28 May 1999

DEBUT OF

QUICK RESISTANCE TEST…

Tests for monitoring

resistance to herbicides and

fungicides will be part of

the IACR exhibit at the

Cereals Event this year.

Louise Impey gets an

update on progress with

procedures for grass weeds

and Rhynchosporium

IACR-ROTHAMSTEDs Rapid Resistance Test, which detects resistance to herbicides in blackgrass, will be displayed at the event for the first time this year.

The new test will be more widely available this summer, after successful evaluation last year by a selected group comprising growers, distributors, agronomists and manufacturers, says Stephen Moss of IACR-Rothamsted.

He also says herbicide resistance to blackgrass now exists in 30 counties across the UK, with more than 750 farms affected. "Weve stopped keeping track of every reported case," Dr Moss admits. "But we know it can occur anywhere."

Concern at rise

The rise in target-site resistance is of greatest concern. "Its more common than growers think and affects the performance of herbicides based on fops and dims."

Resistance in wild oats and ryegrass is also increasing. "We havent tested as many samples but we know both are widespread. Resistant wild oats have been found in 19 counties and ryegrass in 11. But without a national survey, it is impossible to be certain about the extent of the problem."

Dr Moss stresses that the Rothamsted Rapid Resistance Test is not designed primarily for grower use. "The ideal time to do the testing in August/September clashes with harvest and great attention to detail is required. But it is perfect for agronomists, agrochemical distributors or any organisation with basic laboratory facilities. Interpretation of the results is also very important.

The whole test is likely to take about two hours a sample initially, with the time being spread out over several weeks. "It includes collecting, cleaning and counting the seeds, mixing up herbicide solutions, treating the samples and then assessing and interpreting the results. With experience, this time will be reduced to one hour a sample."

Dr Moss believes the new test has advantages over others. "Its certainly quicker than a pot test because the result comes through by September, in time to influence autumn strategies. And its potentially cheaper for the number of herbicides tested.

"It also identifies the mechanisms of resistance, so that an appropriate control strategy can be devised even if a problem is confirmed."

The actual cost for a test will depend on its uptake. "Theres obviously going to be a cost for the equipment needed and for the standard populations required to conduct the test. We are still determining interest in the procedure."

A new simpler system for categorising resistance is also being considered by the Weed Resistance is also being considered by the Weed Resistance Action Group (WRAG), says Dr Moss. "The current rating system has been useful, but it has too many categories and has been modified by many organisations.

"We think that a streamlined version, consisting of four categories would be better."

&#8226 RRR would denote resistance confirmed, highly likely to reduce herbicide performance (previously 5*/4*).

&#8226 RR would be resistance confirmed, probably reducing herbicide performance (3*/2*),

&#8226 R? would be early indication that resistance may be developing, possibly reducing herbicide performance (1*),

&#8226 S would represent susceptible.

"WRAG hopes that all testing organisations will use the new system from this summer," he concludes.

RHYNCHO GIVES RISE TO SNAGS

RESISTANCE in rhynchosporium to some DMI fungicides is difficult to test for because its distribution within a field is not uniform, warns Derek Holloman of IACR-Long Ashton.

"Although we have the technology to detect the problem, we are still not sure of the best way to sample a field so that we can be sure of getting valid results," he says.

Changes in sensitivity to fungicides develop in patches, spreading out from foci, explains Dr Holloman. "And cross-resistance patterns can also make the situation more complicated."

He believes that as modern diagnostic procedures become more commonplace on farms, growers and researchers have to consider how crop sampling should be done. "It would be very easy to walk the classic W shape across a field and not find any resistance in the sample, even though it did exist in the field. So research is now being done on sampling.

"Dont forget that in a farm situation, just one sample has to give the answer."

Growers with rynchosporium resistance will have noticed poor performance of DMI fungicides, such as epixiconazole and flusilazole. "But dont forget that timing of application is also important for good rhychosporium control. So make sure youve discounted all the other reasons for poor control before blaming resistance, says Dr Holloman.

He adds that the problem is widespread in Scotland and wetter areas of the UK. "Its not a wind-borne disease, so it shouldnt spread too rapidly. But where growers have been using DMI fungicides for a number of years, its likely that a resistance problem will exist." &#42