Only microscopic examination of seed potato tubers can truly determine whether it was disease free, agchem manufacturer Certis claims.

The firm has demonstrated how a tuber free from blemishes and visible infection could actually be infected with the rhizoctonia fungus when examined under the microscope.

Being able to see the fungal strands under the microscope while not seeing black scurf with the naked eye on the tuber is actually quite common, according to Harper Adams University’s Matthew Back.

“Ten to 20% infection is common, and if this seed is untreated, then the fungus can cause weakened growth, and black scurf symptoms on daughter tubers.”

He urges growers to take advantage of Harper Adams’ Eye Plug seed test, which measures seed-borne disease levels on potato eyes.

“Typically the incidence is higher than expected.”

Any disease present on the tuber can be treated with a seed treatment, such as RhiNo (flutolanil), Certis’ Peter Shakespeare adds.