• Introduction
• Herbicides
• Insecticides & miticides
• Fungicides & bactericides

• 
• Fungicide & bactericide resistance
• Management principles
• Anilino-
  pyrimidines
• Benzimidazoles
• Carboxylic acid amides
• Demethylation inhibitors
• Dicarboximides
• Dodine
• Morpholines
• Phenylamides
• Qo inhibitors
• Streptomycin
• NZ Plant Protection Society

DMI resistance is common in powdery mildew
Uncinula necator in overseas grape crops

DMI (demethylation inhibitor) management strategy

R.M. Beresford
HortResearch, Private Bag 92169, Auckland, New Zealand

(Revised August 2003)

Introduction

These guidelines are designed to avoid or delay the development of resistance to the DMI group of fungicides in New Zealand. They have been developed from previously published information (Gaunt et al. 1996) in consultation with the New Zealand agricultural chemical industry (Agcarm Inc.) and are based on recommendations from the Fungicide Resistance Action Committee (FRAC) in Europe.

DMI product perspective

DMIs are also known as sterol biosynthesis inhibitors (SBIs) or ergosterol biosynthesis inhibitors (EBIs). DMIs are a subgroup of SBIs that are chemically diverse, but all have a common mode of action in disrupting sterol synthesis at a single biochemical site. Another group of SBIs, the morpholines, have a different mode of action and have a separate resistance management strategy.

Since their introduction in 1969, a large number of new DMI active ingredients have become available. They are highly effective against powdery mildews, rusts and smuts, as well as a wide range of other ascomycete and imperfect fungi (Table 1). They show varying degrees of systemic activity. There is potential cross-resistance amongst all the active ingredients within the DMI fungicides (Table 2).

¹Registered as a plant growth regulator.

Current status of DMI resistance

DMIs have a site-specific mode of action and there is a risk that excessive use will lead to resistance development in pathogen populations. In the laboratory, resistant strains have readily been found in a wide range of pathogens, but in the field, resistance has been reported for only a few species. In relation to their very widespread use in many crops worldwide, field resistance to DMIs has developed relatively slowly compared with some other fungicide groups.

The genetic basis of resistance to DMIs is complex and poorly understood. In many cases it appears to be polygenic and develops as a gradual decrease in sensitivity to the fungicides. The detection of strains with decreased sensitivity does not necessarily imply a loss of disease control in the field. Loss of control depends on the level of resistance and the frequency with which resistant strains occur in the pathogen population. Cross-resistance between different DMI active ingredients appears to be inconsistent, as does the degree of activity of different DMIs against different pathogens. However, alternating or mixing fungicides within this group is not a suitable strategy to avoid resistance development.

Reduced sensitivity to DMI fungicides has been reported in New Zealand for Monilinia fructicola (brown rot) in stonefruit, Pyrenophora teres (net blotch) in barley and Pseudocercosporella herpotrichoides (eye spot) in wheat, although this latter report has not been confirmed. Overseas occurrences of resistance indicate that in New Zealand, particularly careful management of DMI use is required in apples against Venturia inaequalis (black spot), in grapes against Uncinula necator (powdery mildew) and in cucurbits against Sphaerotheca fuliginea (powdery mildew). Resistance in blue mould (Penicillium spp.) in citrus is common overseas.

Resistance management strategy

Observe manufacturers' recommendations for application rate and timing for specific products and the maximum numbers of applications of DMI products for specific crops (Table 3). Exceeding the maximum number of applications per season will increase the risk of resistance development. Wherever practical, apply DMIs in mixture with an effective dose of a fungicide from a different cross-resistance group. Apply DMIs preventatively when disease levels are low, but disease risk is high. Where DMI's are used to control more than one pathogen in a crop (e.g. Venturia inequalis and Podosphaera leucotricha in apples) the maximum numbers of applications allowed per season apply to the whole crop, not to each individual disease.

Implementation recommendations

Product labels should include a statement indicating that there is a risk of resistance development if excessive numbers of DMI applications are made. Some DMI products currently give inadequate resistance management statements on their labels.

Reference

© 2007 New Zealand Plant Protection Society (Inc.)