Late Blight

Late Blight (*Phytophthora infestans*), an exceptionally devastating, rapid-acting disease caused by an oomycete (water mold) belonging to the family *Peronosporaceae* of the order *Peronosporales*, is famous for causing the historic Irish Potato Famine of the 1840s and remains a premier threat to global nightshade crops. Often classified alongside fungal diseases, *Phytophthora* is biologically more closely related to brown algae. It operates as a highly aggressive necrotrophic pathogen that quickly destroys tissues of potatoes and tomatoes.

Identifying late blight involves recognizing large, irregular, water-soaked, dark brown or black oily blotches that appear on leaves and stems. In cool, highly humid conditions, a delicate, white, downy or fuzzy growth consisting of spore-bearing sporangia develops on leaf undersides, forming a ring around the margins of leaf spots. Stems develop dark brown, firm, girdling lesions. Infected potato tubers show a firm, dry, reddish-brown rot, while green and ripening tomato fruits display firm, dark, marbled brown spots.

Late blight primarily targets member species of the Solanaceae family.

The late blight pathogen overwinters as dormant mycelium inside infected potato tubers, volunteer potatoes, or cull piles. In spring, as temperatures rise to 10°C to 25°C and humidity levels increase, the pathogen produces microscopic sporangia. These sporangia are carried by wind currents over several miles or splashed by rain. Upon landing on wet leaves, they either germinate directly or release tiny, mobile, swimming zoospores that navigate water films to penetrate leaf cuticles. The disease cycle is incredibly rapid; a new generation of spores can be produced in as little as 3 to 5 days, allowing it to turn a healthy field into a blackened, rotting mass within a week. For organic management, immediately pull up and destroy the entire infected plant, and apply preventive foliar sprays of liquid copper fungicides or biological formulas with *Bacillus amyloliquefaciens* or *Bacillus subtilis*. Chemical control relies on protective or systemic oomycete-specific fungicides containing chlorothalonil, mancozeb, or metalaxyl, applied preventatively before the disease establishes. Prevent outbreaks by planting certified disease-free seed potatoes, destroying all potato volunteer plants and cull piles, spacing plants to optimize air flow, and watering early at the base of the plant. For comprehensive support, browse our Plant Disease Identifier Hub, or read related guides on Early Blight, Downy Mildew, and Fusarium Wilt.