Late Blight

Late blight has shaped the history of agriculture more than almost any other plant disease. It was responsible for the Irish Potato Famine in the 1840s and continues to cause annual economic losses worth billions of dollars across the world. In India, the disease is particularly destructive during the rabi potato season when prolonged fog, high humidity and cool temperatures prevail. Because the pathogen can complete several infection cycles within a single season, disease outbreaks can escalate from a few infected leaves to complete crop destruction in only a few days. Unlike many fungal diseases, late blight is caused by an oomycete or water mold, which thrives under wet environmental conditions. The disease primarily attacks leaves and stems during crop growth but can also infect developing tubers, leading to serious storage losses after harvest. Among all potato diseases, late blight causes the highest economic losses globally because of its ability to spread rapidly and infect both foliage and tubers.

• 1The earliest symptoms usually appear on lower leaves as small, pale-green, water-soaked lesions near the leaf margins or tips.
• 2These lesions rapidly enlarge under favorable weather, becoming dark brown to black with irregular margins.
• 3During early morning hours or periods of high humidity, a characteristic white cottony growth consisting of pathogen sporulation develops on the underside of infected leaves.
• 4This white growth is considered one of the most reliable field diagnostic features of late blight.
• 5Stem infections appear as elongated dark brown lesions that may girdle the stem, causing wilting and eventual collapse of the entire plant.
• 6In severe epidemics, complete fields may appear burnt within a few days.
• 7Tuber infection usually develops through spores washed into the soil during rainfall or irrigation.
• 8Infected tubers develop slightly sunken brown patches on the skin, while the underlying flesh turns reddish-brown and granular.
• 9Secondary bacterial infections frequently follow, resulting in soft rot during storage.

The disease cycle generally begins with infected seed tubers or volunteer potato plants carrying the pathogen from the previous season. Under humid conditions, the pathogen produces large numbers of sporangia that are easily dispersed by wind and rain splash over long distances. When temperatures remain between 10°C and 18°C and leaves remain wet for several hours, sporangia germinate and infect healthy plant tissue either directly or through motile zoospores. New lesions begin producing fresh spores within three to five days, allowing multiple infection cycles during a single growing season. This explosive multiplication explains why late blight epidemics can spread across entire regions within a very short period.

Late blight is primarily a weather-driven disease. Disease outbreaks are strongly associated with cool temperatures, high relative humidity and prolonged leaf wetness.