Late Blight

Phytophthora infestans is an oomycete (water mould) pest most known for causing the Irish Potato Famine in the 1840s and is the most significant pest to the potato industry. It spreads rapidly, and under its preferred conditions can infect entire fields within a week.

Symptoms:

Initially water-soaked lesions occur on the leaves in a light to dark green colour. These lesions have a lighter halo and white sporulation may appear on the underside of the affected leaves in damper grower regions. Under severe infestations this pathogen can also infect the stems and cause the tubers to rot.

Infected tubers can be totally decayed; however earlier symptoms show as tan or brown-colored discoloration on the surface.

Lifecycle:

• P. infestans is capable of both sexual and asexual reproduction, however in Western Canada there’s only one mating type, so reproduction is asexual
• P. infestans overwinters in infected tubers, cull piles, or potatoes missed at harvest
• Spores wash from lesions, germinate, and travel by water through the rooting zone in the soil. They primarily infect the eyes of new tubers
• Infected tubers transmit to the seedling plants
• Seedlings develop lesions, producing airborne spores
• Airborne spores can travel up to 100 km by wind and storm fronts
  • Temperatures of 10-15C at night and 15-20C during the day are ideal for disease development
• Rain, dew, or overhead irrigation are necessary for the pathogen to infect the plant
• New spores develop in 3-5 days
• Leaf & stem lesions are visible within a few days, lesions expand & sporulate under favorable conditions
• The infection cycle takes 5-7 days, entire fields can be infected rapidly if not controlled

Control:

• Crop rotation
• Isolating cull piles
• Control volunteer potatoes
• Certified clean seed tubers
• Foliar Fungicides

References:

Hielke De Jong, et al. The Complete Book of Potatoes : What Every Grower and Gardener Needs to Know. Portland, Timber Press, 2011. p. 92, 98, 217-218

https://cropipm.omafra.gov.on.ca/en-ca/crops/potatoes/diseases/804fa9d9-c964-4fc1-ab30-153137c3c6e1#identification Date accessed: January 7 2026

https://www.ndsu.edu/agriculture/extension/publications/late-blight-potato Date accessed January 7 2026

https://www.ndsu.edu/agriculture/sites/default/files/2022-05/pp1849.pdf Date accessed January 7th 2026

https://www.alberta.ca/late-blight-of-potatoes-and-tomatoes date accessed January 7 2026

https://www.gov.mb.ca/agriculture/crops/plant-diseases/late-blight-potatoes.html January 7 2026