Resistant varieties benefit from Mildew Control

With the threat of mildew at its highest for several seasons growers should consider varietal response to disease and control methods, as well as a spray's cost and efficacy.

"Growers tend to concentrate on protecting the susceptible varieties," says Dow AgroSciences' cereal fungicides specialist Stuart Jackson. "While, this is, of course, a correct strategy trials have shown that in some circumstances the yield response of mildew-resistant varieties to a cost-effective protectant mildewicide could also be economically advantageous."

Trials carried out by NIAB in 2003 and previously by Dow AgroSciences often demonstrated a yield response in mildew-resistant varieties sprayed with a protectant mildewicide at the crucial GS25-32 stage.

Mr Jackson believes those winter wheat varieties with an inherent resistance to disease, and in this case mildew, have a metabolic disadvantage in being 'resistant'. They pay a yield penalty in fighting the disease.

"The use of a carefully timed fungicide such as Fortress (quinoxyfen) could be freeing the energy normally invested in resisting infection into building additional yield potential. This additional yield has frequently been shown to cover the costs of the spray programme."

Early mildew infection reduces rooting and tiller development, and also diverts nutrients away from the developing grain sites at a crucial time for potential yield development.

The fully replicated trials were designed to evaluate the impact and control of powdery mildew. Ten winter wheat varieties, with mildew susceptibility scores ranging from 4 through to an 8, were assessed.

The programme based around Fortress (quinoxyfen) hit the top in the trials in terms of final yield and on margin over inputs (MOI) for every variety.

"The treatment included a T0 spray of Fortress at 0.15 l/ha + Landmark at 0.5 l/ha, followed by Opus at 0.5 l/ha + Bravo at 1.0 l/ha at the T2 timing. It was the lowest cost programme in the trial series at £40/ha compared to the standard NIAB fungicide treatment at £64/ha. A further fungicide programme costing £54/ha was also assessed," says Mr Jackson.

"The cost-effective Fortress-based programme provided better mildew control than the standard NIAB programme over the early, important growth stages. But what was surprising is that it didn't matter what the mildew resistance score was - the highest yield response on each variety came from the Fortress programme.

"Overall, disease pressure was low, so yield response and the lower cost Fortress-based programme heavily influenced the MOI," he concludes.

The greatest disease pressure was seen at NIAB's Yorkshire Headley Hall site. Yields and MOI for all varieties were highest there even though it had the greatest levels of mildew.

"There was a £36.23/ha increase in MOI for the Fortress-based treatment at Headley Hall over the mean of the other sites," says Mr Jackson.

The T0 spray was applied in mid-April that included Fortress-protected varieties against mildew infection. The first signs of infection came in early May, around GS32. Assessments of disease over the following weeks showed quinoxyfen gave over nine weeks protection against mildew infection.