Common scab Disease of Potato is also known as Corky scab
which is the causal organism of common scab?
fungus called Streptomyces scabies
what are the Symptoms of common scab of potato?
Corkiness of the tuber periderm is the characteristics symptoms.
1/4 inch into the tuber surface are russette appearance.
Light brown to dark brown lesion appears on the infected tuber.
Affected tissue will attract insects.
what are the characteristics of Pathogen causing common scab?
Aerial mycelium in pure culture has of prostrate branched threads.
Sporogenous hyphae are spiral in form.
Conidia are produced by the formation of septa at intervals along the hyphae, which contract to form narrow isthmuses between the cells.
Conidia are roughly cylindrical and hyaline.
The conidia can germinate even at higher temperatures.
The growth of the organisam is good in slightly alkaline medium and is checked at pH 5.2.
What is the Mode of spread and survival of fungus causing common scab?
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.
what are the control measures of Common scab of potato?
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 1.5 hrs 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.
further readings and references