10 September 1999

Wilt risk is high in potatoes

PREMATURE senescence of potato crops this season could be due to the fungal disease, verticillium wilt, say experts.

"Weve seen a lot of it this season," says Hutchinson agronomist Darryl Shailes. "It seems a lot to do with the wet conditions early on."

That is echoed by ADAS Rosemaund research consultant Tom Locke. Nationally, the significance of the disease is under-rated, he believes.

"It can cause 25-60% yield losses in infected patches. The problem is the symptoms are rather vague."

Different causes

Early senescence due to verticillium can appear to be due to blight, virus infection, drought or eelworm damage. "Until growers look closely and notice the way the plant is dying off, verticillium is unlikely to be blamed."

Infection occurs early in the season, through the roots, and the fungus spreads through the vascular tissue of the plant. Irrigation or wet conditions at tuber initiation make plants particularly prone to the disease.

An early symptom is afternoon wilting, which progresses to permanent wilting, yellowing and senescence.

Often these symptoms are limited to one side of the plant, or even one side of the composite leaf, he notes. "It depends which vessels are blocked."

The base of senesced stems on infected plants appears silvery, with a very fine black speckling of sclerotia. "Theyre only visible with a hand-lens," notes Mr Shailes. Black dot has much larger spots.

Sclerotia remain viable in the soil for 5-10 years, and the disease has a wide host range. "The host range includes virtually everything thats not a grass or cereal," says Dr Locke. Linseed particularly seems to exacerbate the problem.

In the Breckland area crisping varieties such as Saturna and Lady Rosetta are worst hit, says Mr Shailes. "Hermes doesnt seem to be affected," he adds.

But Dr Locke stresses no UK varieties are really resistant. "All are susceptible to varying degrees. Estima is particularly prone to infection."

NIABs plant pathology department, or ADASs plant clinic at Wolverhampton, can confirm infection in the crop. ADAS provide a soil test for the fungus. "It could be used to screen out the worst fields for potatoes," he suggests.

VERTICILLIUM WILT

&#8226 Causes premature senescence of potatoes.

&#8226 25-60% yield loss recorded.

&#8226 Persistent soil borne fungus.

&#8226 Wet conditions favour infection.

Widespread early senescence in potato crops this season may well be due to verticillium wilt, says Hutchinson agronomist Darryl Shailes