Caused by : Phytopthora infestans
It is the most destructive disease of Potato that results in crop failures. In the year 1845, there was a huge loss of potato crop due to this fungus infestation. It became an epidemic disease in Ireland as it destroyed the lives of 20 lakh people due to Starvation. Its nothing but the IRIS FAMINE. So proper care must be taken till harvesting in order to prevent disease attack and its spread.
Causes and reasons for its spread
Using infected seed tubers which give rise to sporangial inoculum and infects the nearby plants
Infected tubers do not sprout and their death gives patchy appearance
Previously infected crop will serve as a primary source of infection in spring or late sown season.
Heaps of infected potato tubers near cold stores also serves as a source of infection.
Symptoms appear on leaves, stem as well as on tubers.
On the leaves we can observe small water soaked lesions which later enlarge to become irregular dark brown lesions surrounded by light green halo.
On the undersurface of the leaves we can observe white cottony growth around the lesions during morning hours. It will become dry and necrotic when weather becomes normal(dry).
The whole plant may get destroyed in just 10-14 days during unfavourable conditions like low temperature, high humidity and intermittent rainfall.
Elongated brown stripes will be formed on the stem from below the ground level and encircle it.
Lesions can also be seen on ground part of the stem.
The infection may start at nodes and may extend to other parts of the plant followed by white fungal growth.
On the infected tubers we can observe irregular sunken reddish brown patches.
Internal part of potato will turn spongy and rusty.
Varieties with short stolons bearing tubers near the stem are more liable to fungal infection
Small immature tubers are more prone to infection then larger ones.
Use Mancozeb at 0.25 %
Combination of Dimethomorph and Mancozeb can control this disease effectively