2 March 2001

Weather watching is the key

WATCHING the weather and understanding how septoria develops are the keys to getting fungicide timings right, according to an Aberdeenshire-based agronomist.

"I cannot over-emphasise the importance of understanding how septoria develops," says John Hughes of CSC CropCare. "When disease control goes wrong it is always because growers have not realised how important the weather is in its development."

Knowing how long applications will protect the crop is also crucial, so growers know when the previous dose is running out of steam.

"We are stuck with about 30 days cover with a lot of these sprays, but some products can extend that. Twist or Sphere will give you a little bit extra, but no more than a week."

Most wheat drilled in Scotland last autumn was sown in good time, so fungicide programmes should conform to the conventional timings of a T1 at GS31, T2 at GS37-39 and a T3 at or before flowering, says Mr Hughes. But there are opportunities to tailor that in certain circumstances.

In Morayshire and coastal regions, a T0 mildew spray of spiroxamine or quinoxyfen may be needed. If that is mixed with a triazole such as epoxiconazole or flusilazole at about a third rate then the traditional T1 can be delayed a little, as can the T2 so that it doubles up as both flag and head spray.

"It will depend on temperature and rainfall pattern whether you can extend the flag leaf spray to a shoulder spray."

There is a danger of trying to over-extend the protection provided, he warns.

Whether growers can stretch timings will depend on the weather. Cereal growers should monitor it for septoria control as keenly as potato growers do for blight, he says.

After a period when the crop is sufficiently wet for the disease to spread from leaf to leaf, it takes 200 day-degrees for visible lesions to occur. At GS31 that may be as long as three to four weeks, but at ear emergence just 10 days.

"You have to stop the explosion of disease after an infection period, because if the crop is unprotected or the previous application is starting to run out, you know it is going to come."

A triazole with good kick-back activity such as epoxiconazole, also available in strobilurin mixes, should be applied. At full rate green leaf area can still be saved up to 160 day-degrees after an infection period, he adds. &#42

WEATHER CONTROL

&#8226 Note infection periods.

&#8226 Count day-degrees.

&#8226 Check protection from previous application.

&#8226 Get spray on within 160 day-degrees if needed.

DAY-DEGREES

To count day-degrees add the minimum and maximum temperatures for the previous 24 hours and divide by two.