Late blight, the disease that caused the Irish Potato Famine of the 1840s, is making a resurgence worldwide.
Michael Celetti, plant disease specialist with Saskatchewan Agriculture, said leaves, stems, tomato fruit or potato tubers rot quickly after infection by late blight. It spreads rapidly over great distances, wiping out unprotected potato fields.
“In the early 1980s, a new strain and mating type of late blight was found in the United States. It migrated into Canada and, by 1995, had replaced the old strain in the few infected plants surveyed in a study of Saskatchewan potatoes,” he said.
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A fungicide containing metalaxyl can control late blight, but tests reveal the new strain needs higher doses of this fungicide. Celetti said other fungicides containing different active ingredients are still effective against the new strain.
Dry weather last summer protected many prairie potato growers.
“I don’t think all growers realize how devastating this disease can be or how difficult it is to control once the disease becomes established in a field. Furthermore, it’s a community disease, so vigilant disease management strategies require participation by all potato growers, including home gardeners,” Celetti said.
To prevent the disease: Plant certified disease-free seed; deep-bury cull piles before plants emerge; and plant on land that has not grown potatoes for at least three to four years.
When plants begin to fill the row, apply a registered fungicide to the lower leaves where the symptoms first appear.
Celetti said fungicides should be applied before the disease strikes because effective control of late blight is very difficult after the disease has struck.
“Spraying with a fungicide should continue every seven to 10 days, depending on weather conditions. If the season becomes dry, the spray intervals could be lengthened. With new, more aggressive strains immigrating into the province, however, growers are advised to be careful when stretching the spray intervals.”
Gardeners must also be vigilant. Prevention means removing and burning or deeply burying all potato and tomato plant debris. Tomato and potato plant debris should not be composted because it could be a source of the disease.
Don’t plant potatoes from last year’s crop. Use certified disease-free seed.
There is no potato variety immune to the disease, but some varieties are more resistant, such as Chiefton, Brador and Nooksack. Other precautions include: Planting potatoes and tomatoes in a different part of the garden than last year’s crop; avoiding over-fertilization; and attempting to water the ground, not the leaves.
Regularly inspect for the first indication of late blight – dark brown spots on the plant’s stems and lower leaves.
