Soil test help for powdery scab

Testing soil for powdery scab spore balls could help potato growers with field and variety selection.

Three years of field trials had shown a significant link between the level of soil inoculum and the amount of powdery scab on daughter tubers, Alison Lees, a plant pathologist at the James Hutton Institute told growers at the SAC Association of Potato Producers Conference.

There had been some doubt whether there was a strong link, but the new work suggested there was good evidence for using a powdery scab soil test to help with field and variety selection.

Where the test detected no spore balls, the field could be considered as low risk, and any variety could be grown if healthy seed was planted, Stuart Wale of SAC said.

“If you have a trace up to 10 spore balls you get a moderate risk, and you could use a more resistant variety, or if you are a seed grower use fluazinam.

“At over 10 spore balls then you have a high risk, and you only really have one option: to grow one of the few resistant varieties with a rating of eight. And if I tell you Hermes is rated eight, you will tell me that I’ve seen powdery scab on Hermes, so they are not perfect.”

Field trials had shown that fluazinam could reduce powdery scab in seed crops, but only where disease pressure was not excessively high, Dr Wale explained. “If your soil test shows high levels you’re not going to be able to push a lorry uphill.”

It also only works if you apply it correctly, he said. Dr Lees research had shown that infection of the roots happens almost as soon as they form, provided it is cold and wet.

“So in Scotland we have a challenge as it is often cool and wet,” Dr Wale said. “In order for the fluazinam to work it has to be there in the soil, stopping the swimming spores when they germinate getting to the roots.

“So incorporation is very important and you need to do that shortly before planting. If you do it a week before planting, the chemical residue declines and you’ve lost a week advantage of using the chemical.”

The product should not be sprayed for this purpose, he reminded growers. “It has to be applied as a downward directed drench as you’re putting on 10 times the dose you would as a blight spray, and there is more risk of water contamination.”

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