4 December 1998

German super-mildews threat to strob fungicides

By Andrew Blake

A FIRST shadow of a cloud over strobilurin fungicides has been detected in Germany, reinforcing the message not to cut doses, says manufacturer BASF.

Repeated low doses of a Landmark (kresoxim-methyl + epoxiconazole) look-alike, Juwel, on heavy early mildew infection are thought to be behind the appearance of strains resistant to the strob kresoxim-methyl.

Similar strains have not been recorded outside a small local area in northern Germany. but the discovery serves as a warning not to experiment with low dose rates, says the company. Growers should not rely solely on strobilurin fungicides for mildew control, it adds.

"There was always a concern that resistance could develop, thats why we went the co-formulation route in the first pace," says marketing manager John Bedford. The German find comes after doses as low as a third were sprayed three times a season, he believes.

John Sellars for Dow Agrosciences notes that a new German tank-mix approval for adding mildewicide quinoxyfen to the strob has recently been rushed through. There are no plans for a co-formulation in the UK, he says. "But the strobilurins offer big yield potential and must be protected. Quinoxyfen has a unique mode of action which is ideal for anti-resistance strategies."

A Zeneca spokesman says mildew control has never been a main claim for azoxystrobin (Amistar) – mixture partners always being recommended.

ADAS pathologist Neil Paveley is disappointed at the news from Germany. "I am surprised it has happened so quickly. My concern is that all the other strobilurins on which we pin so much hope could be compromised before they are even released.

"It really re-emphasises the need to differentiate between low and appropriate doses. If you use a dose that is so low that a substantial proportion of the disease population continues to survive you increase the risk of resistance."

Bob Mills of Banks Agriculture is keen to know more about the German findings. "We were always concerned about the strobilurins single site mode of action. But the response was always dont worry."

A key advantage of Landmark and its morpholine-boosted stablemate Mantra has, until now, been their activity on mildew at the T1 spray timing, he notes. &#42


&#8226 Local German find.

&#8226 Resistant mildew.

&#8226 Beware low doses.

&#8226 Tank-mix need?