Early seed potato harvest presents dry rot risk

THE Scottish seed potato harvest began a fortnight ahead of normal this year, increasing the threat from dry rot.

SRUC consultant Dr Stuart Wale pointed out that while an early harvest is likely to reduce the risk of silver scurf, black dot, gangrene, and skin spot developing in store, the risk of dry rot can be more of a problem on susceptible varieties.

“Poor skin set, warm tubers, and store condensation all increase dry rot risk. In seed crops this can be minimised by treating with a fungicide into storage or at grading. Judging the need for treatment should be based on varietal susceptibility to the disease, previous experience of disease on a specific variety and presence of the disease in the seed from which the crop was grown.”

Stuart recommended two fungicide options in this situation: Gavel (imazalil) and Storite Excel (thiabendazole) which can be used alone or in mixture. “Bear in mind that Fusarium sulphureum – one pathogen that causes dry rot – silver scurf, and skin spot have all been found to develop resistance to thiabendazole, whereas there has been almost no resistance detected to imazalil.”

Certis Technical Manager Laurence Power said that for the past two springs dry rot has been a contributory factor to gappy emergence of ware crops. 

“For maximum disease reduction in storage, Gavel should be applied as soon as possible after harvest, preferably within 48 hours. If you leave the conversation with your seed supplier until the spring there may not be any treated seed left and there are no options for treatment out of store or at planting,” he said.

A full list of varietal susceptibilities to dry rot can be viewed at http://varieties.ahdb.org.uk  

Photo: Dry rot in seed (within 0.5% rots tolerance) may go unnoticed and cause gappy emergence.