Decision dilemma

27 November 1999

Want to know more about strobilurins?

DO you have the answers?Come along to one of our Strobilurin Question Time sessions where the experts will be on the spot to help. Weve invited a number of leading agronomists and specialists to field the questions set by you.

In the hot seat will be Dr Mike Carver from the ARC, Doug Stevens from Morley, Keith Norman from Velcourt, Simon Phillips from New Farm Crops and Tom Robinson from Novartis, among others.

Organised by Crops and Novartis, these 9.30am-12 noon meetings are FREE to Crops readers, and this includes a hearty breakfast, from 8.30am. Just choose the most convenient venue.

Some of the commonly asked questions are:

&#8226 What rate should I use?

&#8226 When should I spray?

&#8226 Whats the latest gen on strobs as an ear spray?

&#8226 What nozzles, spray quality and water volumes should I choose?

&#8226 How should I manage the "greening" effect?

&#8226 Whats the interaction between strobs and lodging?

&#8226 How much nitrogen do I use?

Ask those key questions in person or by proxy

YOU can come along and ask your question in person – or ask by proxy, using the registration form here. Either way wed like to hear your questions in advance, so make sure you fill in the relevant section.

To register your interest in attending – or simply to throw your individual questions into the pot – simply complete the details now and send it to the FREEPOST address, or fax it to Novartis on 01223 493847.

Entry price is £10, but Crops readers who register before 15 Jan can attend FREE.

Decision dilemma

Its time to turn previous thinking on fungicide sprays on its head. Lucy Stephenson reports.

SPRAY decisions are easy when its just a case of controlling diseases that you can see. Triazole fungicides which have excellent eradicant activity fit the bill perfectly.

But choosing to use a strobilurin that prevents disease presents more of a dilemma: "You have to get the timing just right with protectant fungicides. You cant spray a week late with a strobilurin: if disease is there the strob wont wipe it out," says Bill Clark of ADAS.

Strobs also offer something more – that extra yield potential. This complicates the decision. So how do you work out whether an application will be worthwhile?

Common sense can be misleading. Take these two misconceptions:

&#8226 high-yielding crops need more fungicide to protect their potential

&#8226 thin crops wont yield well so need less fungicide.

"Both of these seem obvious but both are completely wrong. In fact there is no relationship between yield potential and the fungicide needs of the crop," says Mr Clark.

Whats important is the difference between the yield if left untreated, and the yield if treated. This difference is affected by the condition of the crop, the variety, and the dose rate.

Lower leaves are more valuable in thin crops, because they capture more of the light that penetrates to the bottom of the canopy. Spraying at first or second node is critical to keep lower leaves in good condition, yet this is just the time that the farmer walks through the thin crop and decides to cut back on inputs, says Mr Clark.

In thick crops the flag leaf and the ear intercept three-quarters of the light available; older leaves lower down are less important. But beware cutting rates: disease progresses more rapidly through thick crops. Instead of guessing the yield of a crop, farmers should assess how responsive the variety is likely to be to a fungicide. "For responsive, read dirty, says Mr Clark.

Responsive varieties need more fungicide. With triazoles, the appropriate dose is easy to see. In ADAS trials, yield flattened out at about 7.5t/ha with the full rates of Alto (cyproconazole) and Opus (epoxiconazole) applied to responsive and unresponsive varieties respectively. With a mixture of triazole and strobilurin – Landmark (epiconazole plus kresoxim-methyl), the yield plateau of 9t/ha came at full label rate for responsive varieties and at three-quarter rate for unresponsive varieties.

However, with strobilurin alone – Amistar (azoxystrobin) – full rate yielded about 8t/ha but with no sign of yield levelling out. At double the dose, yield reached 9t/ha but there was still no sign of having reached the optimum.

Although this dose isnt legal, it gave an economic return. The increasing yield is nothing to do with disease control, says Mr Clark. "We still do not know exactly how strobilurins work."

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