Gardeners should be on the lookout for late blight

Infection The disease can be carried by wind, but 
infected material from last year can be a local source

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In 2010, the introduction of the late blight pathogen into Alberta combined with wet weather in many areas resulted in an outbreak of late blight across much of southern and central Alberta. This outbreak included some commercial potato fields and market gardens and many urban residential gardens and plantings. In 2010, disease samples fell into one of either two strains — US-11 and US-23.

In 2011, due to a great deal of effort to clean up potato seed stock prior to the season, as well as increased awareness, monitoring and management practices in all industry sectors, late blight disease levels were greatly reduced. A few positive samples were detected in home gardens and commercial locations ranging from the Peace region to southern Alberta. Response was rapid, and management practices were quickly implemented. In 2011, US-23 strains were not observed and all samples fell into either US-11 or US-24 strains.

“Without knowing what the 2012 growing season is going to be like, it is recommended that ALL growers of potatoes and tomatoes (field or greenhouse, commercial or private/public) be extra vigilant to try and catch any diseased material early on, before a significant outbreak can occur,” says Robert Spencer, commercial horticulture specialist with Alberta Agriculture and Rural Development.

“In the early season, growers and gardeners should watch for tomato transplants and newly emerged potato shoots with water-soaked leaf lesions. Plants may later develop lesions, if the disease is introduced and if conditions are suitable. Water-soaked lesions can grow and spread rapidly, with lesions not contained by leaf veins. In wet conditions, a fluffy growth may develop on the underside of leaves on the margins of lesions. 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 that are often first found around the eyes. Tomato fruit and potato tuber rot can penetrate deeply into the tissues and have a reddish-brown colour.”

Sources of infection

On the Prairies, late blight does not form an overwintering spore type. Rather, the pathogen overwinters on living tissues and the disease is carried forward from one season to another on infected seed potatoes, cull piles and volunteer potatoes. As the season progresses, the risk of late blight being introduced and then developing in crops will increase if late blight has been found/reported in the region, as spores can travel up to 100 km on storm fronts. Wet and humid conditions and moderate temperatures are favourable for disease development. Prolonged periods of leaf wetness caused by dew, rainfall, or overhead sprinkler irrigation also favour disease development and spread.

“If you find plants showing suspicious lesions, it is recommended that you dispose of infected material as quickly as possible, removing diseased parts (small scale) or killing out plants so disease cannot develop further,” says Spencer. “Bury or bag up infected plant material, as spores will continue to be produced as the tissues die, infecting adjacent living tissues. In some cases, the application of protective fungicides can be made if conditions favour disease and if disease is known to be present in the province. Home gardeners should consult local suppliers (garden centres, etc.) for available, registered products.”

If you have suspect plants, you should consult a local horticulturalist for assistance. Commercial operations (potato growers, market gardeners, greenhouse/garden centres) can contact 310-FARM (3276) to determine if further testing is required and to discuss management. Do not hesitate to report an incidence, as early awareness will help to prevent and contain an outbreak and can help others to protect their crop.

“Late blight is a community disease, with the potential to affect many industries and individuals,” says Spencer.

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