Identification of effective strategies for mixing novel and conventional fungicide groups to control powdery mildew in wheat

Abstract

This was a six month project the aim of which was to test a limited number of wheat powdery mildew isolates that had clearly defined differences in sensitivity to conventional fungicides for their sensitivity to novel fungicide groups. Six isolates were tested using a detached leaf method to determine the sensitivity of the isolates to the conventional fungicides tebuconazole, fenpropimorph, fenpropidin, and to the novel fungicides, azoxystrobin, kresoxim-methyl, spiroxamine, cyprodinil and quinoxyfen. EC50 values were determined by fitting dose response curves for each of the isolates. Based on the patterns of fungicide resistance determined, mildew control strategies were developed to minimise the risk of fungicide resistance.

The results showed that there was a wide range in sensitivities within the wheat mildew population to the morpholine fungicides, to the triazole fungicides and to spiroxamine. The isolates tested were all found to be highly sensitive to the fungicides azoxystrobin, kresoxim-methyl, cyprodinil and quinoxyfen.

A cross resistance pattern between the morpholines and spiroxamine was determined in the work. This means that these fungicides should be regarded as belonging to the same resistance group and would not form an effective anti-resistance strategy if used in mixture or in alternation. There were no other cross resistance patterns determined in the work but the number of isolates tested was small, and did not include the newly identified resistant isolates to the strobilurins or isolates resistant to quinoxyfen. More work would be needed to see if there is any cross resistance between the strobilurins and other chemistry or quinoxyfen and other chemistry. There are no reports of resistant strains of mildew to cyprodinil.

In conclusion, the use of mixtures would appear to be one of the most effective anti- resistance strategies in wheat powdery mildew. The mixing partner should come from a different fungicide group and should be used at a rate where it gives effective control. Particular care should be taken when treating crops that are already heavily infected with mildew when a high dose of an eradicant fungicide should be applied in a mixture.

There is no evidence that the use of low doses increases the risk of resistance to conventional chemistry such as the morpholines or the triazoles, unless they are used as part of multiple split applications. There may be a risk, however, associated with the use of low doses of strobilurins and until this is further investigated they should be used at an effective dose. The best evidence available at the moment would also point to them always being used in mixtures.

There is evidence that multiple applications of morpholines and triazoles increases the risk of resistance, and the number of applications of strobilurins is restricted to two per cereal crop. Quinoxyfen is restricted to use in the early part of the season and this is a particularly effective approach to managing resistance to this product.