8 March 1996

Changing tack can beat threat to potato tubers

Stem blight in potatoes could be more significant than many growers realise. Edward Long delves into some recent research findings

By detecting stem blight in an apparently healthy crop and switching fungicides, potato growers can greatly reduce the risk of tuber infections causing rotting in store.

So says Scottish plant pathologist Dr Stephen Holmes of Adgen Diagnostic Systems of Auchincruive, near Ayr.

"In recent seasons there have been several reports of high levels of tuber infection – up to 40%, when there was no obvious foliar blight on crops," he says.

"We could not work out why, so started a research project to find out. We spotted little brown stains on the base of stems in fungicide-treated crops. These were unimpressive and did not look like the stem blight we knew could kill off tips of stems. But tests confirmed it was the fungus."

In a trial, plants were artificially inoculated with blight. Stem lesions continued to develop in spite of a programme of fungicides sprayed at 10-day intervals. By mid-Aug 73% of stems were producing spores, notes Dr Holmes.

As soon as the canopy opened up, spore production increased dramatically. "This was obviously a high risk time when infection could so easily be washed onto soil to contaminate tubers."

The trial, part of a three-year collaborative project with Scottish Agriculture Colleges, highlighted the need to match fungicide to the type of blight.

With no protection, 98% of the stem was diseased in late September. This was cut to 48% by mancozeb, to 28% with metalaxyl + mancozeb, to 13% with oxadixyl + cymoxanil + mancozeb, and to just 3% by a tin-based compound.

"When fungicides are being developed their effectiveness is assessed against foliar blight and tuber infection," says Dr Holmes. "But often stem blight is forgotten. This may account for why blight in tubers can be so serious when none is obvious on the foliage."

Infection can get onto stems at any time, he explains. If it comes in early it can be hidden by the canopy and held in "tick-over" mode. The critical high risk time comes towards the end of the season when leaves are dead but stems are alive. This is when growers usually relax blight control programmes.

Using a diagnostic kit to confirm stem blight could highlight the need to maintain spray pressure, and the need to switch to a more appropriate fungicide, suggests Dr Holmes.

The SAC, through its diagnostics business Adgen, has helped develop the Alert kit which it markets throughout the world. For about £12 a test a grower can know within 10 minutes whether a change of tack is needed.

"If required the programme should be maintained with an extra spray, and a switch made to a tin-based material or a fungicide such as the fluazinam-based Shirlan," Dr Holmes concludes.

Diagnostic kits can help fine-tune blight control programmes, according to the SAC.