31 October 1997

Crop differences help in septoria spray strategy

Septoria risk and spray

buffer zones were two

topical issues at the autumn

meeting of the Arable

Research Institute Association.

Andrew Blake reports

CROP structure as much as rainfall and disease pressure determines the ability of wheat crops to resist to Septoria tritici attacks, according to an IACR-Long Ashton researcher.

Most modern varieties are fairly short but differ a lot in their growth habit, Darren Lovell told a meeting of the Arable Research Institute Association. Depending on the weather such differences can be exploited to fine-tune spray decisions, he believes.

MAFF-funded LARS work has shown rain splash is not always needed to spread disease to the yield-building upper leaves. Horizontal spore transfer from infected leaves to those emerging just below must also be taken into account.

The key is the amount of overlap between lower lesion-ridden foliage and the newly emerging top three leaves. That depends on variety – the overlap in Riband is greater and lasts longer than in a less susceptible variety like Apollo.

This means some varieties with the right growth habit can outrun the disease. But recent experiences with 18 varieties show the weather still has an important role.

Escape disease

In 1995, during a dry April and May, the disease remained in the lower leaves and the upper ones escaped infection. A year later, when it was very wet, events were different. "The pathogen grew faster than the crop and was above the emerging flag leaf in four varieties and close to it in several others."

Big differences in the risk to flag leaves emerged between varieties, with Cadenza and Hereward at least risk. In 1995 Cadenza required a 40cm (15.7cm) high rain splash to induce flag leaf septoria, he notes. "In contrast, only a light dew was needed to cause infection of the flag of Chianti in 1996."

The findings could aid spray decisions, Mr Lovell believes. Two 10min monitoring sessions a week to check the height of the highest septoria lesion in crop samples should be enough to quantify the risk in a particular variety, he says.

This experimental electronic meter, developed in Poland with LARS collaboration and able to monitor splash density as well as height, could lead to practical in-crop equipment to guide spray decisions. But canopy inspection will still be vital, says Darren Lovell.

SEPTORIA RISK

&#8226 Strong canopy effects.

&#8226 Rain splash not essential.

&#8226 Scope to out-run disease.

&#8226 New monitoring approach.