Dry weather brings scab threat for potato growers

Lack of rain has affected most of England and Wales and unusually dry ground may result in higher levels of common scab than in the last few years, says Potato Council technical executive Chris Steele.

"The disease costs the industry about £4m a year in rejections and remains quite a problem.

It mostly affects the fresh sector that depends more on the cosmetic appearance of tubers. Most supermarkets draw the line at 5% infection."

Although the processing sector is more tolerant, pitted lesions can cause problems. "If potatoes have to be peeled more than four times, they are likely to be rejected," he points out.

Infection is most likely to occur for six weeks after the start of tuber initiation where soils are dry, when Streptomyces scabiei, the causal agent, multiplies significantly on and around developing tubers. As tubers grow, the lesions enlarge and infected areas of the potato respond, producing a corky layer, eventually producing rough, brown mature scab lesions.

Good irrigation management is vital, emphasises Mr Steele. "Irrigation is an important method of control that should be considered part of a management programme starting much earlier," he says.

"Irrigation should be scheduled, using an accurate, validated system. Be prepared to apply every four to six days, depending on rainfall and evaporative demand."

Soil moisture deficits should not exceed 9mm on sands and 17mm on silt loams, and should be maintained for three to four weeks after tuber initiation.

"If soils are dry, start irrigation when the first plants initiate tubers, so no more than 50% reach that stage by the time the field has been irrigated. A repeat application may be needed within three to four days if ridges remain dry."

"Whatever you do, do not do half a job – work has shown that ceasing irrigation after two weeks will still allow scab to infect tubers, and could actually exacerbate disease levels," Mr Steele warns.