Late blight of tomato

Late blight, caused by the oomycete Phytophthora infestans, is arguably the most devastating disease affecting tomato and potato crops worldwide. This pathogen is known for its ability to spread rapidly across fields, potentially destroying an entire harvest in a matter of days if left unmanaged.

While tomatoes are a primary host, the disease also readily infects potatoes, peppers, and various nightshade weeds. Because of this broad host range, managing late blight requires careful monitoring of all related plant species in the proximity, as the pathogen can easily cycle between these different crops.

Symptoms typically manifest as water-soaked, dark spots on the leaves, which quickly darken and turn necrotic. Under high humidity conditions, a distinct whitish mold—consisting of sporangia—develops on the underside of infected leaves. Eventually, the disease spreads to stems and fruits, causing severe, firm, brown decay.

Development and spread are strongly correlated with cool, moist weather conditions, particularly high humidity, morning dews, and frequent rainfall. Temperatures between 15°C and 20°C are optimal for the pathogen to thrive, allowing its spores to be transported long distances via wind and water splashes.

Economic damage is severe, often resulting in complete crop loss. Control strategies must be preventative, focusing on:

• application of protective and systemic fungicides;
• utilizing resistant cultivars;
• ensuring adequate plant spacing to promote airflow;
• prompt destruction of infected plant debris;
• avoiding overhead irrigation to reduce canopy moisture.