3 May 1996

Nows the time to spray but note the changes – ADAS

WHEAT crops are now highly vulnerable to septoria infection and any delay in the first spring fungicide could compromise yield and profit. But seasonal complications mean ADASs advice has changed for 1996. Failure to note the changes could lead to unnecessary economic losses.

"The cold, late start to spring has slowed the growth of crops and upset normal leaf development patterns, which could confuse wheat growers and agronomists," says ADAS Cambridge-based plant pathologist Bill Clark. "Crops are much more backward than usual and most have only reached growth stage 31, instead of 32 which is normal at this time of year. Yet disease threatens as septoria spores are about."

A further complication is that plants have one fewer leaf than normal. That means they are now at risk as leaf 3 is emerging. In a more normal year ADAS says it is best to wait until GS32 for leaf 3 before applying the early season fungicide.

"So with plants somehow having dropped a leaf, earlier treatment must be the norm in 1996. Septoria infection is now active on leaves 5 and 6 low down on the plant and these are brushing up against leaf 3. This did not happen last year. There is a significantly greater risk of the highly damaging disease gaining a toehold in the centre of the canopy and threatening yields," says Mr Clark.

The confusion is compounded by the wide spread of physiological growth stages. Some wheats are at GS32 whilst many linger at 31. There are also differences within individual crops, with a wider than normal variation between main stems and secondary tillers. Plants should be dissected to establish which leaves are emerging.

Mr Clark urges growers to get on early with chlorothalonil (Bravo) in a mix with the chosen triazole. "The Bravo component is needed to provide the protection for crops where leaf 3 is not fully expanded, or where there is a lot of variability in growth stage between tillers. As leaf 3 can come out over a 10-day time span, it is vulnerable for a long time so must be protected.

"If the growth stage is not known then my advice is to get on and spray sooner than normal to ensure disease is kept at bay. Failure to do this could have serious consequences. The lower leaves already have septoria on them so leaf 3, which may look healthy, is already infected. If nothing is done now it will provide the `reservoir of spores to threaten leaf 2 and the flag, the most important yield-generating leaves, at a particularly vulnerable stage for the crop."

Compromise treatment

Failure to provide adequate early protection could compromise the effectiveness of reduced rate triazole treatments later in the season. If a comprehensive fungicide programme is not used, or a poor product chosen for the early treatment, cutting rates of triazoles at the flag stage could result in lower yields, Mr Clark suggests.

"The crop may look reasonably clean but still suffer a 1-2% cut in yield. This would not be obvious so no one would know this loss has been suffered. It is easy to make savings of £5-6/ha (£2-3/acre) by cutting rates by a quarter, but 1% less grain worth £115/t off the combine could lead to losses which well exceed the moderate savings achieved."

&#8226 Growth stages late.

&#8226 Leaf 3 emerging early.

&#8226 Rethink Septoria control.

&#8226 Spray now, with protectant.

Hit septoria now to keep crops clean later in the season, advises ADASs Bill Clark.