Late Blight

Late blight typically appears as pale green to brown, water-soaked lesions on leaves, often starting at leaf margins or tips and expanding rapidly. In humid conditions, a white, fuzzy sporulating growth can often be seen on the underside of affected leaves, which helps distinguish it from other leaf blights. Stems can develop dark brown to black lesions, and infected tubers show a coppery to reddish-brown surface discoloration with a firm, dry rot extending into the flesh. On tomato fruit, infection produces firm, greasy, dark blotches.

As an oomycete, Phytophthora infestans differs from true fungi in fundamental ways, including the chemistry of its cell walls. Its disease cycle centers on sporangia produced on infected tissue, which are readily dispersed by wind and rain. Under cool, moist conditions, sporangia can release swimming zoospores that are attracted to fresh plant surfaces, where they encyst and infect.

Under favorable weather, this cycle can repeat roughly every five to seven days, allowing epidemics to develop very quickly. Between growing seasons, the pathogen survives mainly in infected seed tubers, cull piles, and volunteer potato plants; where both mating types of the pathogen are present, it can also survive in soil as long-lived oospores produced through sexual reproduction.

• Infected seed tubers or transplants carried over between seasons
• Cull piles and volunteer potato plants left in or near fields
• Soil-borne oospores, where both pathogen mating types occur together

Potato and tomato, both members of the Solanaceae, are the principal hosts of late blight. Some related wild and weedy solanaceous plants can also be infected and, in some regions, act as an additional source of inoculum between cropping seasons.

Late blight is historically significant as the cause of the Irish potato famine in the 1840s, an event that reshaped agricultural science and disease surveillance. The disease remains a major recurring cost for potato and tomato production globally, and pathogen populations continue to be tracked because their composition can shift over time and by region.

• Cool temperatures combined with high humidity or extended leaf wetness
• Overhead irrigation, fog, or dew that keeps foliage wet for long periods
• Use of infected seed tubers or transplants
• Nearby cull piles or volunteer potato plants carrying the pathogen over
• Dense canopies that trap moisture and slow drying

Because epidemics can develop within days once the pathogen is established, management emphasizes preventing introduction and buildup of inoculum ahead of favorable weather, rather than reacting after a crop is already heavily infected.

• Plant certified disease-free seed potatoes and transplants
• Select resistant or tolerant varieties where available
• Destroy cull piles and volunteer plants promptly
• Use weather-based forecasting and monitoring tools from local extension services to time scouting
• Avoid overhead irrigation practices that extend leaf wetness
• Scout fields regularly, especially during cool, humid weather windows