Features Fruit Production
Late blight of potatoes, tomatoes In Alberta

September 14, 2010 – Late blight has been reported in many areas across
southern and central Alberta, with indications that it has been found
in some commercial potato fields.

September 14, 2010 – Late blight has been reported in many areas across southern and central Alberta, with indications that it has been found in some commercial potato fields.

“The largest number of reports has been from urban residential plantings of potatoes and tomatoes,” says Robert Spencer, commercial horticulture specialist with Alberta Agriculture and Rural Development. “Due to late identification, limited control options and excellent conditions for disease development, the disease has been largely unchecked in residential locations and has spread rapidly. There is the potential for continued spread of the disease, due to air-borne spores that are being produced. There is also a risk of spread into greenhouse tomato operations.”

Late blight is a serious plant disease caused by the fungus Phytophthora infestans and is found in most potato and vegetable-growing areas of Canada, although it does not occur every year on the Prairies. It affects potatoes, tomatoes, eggplants and related weeds. Late blight is an aggressive disease that, if left unchecked, can cause significant and rapid crop losses, both in the field and in storage.

“Initial symptoms are typically noted on older leaves, appearing as dark, water-soaked areas (lesions) that move in from leaf tips/margins, becoming brown and brittle within a couple of days,” says Spencer. “Late blight lesions are not contained by the leaf veins as they are with another common foliar disease called early blight. Lesions may also develop on plant stems and on potato tubers and tomato fruit.”

Late blight develops quickest in warm and wet/humid conditions and can spread very rapidly through a planting. Plants may be rapidly defoliated and die. Potato tubers can be infected by spores on the foliage. Infected tubers may have irregular, sunken lesions that are often around the eyes with the rot penetrating deeply into the potato. The rot has a reddish-brown colour and the disease can spread from diseased to healthy tubers in storage.

“On the Prairies, late blight does not form an overwintering spore type,” says Spencer. “Instead, 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. In-season spread is by spores produced on infected tissues and diseased crop debris. Spores can move considerable distances on the wind or will move within the fields by rain or water splash.”

Late blight can be managed in commercial fields by using protective fungicidal sprays applied when conditions favour disease development.

If infected crop debris is composted, it should be covered with a tarp or soil until it has frozen to minimize spore distribution. Killing potato tops can help to minimize tuber infection, as this encourages tuber skin set and stops top growth. Tubers can be harvested a couple of weeks after the tops are killed.

“Only harvest from healthy stands, do not keep any infected plant material, and tubers should be heavily graded before storage in an attempt to prevent entry of the disease into storage. Dispose of infected tubers appropriately,” notes Spencer. “It’s always recommended that growers and gardeners purchase clean, certified seed potatoes each season, rather than keeping their own tubers over for seed.”

Once the tops of plants are dead or have been removed, the potential for continued infection is reduced or eliminated

“As the season progresses into fall, the risk of late blight infection should decrease dramatically,” adds Spencer. “Potato and tomato vines will stop growing and begin to dieback naturally, thus reducing the amount of living tissue available for infection and reproduction of the pathogen.”

By carefully managing any infected plant debris, people can help to prevent late blight from overwintering and avoid a potential repeat infection of susceptible crops and weeds in future years.