Late Blight of Potato: Identification and Management

Identifying Late Blight Symptoms

Recognising late blight early is key to its control. Look for the following signs on your potato plants:

• On Leaves: Initially, small, irregular, water-soaked spots appear, usually at the leaf tips or margins. These spots quickly enlarge, turning dark brown or black. During humid conditions, a white, cottony fungal growth can be seen on the underside of the leaves, especially around the edges of the spots. Infected leaves shrivel and die, giving the field a burnt appearance.

• On Stems: Dark brown to black lesions can develop on stems and petioles. These lesions can girdle the stem, causing the plant parts above the lesion to wilt and die.

• On Tubers: Infected tubers show irregular, purplish-brown lesions on the skin. When cut open, the flesh beneath these lesions is reddish-brown, granular, and extends inwards. This rot can progress rapidly, especially during storage, leading to significant post-harvest losses.

Factors Favouring Disease Development

Late blight thrives under specific environmental conditions. Indian farmers should be particularly vigilant when:

• Temperature: Cool to moderate temperatures (10-20°C) are ideal for the pathogen.

• Humidity: High humidity (above 90%) and prolonged periods of leaf wetness (4-8 hours) from rain, dew, or fog are critical for spore germination and infection.

• Rainfall: Frequent rainfall helps spread the spores from infected plants to healthy ones.

These conditions are common during the winter growing season in many parts of India, making late blight a constant threat.

Prevention and Management Strategies

An integrated approach combining cultural practices and chemical control is most effective against late blight.

Cultural Practices:

• Use Healthy Seed: Always plant certified, disease-free potato tubers.

• Resistant Varieties: Where available, choose potato varieties known to have resistance or tolerance to late blight.

• Proper Spacing: Maintain adequate plant spacing (e.g., 60 cm between rows, 20 cm between plants) to ensure good air circulation, which helps reduce leaf wetness.

• Field Sanitation: Remove and destroy any volunteer potato plants or infected plant debris from previous seasons, as these can be sources of infection.

• Hilling: Ensure proper hilling (earthing up) to cover tubers adequately, preventing spores from washing down from infected foliage and infecting tubers.

• Irrigation Management: Avoid overhead irrigation late in the day, which can prolong leaf wetness. Drip irrigation is preferable.

Chemical Control (Fungicides):

Fungicides are crucial for managing late blight, especially during favourable weather conditions. A combination of contact and systemic fungicides is often recommended.

Fungicide TypeExample Active IngredientDosage (per acre)Application IntervalContact FungicideMancozeb 75% WP800-1000 grams7-10 days (preventive)Systemic + ContactMetalaxyl 8% + Mancozeb 64% WP500-600 grams5-7 days (curative/preventive)Contact FungicideChlorothalonil 75% WP600-800 grams7-10 days (preventive)

Start spraying preventively when conditions become favourable or at the first sign of disease. Rotate different types of fungicides to prevent the development of resistance in the pathogen. Always follow the manufacturer's instructions for dosage and safety precautions.

Best Practices for Control

Regular field scouting is vital. Inspect your potato fields frequently, especially after rain or prolonged periods of dew. If late blight is detected, act immediately. Remove and destroy severely infected plants to prevent further spread. Combine cultural practices with a timely and appropriate fungicide spray schedule. Monitoring local weather forecasts can help you anticipate high-risk periods and plan your preventive sprays accordingly.

Late blight can quickly devastate a potato crop, but with careful monitoring, good cultural practices, and timely application of appropriate fungicides, farmers can effectively manage this disease and protect their yields.