Potato growers are being urged to pay more attention to early blight control, especially after the wettest May since 1983 disrupted spraying programmes in many areas.

“June is often a critical month for blight control and historically growers tend to put too few sprays on,” says Agrovista’s Mark Palmer.

Some blight cases have already been reported on outgrade piles, and moist soil conditions, combined with warm temperatures, mean systemic protection of rapid haulm growth will be more critical than usual, he says.

“This year will be particularly good for systemic fungicides.”

Products such as Consento (fenamidone + propamocarb hydrochloride) and Tattoo (mancozeb + propamocarb hydrochloride) have performed particularly well in trials and can be highly mobile systemics if good spray coverage is achieved, he says.

Broad-spectrum systemics may also give some early tuber blight control, which could be particularly useful on more susceptible varieties.

Lincolnshire-based agronomist John Keer has seen a “horrendous” patch of blight in a small area of potatoes grown under polythene for supply to a local farm shop in south Lincolnshire.

“It’s only one example, but the patch was so severe, it’s a good indication that conditions over the past fortnight have been unusually optimal for the development of blight.”

Warm, dry conditions are likely to help keep a lid on the disease, but warm and humid weather will heighten the need for growers to keep spray intervals tight, to about six or seven days, he advises.

Where blight has already been detected, curative cymoxanil-based products such as Curzate or C50 + partner, for example Ranman (cyazofamid), are likely to be more beneficial than systemic fungicides, he says.

“If there has been a delay in spraying or there are visible lesions in the crop, you will need a good curative treatment.”

Low risk in the north

Low night-time temperatures in Scotland mean blight risk is currently low, but humid, showery weather could easily change this, says the Scottish Agricultural College’s Ruairidh Bain.

“Although May was wet, the past week or so has been bone dry and, combined with low night temperatures (2-5C), I don’t anticipate any major problems.

But if the weather turns humid and wet, it could kick things off.”

Delayed planting due to wet weather means there is a significant range of crop growth stages across Scotland, he adds.

Some late-planted seed crops are just emerging, while a number of first early growers have started lifting crops, he says.

Dr Bain says this range of growth stages can make blight fungicide choice more difficult, but he favours systemic products during periods of rapid haulm development to protect new growth.

“If the weather turns, it can be difficult to get sprays on.

“There’s no real pattern to blight and sometimes it doesn’t come in until mid-July.

Last year was relatively low risk right through the season, although there were localised pockets of blight.”

paul.spackman@rbi.co.uk