Late blight is a fungal infestation. The first symptoms of the disease in the field are small light to dark green circular to irregular-shaped water-soaked lesions on the potato plant’s leaves.

The circular or irregular leaf lesions are often surrounded by a pale yellowish-green border that merges with healthy tissues of the leaf. Lesions enlarge rapidly and turn dark brown to purplish-black. These lesions usually appear first on the lower leaves within 3-5 days of infection.

White mould

During periods of high humidity and leaf wetness, a cottony, white mould growth is usually visible on lower leaf surfaces at the edges of lesions.

Spores formed on the mould spread to neighbouring fields by air, irrigation thus beginning a continuous cycle of the disease.

Good soil protection measures provide better protection for the crop against this infestation.

Harvest should not be started until all vines in the field are completely dry. Harvest should be done minimum two weeks after all the vines have dried up. In fields where late blight is confirmed, a minimum of two – three weeks should pass. Late blight will not survive on dead vegetation so tubers that are exposed after harvest are less likely to be infected.

Remove infected tubers

Remove infected tubers before storage to reduce additional losses from soft rot. Tubers should be dry when placed in storage. The fungus can only survive in living infected potatoes, including potatoes in storage.

There are several varieties that are resistant to late blight. These varieties will slow down, but not prevent, the disease.

The most widely used fungicide is Dithane M45 (mancozeb) at 3gm/lit. The systemic fungicide ridomil (metalaxyl) at 2gm/lit can also be used. Application of Ridomil (MZ 58), Ridomil once and Dithane M-45 (Mancozeb, 80 per cent WP) subsequently is recommended for this infestation.

(Utpal Dey, scholar and Dr. D.N. Dhutraj, Professor & Head, Department of Plant Pathology, Marathwada Agricultural University, Parbhani, Maharashtra, email:, Mobile No. 8275824103)

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