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Late blight confirmed in Alberta

Specific steps needed to prevent carryover of disease to future crops

Late blight is one of the most serious diseases of potatoes and tomatoes worldwide, resulting in significant yield and quality losses annually. In general, in Alberta, late blight occurs infrequently, but can have devastating impacts in the years when it reaches epidemic levels.

“In the summer of 2013, late blight was confirmed in parts of Southern Alberta and spread eastward through that region,” says Robert Spencer, commercial horticulture specialist, Ag-Info Centre, Stettler. “In 2014, with the intent of returning to late blight-free status, increased awareness efforts were instituted in both the industry and the public. Everyone was encouraged to increase early and season long monitoring of fields, gardens and greenhouses in order to protect crops from late blight, as well as have a quicker response to any perceived infections.”

However, as the 2014 season starts to head towards the finish line, late blight has been confirmed in parts of Alberta. “Therefore, it’s recommended that all growers of potato or tomato (commercial or home garden) take specific steps to prevent the carryover of disease into future years, as a number of valuable crop industries in Alberta could be impacted by repeated outbreaks.”

In the late parts of the growing season, growers should ensure plants die down quickly by using top-killing treatments such as chemical desiccant (diquat) or mechanical treatments.

“At the end of the growing season, gardeners should dispose of all above-ground plant materials (stems and foliage), whether infected or not, either by burial, freezing or composting,” says Spencer. “The purpose is to ensure that living tissues do not survive the winter and will break down completely, thus preventing carryover of the late blight pathogen. Avoid placing infected materials in uncovered compost piles as spores may be produced and spread the disease to nearby plantings of susceptible crops. Piles may be covered with a tarp until the materials have frozen and are completely dead.”

Since tubers represent the primary method of disease carryover in potatoes in Alberta, every effort should be made to prevent the survival of infected tubers. “Recognize that some of the recently prevalent strains of the late blight pathogen are more aggressive on tubers,” explains Spencer. “Carefully grade and sort harvested potato tubers in an effort to remove any infected tubers. Commercial seed growers should be prepared to further grade seed tubers in the spring, and mancozeb-based seed treatments should be applied to try and protect developing crops from seed-borne late blight.”

Culled tubers should be disposed of in such a way as to encourage them to breakdown over winter. “Culled tubers can be fed to livestock or may be chopped, incorporated and buried, or can be placed in covered piles until they freeze completely. Ensure that potatoes do not volunteer (grow in another crop).”

The late blight pathogen normally cannot survive away from living tissues. While the disease can survive for a time on tomato fruit, spores will not carry over on tomato seed. The disease can be introduced on living tomato transplants that are brought in from areas where late blight survives the winter.

“In Alberta, the late blight pathogen does not survive or overwinter in the soil, so growers should not worry about re-infection by planting in or adjacent to a field where late blight has occurred, provided there are no surviving tubers that could reintroduce the disease through infected volunteer plants,” says Spencer. “However, rotating between locations is always recommended, whenever possible, to prevent the build-up of other diseases.”

All growers should take the time to assess the past growing season and the level of risk of late blight infection or re-infection that they will face for the next growing season. “Determine where disease might have come from and put preventative measures in place to protect against infection,” says Spencer. “It is in EVERYONE’S best interest to manage late blight, as this is a community disease. It is also critical that everyone take an active role in submitting suspect material to improve detection and management.”

Click here to read Alberta Agriculture and Rural Development’s FAQ on late blight of potatoes and tomatoes.

For more information on identifying or dealing with late blight, or to submit a sample for testing, contact the Alberta Ag-Info Centre at 310-FARM (3276).

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