Identifying Scab- Infected Potatoes

Press, Volume CVI, Issue 31088, 17 June 1966, Page 6

Identifying ScabInfected Potatoes

There are two specific diseases of potatoes which could easily be mistaken if one is not aware of the distinctive identifying feature of one of them. The pathogens we are here concerned with are Powdery Scab and Common Scab, and both attack the tuber only.

Powdery scab only serious in cold, wet soils and those that are poorly drained. It shows up on infected tubers in the form of small, roughly circular, areas which, in the case of severe attacks, may run together. These areas, so called scabs, have distinctive raised margins consisting of the torn skin of the tuber and a centre filled with a very fine rusty brown coloured powder which is in fact a mass of spores.

Annual carry-over in the soil provides the source of infection for following crops but extreme resistance of the spores necessitates very long

rotation. The disease can also be carried over from year to year by infected tubers. The application of lime increases incidence of powdery scab. Common scab is induced by liming, a practice which should at any rate not be associated with potato growing as they prefer a near acid soil. It can be inhibited somewhat by the use of acid fertilisers such as sulphate of ammonia.

Irregular scabbed areas which may be depressed below or raised above the surface and are firm or corky in texture often cracked quite deeply are the indentifying symptoms of common scab. The mass of powdery material characterises only powdery scab. The disease is both soil and seed borne and again rotation will greatly reduce the trouble. Lowering the pH of the soil to about 5.6 will also improve resistance. Whilst infected seed should never be used others can be dipped in a proprietary mercury dip.