13 March 1999

STILLMUCHTOLEARN

Strobs: should you splash them on all over, or be fussy about which crops you spray? Gilly Johnson heard the answers at a Cambridge conference organised

by the ProCam Group.

THEYRE expensive, and as yet no-one fully understands how they work. So whats the best way to use the strobilurins? Should every cereal crop be given a spray – or should the strobs be targeted to certain sites only?

Dr David Ellerton of ProCam has formed some opinions thanks to company trials and an extensive grower data bank recording results from farmer customers. But he admits the industry has a lot to learn. "Were only at the beginning with strobs as we were in the early 80s, with the triazoles."

Two years down the road since strobs arrived, trials experience has convinced him a targeted approach is the best strategy, but its how to make the decision that is the difficulty.

First, the good news – those much vaunted yield benefits do exist in the real world outside a trial situation. The data from ProCams computerised record system shows strobs gave average 11% and 9% yield increases for wheat and barley, respectively, last season, on commercial farms. But these averages hide a wide variation. Wheat gained from 0.77t/ha to 1.61t/ha (6-13cwt/acre). The spread with barley was not as great, from 0.17-0.83t/ha (1-7cwt/acre).

Variation

Dr Ellerton has analysed the data, seeking clues to whats causing the variation. "It had been thought that the most responsive varieties to fungicides – those with the greatest susceptibility to disease – might give the biggest yield improvements to strobilurins. Sadly, its not that simple. One variety might give the top response on one site in one season, and the least response in another. This lack of consistency gives us a problem."

The next step was to consider soil type. This brought a glimmer of light; his computer records threw up indications of some varieties such as Hereward responding to strobs more on light soil than on heavy land. It was the opposite with Abbot. And with Rialto, there was little response over and above that from conventional fungicides on either heavy or light land.

"This might go some way to explaining the variation between varietal responses to strobilurins," said Dr Ellerton. With rotational position, there were signs that first wheats did best with the new chemistry – but it was a bad season for take-all, which might have skewed this result.

Given that strobilurins can boost specific weight, without adversely affecting grain nitrogens or proteins, he considered it sensible to target strobilurins on cereals where quality standards must be met.

On timing and dose rates, Dr Ellerton was on more certain ground. "Its best to stay with conventional T1 and T2 timings, and a sequence is consistently better than a single spray. At the T3 timing, Amistar is simply the best ear fungicide I have ever seen."

Dr Ellerton might add mbc or a triazole to an Amistar (azoxystrobin) ear spray, depending on the mix of diseases present. He confirmed the manufacturers claim that yield response is related to dose rate, although raising the rate may not always be the most cost effective strategy".

A ProCam wheat trial yielded 7t/ha (2.8t/acre) under conventional triazole management, and when given two three-quarter doses of Landmark (kresoxim-methyl + epoxiconazole), it gave 7.5t/ha (3t/acre). The yield rises to 7.66t/ha (3.1t/acre) after two full doses of Landmark.

Pushed to choose between the two strobs, Dr Ellerton saw a greater response from Landmark as opposed to Amistar in his results, but he points out a higher Landmark dose might explain the difference. "They do seem to be reasonably exchangeable products, provided you choose the right triazole partner for Amistar."

No to mixing

Mixing Amistar and Landmark might not be a good idea. Last years results showed a sequence of two sprays with full-rate Landmark matched a cheaper two-spray sequence of half-rate Landmark mixed with 0.2 litre of Alto (cyproconazole) for performance, giving 0.67t/ha (5cwt/acre) over the triazole-treated control. Half-rate Landmark with Amistar in a two-spray programme, didnt do as well as either, only giving a 0.49t/ha (4cwt/acre) boost.

In sequence, the story is somewhat different. Best strategy in last years wheat trials was a T1 of half-rate Landmark, followed by a T2 of three-quarter rate Amistar mixed with quarter-rate Opus (epoxiconazole). "In some circumstances, a sequence of the two may be better than one or the other."

Finally, strobilurins DO affect harvest date, insisted Dr Ellerton. "We were told they would not delay harvest – that is simply not true. Although the grain was at a low moisture and fit to combine, the stems were green for longer. We measured stem moisture on strob-treated crops and it was 31% when the untreated crop was at 15%, and the triazole-treated wheat was at 25%. Pre-harvest glyphosate might be worth considering for quality crops where you darent risk losing hagberg."

Strob summary: In wheat

&#8226 Landmark/Mantra are similar to Amistar plus a suitable triazole

&#8226 Landmark is better on curative action and on mildew

&#8226 Amistar is better on sharp eyespot and ear disease

&#8226 Mix Amistar with appropriate partner at T1 and T2

&#8226 Mixes of different strobilurins may be inadvisable

&#8226 Strob sequences are best

&#8226 Dont go below a half dose at a single timing, and use at least full dose over a T1/T2 sequence

&#8226 Low rate Landmark with an appropriate triazole could be as good as a higher dose Landmark

&#8226 Use PGRs with Landmark, but might be able to trim PGRs with Amistar

&#8226 After strobs, the following crop may need more N

&#8226 Ensure sufficient trace elements when using strobs

&#8226 Harvest may be delayed, consider glyphosate?

&#8226 Beware – low rates could increase resistance risk. Use alternative modes of action where possible

Source: ProCam Group