29 September 2000

Eye testing for disease advised

GROWERS should eye test potato seed for disease when selecting stocks, says SAC.

"Eye tests have come on from being just for research," says SAC potato specialist Rob Clayton. "They are now a practical advice tool."

Washing and visually inspecting a sample of the seed stock should be every growers first step, he says. But diseases such as powdery scab, skin spot, and black scurf can be present on seed at invisible levels that an eye test will identify.

For high value pre-pack or punnet crops or high grade seed growers, that information could be vital, he says.

"As the level of eye infection increases then you need to start thinking about using a fungicide, or looking for another seed stock."

Equally, results may be used in price negotiations, he adds. That could work both ways, putting a higher value on disease free stocks or discounting diseased samples.

"If it is your own seed, then it is bad luck. But the results might point to where and why stem canker occurs," he says.

SAC trials at SCRI Invergowrie indicate a link between eye rhizoctonia infection and stem canker (see table). However, in the trial, high levels of the skin spot fungus, polyscytalum, resulted in the most serious stem canker.

"The skin spot may have greatly weakened the stock and predisposed it to rhizoctonia," he says. "It also produced a high proportion of small tubers, so again, the advice would be dont buy it."

Samples for eye testing, or visual assessments, should be taken from well into the seed store, avoiding the front row of boxes which frequently contain seed from end rigs, and are more freely ventilated, he continues.

"You have got to get into the stock and avoid the surface of boxes. A good indication of the worst will be in the top row of boxes, 6-10 inches below the surface."

Each sample for testing should be made up of 100 tubers, drawn at random from a range of boxes for each stock. "Ten tubers from 10 boxes is good."

At SAC, a basic disease test on seed costs about £30/sample, plus £15/sample for an eye test. Results are available in 9-10 days. ADAS, Sutton Bridge, Cambridge University Farms also offer similar testing services, he adds. &#42

Pre-pack, punnet and high grade seed growers should consider eye testing seed stocks, says SACs Rob Clayton.

Eye test trial results

Stock A B C D E

Rhizoctonia <1 4 16 30 62

Skin spot 22 0 0 0 0

Silver scurf 2 4 12 0 8

% emergence 86 79 96 100 91

Stem canker index 20.3 1.2 4.2 4.4 5.4


&#8226 Random 100 tubers/stock.

&#8226 Draw from heart of store.

&#8226 Wash and inspect.

&#8226 Pre-pack, punnet, high grade seed? Consider eye test.