Although blight pressure on potatoes was not unduly severe earlier this week, there was a Smith period in Exeter last week, noted Masstock Arable's Malcolm Smith. And in East Anglia rain at the weekend meant blight could soon be troublesome, according to Agrochemex agronomist John Keer.

Last year's difficulties in controlling blight and the emergence of new potentially more damaging strains have led some advisers to urge producers to adopt tighter spray intervals. "There's been a massive shift in populations in two years," said Mr Smith.

Last year the A2 strain, more aggressive than the A1, dominated samples analysed by the SCRI in the BPC's Fight Against Blight campaign, accounting for 82% of the total.

And 71% of all samples, including the A1s, were of the so-called 'blue-13' type against which phenylamide fungicides are relatively ineffective because of resistance.

Keep spray intervals tight & be prepared to alternate products to stay on top of blight, advises Malcolm Smith

Indeed the SAC no longer recommended phenylamides this season pending results of a review of how they performed against the new genotypes.

"Blue-13 is very aggressive, has a shorter life cycle and can reproduce at both higher and lower temperatures," warned Mr Smith. "Masstock Arable is now suggesting a maximum of seven-day intervals irrespective of the blight pressure."

There were two main approaches to mid-season control - 'alternate' and 'block', he explained. The latter, possibly simpler, involved several successive protectant treatments followed by several sprays using products with curative action.

"There are data to support both techniques. But we always advocate alternate programmes because they're best for resistance management and give you more flexibility when it comes to the maximum number of applications in sequences."

Whatever the approach, if blight pressure became intense, Mr Smith recommended adding cymoxanil to boost the treatments' 'kick-back' curative action.

Unlike cereal septoria sprays such treatments only dealt with blight infections up to about 48 hours beforehand. "But that can be very significant in seven-day schedules," said Mr Smith.

Several curative products, including Merlin and Infinito, had label doses linked to blight pressure, he noted. "That means you can tweak rates accordingly."

Mr Keer believed seven-day intervals had to become the norm. "If conditions are conducive [to blight] you've got to be on narrow intervals and using the better products such as Infinito, Revus and Ranman. They're the three key ones when the going gets tough. Provided intervals stay tight there's no reason why good protective products won't work."

Should intervals become stretched, as they did for many growers last year, Dr Keer advised Curzate for its curative action in tank-mix with the modern protectants.

Valbon appears to offer useful 'kick-back' notes Mr Smith.

"2007 was the first year to give Valbon a true test against strong blight pressure."

Potato fungicides

Phenylamides

• Epok, Fubol Gold + others (metalaxyl-M)
• Galben M, Intro Plus + others (benalaxyl)

Mainly protectant

• Dithane + others(mancozeb)
• Electis, Roxam (mancozeb & zoxamide)
• Ranman (cyazofamid)
• Shirlan + others (fluazinam)

Curative & protectant

• Curzate & others (cymoxanil + mancozeb)
• Infinito (fluopicolide + propamocarb hydrochloride)
• Invader (dimethomorph + mancozeb)

Merlin (chlorothalonil + propamocarb hydrochloride)

• Revus (mandipropamid)
• Tanos (cymoxanil + famoxadone)
• Tattoo (mancozeb + propamocarb hydrochloride)
• Valbon (benthiavalicarb-isopropyl + mancozeb)

Curative mixture addition

• (Option, Sipcam C50) (cymoxanil)