19 March 1999

FRACs strategy to cope with disease resistance

By Andrew Blake

SPRAY strobilurins no more than twice on any cereal crop. That is the key advice from manufacturers in new guidelines (Arable Mar 12) aimed at managing disease resistance, particularly in powdery mildew.

The latest five-point strategy, from the Fungicide Resistant Action Committee, comes after sampling in northern Germany last season found nearly half the wheat mildew spores at one site were strob-resistant.

Tests on field strains of vine downy mildew confirm that cross-resistance exists between the two strobilurins currently available in the UK and three related compounds currently seeking approval.

"We have a new cross-resistance group," says Zenecas Steve Heaney, chairman of FRACs new strobilurin and related compounds (STAR) working group. "Its clear cut." None of the fungicides offer even 50% control of a resistant strain of vine mildew.

The group includes azoxystrobin (as in Zenecas Amistar), kresoxim-methyl (BASFs Landmark and Mantra), Novartiss trifloxystrobin, Du Ponts famoxadone and Rhone Poulencs fenamidone.

None, however, displays any cross-resistance to other fungicides such as the triazoles, morpholines, quinoxyfen (Fortress) or cyprodinil (Unix), stresses Mr Heaney.

Neither has strob resistance yet been discovered in any other cereal diseases, including powdery mildew in barley in the same part of northern Germany.

Several factors are thought to have encouraged wheat mildew resistance in northern Germany. Growers were applying three strob sprays a season, often at less than half dose, with ineffective tank-mix partners on highly susceptible varieties, says Mr Heaney.

&#8226 KEEPING the dose up is important because of the way STAR fungicides work, says Steve Heaney.

In any one cell there may be one mitochondrium with altered DNA making it immune to the fungicide. As long as the dose is kept up to kill the others, it does not have enough energy for the cell to reproduce.

But lowering the dose may allow some of the susceptible ones to survive with enough energy to let the resistant cell multiply, he warns.

1 Apply STARs at manufacturers recommended rate for target disease (or complex) at specific crop growth stage indicated. Lowering doses may increase risk of losing control of large disease populations and for STAR compounds may be critical in delaying resistance evolution.

2 Apply a maximum of two STAR containing sprays per cereal crop.

3 Where strong powdery mildew attacks common (due to location, variety, cultural practice) always apply STARs in mixture with effective dose of partner fungicide from different cross-resistance group (eg triazoles, morpholines, cyprodinil, quinoxyfen).

4 Where strong powdery mildew attacks uncommon always apply STARs, solo or in mixtures, before mildew is established and at recommended rates to maximise disease control and minimise risk of resistance developing through selection pressure on large populations.

5 Where any cereal disease, is already established at application STARs should always be mixed with an effective dose of an effective curative partner.

Most manufacturers endorse guidance

Du Pont, which has not taken part in FRACs STAR meetings, has evidence from work on yeast to suggest that famoxate (the registered trade mark for famoxadone) binds to the mitochondrial target differently to strobilurin, says Andy Selley.

"That may or may not have a role to play in more effective resistance management options." Famoxadone is not active against cereal powdery mildew and will not be used alone in any crop, he notes.

BASF fully endorses the FRAC guidelines, says spokesman Tony Grayburn. "Strobilurin fungicides should be applied at full rate and in combination with partner fungicides offering alternative modes of action.

"Partner fungicides should be applied at robust rates and offer a complementary spectrum of disease control, including eradicant and curative activity where any disease is already established at the time of application."

Novartiss Neil Waddingham, who anticipates a 2000 launch for a trifloxystrobin product, is also fully behind the guidelines.

Asked to comment on fenamidone, Rhone Poulenc spokesman Jean-Marie Gouot referred farmers weekly to Steve Heaney. "I am not sure if fenamidone will be developed on cereals, but the data shows the compound is in the same cross-resistance group as the strobilurins and famoxadone," says Mr Heaney. &#42

Agronomists give cautious thumbs up

ADAS pathologist Neil Paveley believes the FRAC approach is right in principle, but takes issue with the reference to manufacturers recommended rate. "It is important to use the right rate to ensure control. If you have a small readily controlled population you have to ask whether you need a blunderbuss."

Keith Norman, technical director of Velcourt, says the latest advice is sensible, reflecting his own companys policy. "Its what we have been doing already because we want to get the full benefits of strobilurins.

"Anybody using quarter doses more frequently is on a hiding to nothing, because they wont get the best out of them. We always use at least half dose each time, with partner products, so we cant afford to use them more than twice."

Dick Neale of Cambs-based distributor Hutchinson agrees. "Know-ing the basis of the decisions they seem extremely sensible and very fair when it comes to dose rates." The key point is that although no strob resistance has yet been detected in diseases other than mildew, the possibility exists, he says.

Steve Cook of Hampshire Arable Systems reckons he will take the FRAC advice on board. "We were planning to use three sprays at quite reasonable rates – half to three-quarter doses. Now we shall have to evaluate which is the most important timing depending on variety."

For feed wheats the strobs are most likely to be used early. "For milling varieties we shall probably use one at flag leaf with the option to come back again on the ear."

Suffolk-based crop consultant Johan Zethraeus is keen to learn more about STAR resistance. "We need proof that the resistance is different to that for triazoles. Without it, it is difficult to expect farmers and advisers just to buy the dose rate message."