Reference: Department of Agriculture and Aquaculture Canada
Potato late blight, caused by the fungus Phytophthora infestans, originated in Mexico and spread to Europe and North America around 1845. It triggered devastating epidemics and the Irish Potato Famine, where potatoes were a dietary staple. The disease was first reported in Canada's Maritimes between 1845 and 1847.
Recently, more aggressive lineages have emerged in the Maritimes. These strains infect plants earlier, cause severe damage, and persist through dry periods.
Infection requires three key factors: a source of inoculum, a susceptible host, and favorable environmental conditions.
Late blight overwinters in infected tubers. Sources include stored potatoes, volunteer tubers in gardens or compost that survive freezing, and infected tomatoes discarded in compost. Wind disperses spores up to 80 km, commonly 8-16 km, to new potato or tomato shoots.
Nightshade family members like potatoes, tomatoes, and peppers are vulnerable. While some potato varieties show partial resistance, none are fully immune. Advances in genetic engineering promise more resistant options soon.
Cool, wet weather promotes late blight. Ideal conditions include nighttime temperatures of 10-15°C, daytime 15-20°C, humidity above 90%, and 30 mm+ rainfall over 10 days.
Spores infect wet leaves, stems, or fruits, producing lesions in 4-5 days. Leaves show dark green, water-soaked spots that turn brown with pale green borders. White, fuzzy sporulation appears on leaf undersides. Stems develop dark lesions with white sporulation, often at leaf bases.
Heavy rain washes spores to tubers, causing reddish, granular rot up to 2 cm deep. White spore masses may form on stored tubers in humid conditions.
Tomatoes exhibit similar leaf and stem symptoms, with fruits developing pale brown spots that darken, wrinkle, and typically appear on the upper surface.
Avoid using home-saved or grocery store potatoes as seed; opt for certified, inspected seed potatoes. Discard any with rot before planting.
First infections often hit in early to mid-August. Harvest early by:
Bury or remove old potatoes and tomatoes from compost; eliminate volunteer seedlings.
Promote dry conditions and airflow:
Limit nitrogen fertilizer to avoid lush growth that reduces resistance and delays maturity. Separate tomatoes from potatoes. Hill up potato plants to shield tubers.
For fungicides, apply weekly from early season to delay onset.
Daily remove infected leaves/stems to curb spore spread, especially in dry weather.
If uncontrollable, on a dry day, pull all tops (stand astride rows to avoid disturbing tubers), bag in sealed plastic, and discard. Leave tubers in ground 2+ weeks before harvest.
As a soil-friendly alternative to Bordeaux mixture (which accumulates copper), horsetail manure shows promising results.
Further insights from a 4-year FREDON Nord-Pas de Calais study (1999-2002):
Pre-Planting Prevention:
During Growth:
Resistant Potato Varieties: Aziza, Ballade, Bondeville, Corolle, Eden, Gasoré, Innovator, Naturella, Raja, Santé.
Moderately Resistant: Caesar, Santana.
Susceptible: Agata, Artemis, Bintje, Charlotte, Chérie, Daisy, Franceline, Russet, Shepody, Victoria.
Treatment Alternative: Horsetail manure effectively replaces Bordeaux mixture without long-term soil copper buildup.
