Favourable Conditions for Late Blight

• In Punjab, late blight generally makes its first appearance in first or second week of November when the main season crop is about 40-50 days old.
• Early planted crop (September planted) generally escapes the attack of late blight because of relatively warmer climate.
• The sporangia of the fungus are killed at temperature above 30⁰ C.
• The main season crop planted in October and the successive crops are invariably attacked due to favourable climatic conditions and the disease may continue up to March.
• The mean temperature and relative humidity during the main crop period usually range between 15-18⁰ C and 60-80 % respectively coupled with dew deposition during night and accompanied by sunny days. Under such conditions, the disease may not develop severely and it may be restricted and localized or remain confined.
• When winter rains occur raising the humidity to > 90% and cloudy or foggy days persist for 6-7 days with atmospheric temperature in the range of 10-20⁰ C (during November-December), late blight develops rapidly on upper leaves, stems and multiple infections take place on the adjoining plants and then on the entire field through spread by the air-borne sporangia of the fungus.
• Due to rapid disease spread, epidemic situation may develop, if humid weather conditions persist for longer duration. Normally five infection cycles are completed before uniform spread of the disease is observed in the crop.
• It may take about 3 weeks from the first disease appearance for development of an epidemic situation, if favourable weather conditions continue. With the return of dry conditions and sunny days, development and spread of the disease is slowed down.
• Even one infected tuber in 1 ha crop is enough to create severe disease situation under favourable climatic conditions.