Blight threat from unlifted crops

25 May 2001



Blight threat from unlifted crops

By Andrew Blake

BLIGHT inoculum from unprecedented numbers of abandoned potato fields is a real danger to this seasons crop, warns the British Potato Council.

An estimated 8000ha of unlifted and poorly harvested potatoes from the 2000 campaign present a potent source of early disease, it warns.

Much of this springs late-planted crop is only just emerging and is under threat, unless the situation is managed correctly, says the BPC.

“It is a significant extra threat,” says the BPCs Mike Storey. Warm, humid conditions mean newly emerged foliage in nearby crops will be at particular risk.

Despite warnings, every year initial infections often come from uncontrolled growth in dumps of waste tubers.

“Dumps should not be a problem, but every year without fail, they cause trouble,” says Mr Storey.

This season there are effectively whole fields ready to act as dumps, says agronomist colleague Rob Clayton.

“The key is to prevent any volunteers from developing, which probably means repeated use of herbicides.”

Simply ploughing down lost crops will not avoid the risk of blight arising, he says.

Several treatments of glyphosate at 14-day intervals will be needed to deal with emergence from tubers at different depths, he advises.

For volunteers on truly waste ground Casoron G (dichlobenil) is a more effective controller.

However, nothing may be sown or grown for over a year thereafter, notes former BPC research and development committee chairman, John Chinn.

The main problem on his Herefordshire farm are widespread volunteers in subsequent peas and sugar beet drilled after two fields of potatoes went unlifted.

Most buried tubers rotted, but after a light cultivation in February, those near the surface were only partially killed by frosts.

He said: “It is a big worry. Are we going to have to put a fungicide in with the sugar beet spray every time we go through?”

To minimise the threat in vulnerable areas it may be sensible to include a fungicide with the herbicide used to destroy them, says Dr Clayton.

“But you need to get MAFF permission to do so on the grounds of plant health risk,” he added.

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