Archive Article: 1997/07/19

19 July 1997




GET under the canopy and look for tell-tale symptoms that could spell tuber blight this season.

Recent long periods of wet and humid weather mean the risk of both foliar and tuber blight for UK potato growers, according to plant pathologist, Dr Ruairidh Bain, of SAC Auchincruive.

He is convinced that effective tuber blight control must be achieved by looking at the whole spray programme – not just at the end of the season when the crop canopy begins to senesce.

"Tubers can actually become infected with the blight fungus much earlier than some growers might think," he says.

"Only a small percentage of the foliage needs to be infected for tuber blight to take hold. Above average rainfall this June means that growers who have not adopted an early spray programme will be at greatest risk from tuber blight at the end of the season."

Dr Bain says that although foliar blight can be related with reasonable accuracy to Smith periods it is much more difficult to forecast the risk of tuber blight. For example, 1995 was a very bad tuber blight season in some parts of the UK even though there was only low risk of foliar blight.

"The main risk factors for tuber blight include variety resistance, end of season rain, unprotected tubers and stem lesions," he says.

"In the past agronomists have missed the stem lesions because they only looked at the top of the canopy. More often than not the disease develops on the stem or at the base of the canopy and goes undetected."

Dr Bain believes growers now realise the key to protecting crops from tuber blight is to provide protection as early as possible.

Traditionally, potato growers relied on mancozeb plus phenylamide-based products applied early in the season. But, extensive use of these products in the 1980s resulted in potato blight resistance.

However, a reduction in the number of phenylamides being applied by growers and a tendency to source alternative products means levels of resistance have reduced.

According to Dr Bain, growers can still use phenylamides to make a significant contribution to tuber blight control.

"The key is not to use them for more than three sprays to avoid selection pressure or resistance build up," he points out.

"However, some growers now prefer to use cheaper contact fungicides such as fluazinam (Legacy/Shirlan). Also lower rates of active ingredients means fluazinam fits in with a change to using lower quantities of pesticide."

In trials and in commercial use, programmes using fluazinam have shown prolonged early season activity against foliar blight and good late season protection against tuber blight.

The product can be applied at seven day intervals under extreme blight pressure – with up to a maximum of 10 applications in a season.

Low levels of tuber blight are usually related to good control of foliar blight through the season.

Fluazinam has multi-site activity which prevents spore germination and growth and the release of infectious zoospores. It also inhibits spore motility and production.

"Controlling motile zoospores is important as it not only prevents the infection of foliage, but also controls zoospores washed on to soil by rainfall, which would otherwise develop into tuber blight," says Dr Bain.

"In areas of the country where very early infection is likely, contact fungicides might be necessary prior to the foliage meeting in the row."

Once there is sufficient haulm to justify the use of more expensive products, SAC recommends an effective foliar and tuber blight programme starting with two or three systemic sprays such as Trustan (cymoxanil + mancozeb + oxadixyl) or Fubol (mancozeb + metalaxyl) at 10 to 14 day intervals when haulm growth is rapid.

Application

However, fluazinam applied at seven day intervals should also be considered where suitable spray days are not limiting.

"These products provide effective protection against foliar blight and should also provide a knock-on effect to help reduce the risk of tuber blight," says Dr Bain.

He adds that mid-season control – between the time haulm stops growing rapidly and senescence – should include a contact fungicide such as Legacy or mixtures of translaminar and protectant fungicides such as Curzate M (cymoxanil + mancozeb) and Invader (dimethomorph + mancozeb).

ADAS confirms that spray schedules thrown out by wet weather should be brought back as soon as possible, given unsettled conditions.

Highest risk areas this season are North Yorkshire, Nottinghamshire, Lincolnshire, most counties of East Anglia and the south west of England where blight sprays should applied every seven days if the product label permits.

Market research shows most farmers apply blight sprays every 9-13 days, says Dr Antony Goulds, of Cyanamid. He argues that the anti-sporulant activity of dimethomorph contributes to growers peace of mind if they cant get the spray on when intended.

Dr Bain says tin-based products conventionally make up the final two sprays; the first being applied seven to 10 days before desiccation and the second with the desiccant.

"An alternative to two tin-based sprays is to include fluazinam as the last three or four sprays," he adds.

Your potato fungicide programme could be missing a major underground target.


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