Late blight is a destructive plant disease that primarily affects potatoes and tomatoes. This disease has a historical impact, causing the Irish Potato Famine in the 1840s, which led to widespread crop failure and societal consequences. It remains a persistent concern for growers worldwide.
Understanding Late Blight
Late blight is caused by Phytophthora infestans, an oomycete, or water mold, that is a fungus-like microorganism related to algae. This pathogen survives mainly as mycelium in infected potato tubers left in fields or in waste piles, a primary source of inoculum for new crops.
The disease spreads through spores, sporangia, which are dispersed by wind and rain. When these sporangia land on wet leaves or soil, they release zoospores that swim in water films to infect plants. Optimal conditions for sporulation and rapid spread include temperatures between 12°C and 18°C and humidity consistently above 90%. A disease cycle can complete in as little as four to six days under favorable conditions, causing rapid infection.
Recognizing Late Blight Symptoms
Identifying late blight symptoms early is important for management. On leaves, initial symptoms appear as small, irregular, water-soaked spots that are light to dark green. These lesions quickly expand, turning dark brown or black, often appearing greasy. Under humid conditions, a fuzzy white mold may be visible on the underside of the leaves, particularly at the edge of the lesions.
Stems can also develop dark brown to black lesions that may girdle and kill the stem. These stem lesions may also show whitish fungal growth under humid conditions. If the disease progresses unchecked, it will reach the tubers.
Infected potato tubers exhibit irregular, purplish-brown lesions on the skin. Cutting into the tuber reveals a reddish-brown, granular rot that extends from the surface inward. Initially firm, the rot can later soften due to secondary bacterial infections.
Strategies for Control
Effective late blight control involves preventative measures and management strategies. Choosing resistant potato and tomato varieties can reduce disease severity, though no variety is entirely immune. Proper plant spacing improves air circulation, helping foliage dry quickly and discourage disease development. Avoiding overhead watering, especially in the late afternoon or evening, also reduces leaf wetness periods that favor the pathogen.
Hilling potato plants provides a protective soil barrier over tubers, preventing spores from washing down and infecting them. Removing volunteer potato and tomato plants, as well as controlling weed hosts like hairy nightshade, eliminates potential sources of inoculum. Using certified disease-free seed potatoes is important to avoid introducing the pathogen into the garden. Crop rotation, not planting solanaceous crops in the same spot annually, helps break the disease cycle.
If late blight symptoms appear, prompt action is needed. Immediately remove and destroy any infected plant material; do not compost it, as spores can survive and spread.
Fungicides can be used for severe outbreaks, but are most effective when applied preventatively or early. They work by preventing spores from germinating, so apply them to the plant surface before infection. Always follow label instructions for application rates and intervals, as some P. infestans strains have developed resistance. Cleaning tools after working with infected plants also prevents further spread.