cull piles, volunteer potatoes or tomato transplants.

“In early spring and into summer, the priority is for regular scouting and monitoring of emerging plants, and new plant material,” said Spencer. “Early detection is critical for minimizing the impact of the disease and preventing further spread and significant outbreaks.”

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