Disease management re-evaluated

08-09-2010 | Sandra Taylor

Disease management in cereal crops is changing in relation to crop-management practices and trials are showing a yield response to later season fungicide applications.

Nick Poole, research co-ordinator with the Foundation for Arable Research (FAR), believes there is a case for looking at disease control differently given the increasing use of irrigation and this country's crop disease status.

New Zealand is different to the UK and Europe in that the disease Septoria leaf blotch is not endemic or at the same levels experienced elsewhere and milling wheat is usually not in the ground until May, also reducing its susceptibility to disease.

Leaf rust is a problem in many cultivars and this has resulted in disease building up later in the season.

This means that in dryland and irrigated crops there is potential for disease-control programmes to begin later than has been standard practice.

Traditionally, the first fungicide treatments (T1) have been applied at growth stage (GS) 31-32, as this was based on the UK practice to control Septoria, but as this disease is not such a problem in NZ, there is potential for this treatment to be delayed.

Poole says irrigation has changed the disease profile of cereal crops, as the more water available the longer the green leaf is retained, the higher the humidity in the canopy, and the greater the disease pressure. Potentially the canopy of an irrigated crop will also run longer so there is more opportunity for disease to build up.

Thus irrigated crops need more protection later in the season, particularly during the extended grain-fill period.

"At the moment we are probably not adequately protecting our crops at grain fill," he says.

Poole says realisation is growing that the traditional reduced rate T3 fungicide application, on the ear at flowering (Growth Stage 61-69) does not have the persistence to cover a full eight to 10 weeks of grain fill - particularly on better soils with adequate irrigation.

Trials done in the past two seasons have looked at applying an extra fungicide treatment later in the season to protect susceptible wheat grown under irrigation. These trials showed increased yields of an average .3t/ha.

"This is worth having providing you don't fall foul of withholding periods."

At FAR's international conference in Ashburton, Poole said the problem with T4 sprays (T4 using the UK numbering system, but the third spray if the programme started at flag leaf) was the possibility of contravening the withholding period of the fungicide.

While work continues on the role of the T4 spray, FAR is also looking at increasing the rates of active ingredient used on the ear in order to boost the persistence of the ear wash spray.

In dryland crops, particularly where more resistant cultivars are being used, the effect of later disease build-up and a truncated grain-fill period often means a single fungicide application at flag leaf is the most cost-effective option.

Similar findings have been made in irrigated barley crops in South Canterbury, with evidence that the second fungicide applied at awn emergence (GS49) needs to be robust enough to protect the crop from late epidemics of Ramulari. This disease may not become apparent until 40 days after the traditional awn emergence spray.

The recent release of a new family of fungicides on to the market may be a significant step forward in the management of irrigated cereal crops as these succinate dehydrogenase inhibitors purport to offer an increased period of protection.

NZ growers will get the opportunity to try the chemicals this spring with the release of the Syngenta product Seguris Flexi.

Poole says initial results from trials with the product look promising, particularly as one of its key strengths is the control of leaf rust.

While there are resistance risk issues with these new active ingredients, products from this group of chemicals could fit with the need to control disease over the longer, irrigated grain-fill periods.

This "re-evaluation of fungicides" programme is a continuation of FARs work on a two-spray "straddle" programme. Six seasons of work illustrated that this programme, based on fungicide applications at before and after flag leaf emergence with a second, higher-rate application to the ear and flag combined, was better or equal in profitability to the traditional three-spray programme. This three-spray programme centred on the flag leaf with lower dose applications before and after flag.

Poole says more recent evidence suggests that, with the exception of stripe rust susceptible cultivars, it may be possible to delay disease management programmes until flag leaf.

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