Late blight of potato is a common and serious problem in Nepal which is caused by a fungus (Phytophthora infestans). The same disease is also serious in tomato. This fungus affects leaves, stems and fruits. All growth can be affected stages as well as on whole plant at different stages i.e. pre-emergence, seedling, vegetative growth, flowering, fruiting and post harvest. Infected potato tubers rot.
Small (2-10 mm) brown or black lesions on leaves of irregular shape appear first mainly starting from the margin of leaves. Sometimes patches of fungus can be seen underneath the leaves. These lesions gradually expand and affect the stem and eventually the whole plant leading to death.
This problem becomes severe if the atmosphere is humid for more than 7/8 hrs with temperature ranging between 15-20°C. Temperatures above or below this range will reduce the rate of pathogen growth and disease severity.
There are different ways to manage this disease:
- Selection of resistant varieties if available
- Spray Krinoxyl gold [Metalaxyl (8%) + Mancozeb (64%)] @ 2 gm/l water
- Collect and destroy affected parts at initial stage of infection
- Agronomic practices like crop rotation, wider spacing, and early planting are recommended.
- Select seed from disease free areas/plots
When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval.