Common scab or corkey scab: Streptomyces scabies
Symptom:
- Corkiness of the tuber periderm is the characteristics symptoms.
- 1/4 inch into the tuber surface are russette appearance.
- Slightly pitted on the infected tuber. Light brown to dark brown lesion appears on the infected tuber.
- Affected tissue will attract insects.
Identification of pathogen:
Mode of spread and survival:
- It attacks cabbage, carrot, egg plant, onion, radish, spinach and turnip.
- The causal organism perpetuates in soil and infects the crop every year.
- Infected potato tubers serve as the main source of long distance spread of the disease.
- The pathogen may survive passage through digestive tract of animals and hence it may spread with farm yard manure.
Management:
- Only scab free seed potatoes should be planted as this will help in checking the spread of the inoculum and infection to be subsequent crop.
- Infection of the seed tubers can be removed by 1.5hrs dip in mercuric chloride 0.1% solution or by 2h dip in 1 part formaldehyde in 240 parts of water.
- This disease can be reduced by soil application of PCNB at the time of planting. Four to six years crop roation with alfalfa satisfactory under irrigated conditions.
- The disease incidence can be effectively reduced by green manuring the fields before planting potatoes.
- Common scab is severe in alkaline soil and application of alkaline fertilizers like calcium ammonium nitrate should be avoided.
