23 January 1999

DISEASE DILEMMAS

Fresh information was unveiled at each of the HGCA Roadshows which wound their way from Chelmsford to Inverness and finished up in Haverfordwest. David Millar caught up with the Cirencester meeting.

IN INVERNESS it was ramularia on spring barley that was the roadshow revelation. In Cirencester ADAS fungicide expert David Jones brought his series of trials with new and old fungicides bang up to date with the latest on the breakdown of strobilurin activity against powdery mildew.

Mildew resistant to strobilurin chemistry has not been identified in the UK but it has in northern Germany where kresoxim-methyl (Landmark) has met its match after repeat low-dose applications over three years. Dr Jones admitted information was still scarce but it appeared the single-step activity of strobilurins against mildew had been countered. Even rival strobilurin manufacturers were now agreeing that all strobilurins would face the same breakdown possibility on powdery mildew in the same way that mbc fungicides encountered similar breakdown shortly after their introduction.

"In practice, the only immediate thing we can say is that in the UK almost all strobilurin use will be in mixtures in almost all situations," said Dr Jones. Azoxystrobin (Amistar) is the only strobilurin currently available in the UK as a standalone strobilurin. "I would be very reluctant to use straight Amistar anyway in wheat or barley except possibly as an ear spray on winter wheat," he added.

Valuable weapon

Wheat mildew had not been a UK problem in 1998 but he expected greater reliance on quinoxyfen (Fortress) in situations where growers anticipated mildew this year. "There is no evidence of any shift in quinoxyfen resistance. Quinoxyfen looks as if it will be an extremely valuable weapon for mildew control."

ADAS fungicide trials for the HGCA and MAFF in 1998 also looked at the strobilurins and their use with triazoles and older chemistry. Among the findings was the side-effect of Amistar use apparently decreasing root lodging. This was not, however, enough to justify reducing plant growth regulator use. Dr Jones added that he had not seen the increased lodging of which kresoxim-methyl was sometimes accused although greater ear weight and green leaves might mean greater risk if growth regulator use was inadequate.

A trial with the highly disease resistant wheat Spark on the low disease Boxworth farm in Cambridgeshire had shown a 1.3t/ha response to Amistar treatment in the absence of disease. Similar use of epoxiconazole (Opus) or Landmark (kresoxim-methyl + epoxiconazole) had not turned out economically worthwhile.

Dr Jones examination of partner products for the strobilurins suggested there is unlikely to be a revival of older chemistry. Highest yield response and most economic returns still came from using the latest triazoles, such as Opus, at reduced rates in conjunction with a strobilurin such as Amistar.

Commenting on the reports from Scotland blaming ramularia for severe blotching and yield losses in spring barley, Dr Jones said it was still not entirely clear that all the blame could be pinned on the disease. There were no similar reports from south of the Border.

Dr Neil Paveley, of ADAS High Mowthorpe, reported on yellow rust control for which the strobilurins havent yet established an economic role. Single spray trials have shown that a full dose of the older triazoles at maybe £15/ha is needed to get the same response as cut rates of the newest triazoles at £10-12/ha. Adding in a morpholine will make the treatments more robust and help stave off resistance.

Spray timing is, however, a key issue for yellow rust control with the evidence clearly pointing to an application as close as possible to flag leaf emergence having the greatest yield response when infection was present at that time. A quarter-rate dose at the most effective timing was better than higher rates applied even just a couple of days either side.

Seed treatment, particularly with Baytan (fuberidazole + triadimenol), could stave off the start of a yellow rust epidemic as could the use of slow rusting varieties such as Hereward, Rialto, Savannah, Equinox and others. However, he reminded growers of the ability of the disease to develop races which overcome resistance – most recently in Equinox, Madrigal and Brigadier.

"Resistance can buy time and save fungicide but is not an excuse for poor vigilance," said Dr Paveley. Growers should continue looking for early evidence of disease foci in their fields, use appropriate doses of agrochemical and protect the first three leaves as they emerge.

Dr Rosemary Bayles, head of cereal pathology at NIAB, advised growers to consider the most important disease for their site when choosing varieties with resistance in mind. NIAB has developed a computer model which demonstrates the relationship between fungicide cost, the disease resistance rating of cereal varieties and gross margin.

Profitable

The system can be used to demonstrate the point at which the cost of chemical disease control makes it more profitable to opt for a particular level of inherent disease resistance in a variety. For example, in a comparison of Savannah, with a mixed bag of disease resistance, against Claire, which is very resistant to most diseases bar mildew, Dr Bayles can show Claire having the advantage more commonly on low yielding sites even when the cost of controlling disease in Savannah is low. On potentially high yielding sites, Savannah is more often likely to have the edge.

Dr Bayles recommended choosing clean varieties with good resistance to maximise gross margins, but balance that resistance against yield potential. Resistance is most important when the disease pressure is high and both yield and the price of grain is low.

Oilseeds research funded by the HGCA includes work carried out in Scotland by the Scottish Agricultural College whose Elaine Booth reported on variety and management trials with oilseed rape. There was no evidence, she said, to deviate from the breeders recommendation to keep seed rates low for the more vigorous composite hybrid varieties.

However, these low rates did make the composite hybrids particularly susceptible to pest attack, especially from slugs but also from pollen beetle which could severely affect pollination if left uncontrolled. She suggested growers lower their spray threshold for pollen beetle to one beetle per five plants to avoid potential difficulty.

Dr Booth also warned home-saved seed users to be aware of the potential for carry over of light leaf spot which had been found by SAC in seed kept from untreated plots.