Demethylation inhibitor (DMI) [Group 3] resistance management strategy

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

Updated 2023, R. Beresford and M. Christie

Introduction

The purpose of this guideline is to help maximise the useful life of fungicides in the Fungicide Resistance Action Committee (FRAC) Group 3 for controlling plant diseases in New Zealand by providing relevant information about the nature of pathogen resistance to this group and guidance on practices that may prevent or reduce the development of resistance.

Disclaimer:

The fungicide and bactericide resistance information in the mode of action group table, the mode of action-based strategies and the crop-based strategies is intended to help manage the effects of pesticide resistance on plant disease control by agrochemicals in New Zealand. The New Zealand Committee on Pesticide Resistance (NZCPR) is a subcommittee of the New Zealand Plant Protection Society (NZPPS). The following parties: NZCPR, NZPPS, any people named in the strategies and any organisation the people may be affiliated with, do not guarantee that the resistance information is complete, consistent, accurate or up-to-date, or that resistance will not develop, or that the degree of resistance will decrease, as a result of the resistance information being used, and the parties accept no liability for loss or damage arising from the use of the resistance information contained in these strategies.

Also see the separate crop-based strategies for use of DMIs against Venturia inaequalis (black spot; scab) in apples, here, against Monilinia spp. in Summerfruit (here) and against Erysiphe necator (grapevine powdery mildew) here.

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, C14-demethylase (erg11/cyp51). They show varying degrees of systemic curative activity and varying degrees of protectant activity.

Since their introduction in 1969, a large number of new DMI active ingredients have become available. In the absence of resistance they are highly effective against powdery mildews, rusts and smuts, as well as a wide range of other ascomycete and imperfect fungi (Table 1).

**Table 1:** Pathogens and crops targeted by DMI fungicides in New Zealand.
Pathogen	Disease	Crops
Horticultural crops
Colletotrichum spp.	Anthracnose	Avocados
Colletotrichum (Glomerella) spp.	Leaf spot	Ornamentals
Erysiphe cichoracearum	Powdery mildew	Cucurbits
Fulvia fulva	Leaf mould	Tomatoes
Monilinia spp.	Brown spot	Stonefruit
Mycosphaerella fragariae	Leaf spot	Strawberries
Penicillium spp.	Blue mould	Citrus
Podosphaera leucotricha	Powdery mildew	Apples
Puccinia chrysanthemi	Rust	Chrysanthemums
Puccinia horiana	White rust	Chrysanthemums
Sphaerotheca fuliginea	Powdery mildew	Cucurbits
Sphaerotheca pannosa	Powdery mildew	Roses
Uncinula necator	Powdery mildew	Grapes
Venturia spp.	Black spot	Pipfruit
Sclerotium cepivorum	White rot	Onions
Erysiphe polygoni	Powdery mildew	Peas
Cereal Crops
Blumeria graminis	Powdery mildew	Wheat, barley
Leptosphaeria nodorum	Glume blotch	Wheat, barley
Mycosphaerella graminicola	Speckled leaf blotch	Wheat
Oculimacula yallundae and O. acuformis	Eye spot	Wheat, barley
Puccinia coronata	Crown rust	Oats, ryegrass seed crops
Puccinia hordei	Leaf rust	Barley
Puccinia graminis	Stem rust	Wheat, oats, ryegrass seed crops
Puccinia recondita	Leaf rust	Wheat
Puccinia striiformis	Stripe rust	Wheat
Pyrenophora teres	Net blotch	Barley
Ramularia collo-cygni	Leaf and awn spot	Barley
Rhynchosporium secalis	Scald	Barley
Tilletia tritici	Bunt	Wheat
Ustilago avenae	Loose smut	Oats
Ustilago hordei	Covered smut	Barley, oats
Ustilago nuda	Loose smut	Barley
Ustilago tritici	Loose smut	Wheat, barley
Gloeotinia temulenta	Blind seed disease	Ryegrass seed crops

**Table 2:** DMI fungicides marketed in New Zealand.
DMI group	Common name	Trade name
Imidazoles	prochloraz	Mirage, Octave, Sportak, Trump, Curator, Standbye, Varicur, Prosper, Bolide
Piperazines	triforine	Saprol
Triazoles
	cyproconazole	Alto, Beetrizole, Cypro, Escolta, Garrison Rapid, Mondo, Rivera
	difenoconazole	Cannon, Difference, Digger, Difenol, Divino, Dyfen, Glacier, Score, Revus,
	epoxiconazole	Accuro, Allegro (with kresoxim methyl), Adexar, Avior Gold, Agpro Epoxiconazole, Bolide, Fortify, Opus, Stellar
	flutriafol	Novall, MegaStar
	myclobutanil	Validus
	paclobutrazol	Avocet, Payback
	penconazole	Citrara, Pearl, Pendant, Topas
	propiconazole	Tilt, Aurora, Headway Maxx, Condor, Spotless, Prop-P
	prothioconazole	Aviator XPRO, Caley Iblon, Fandango (with fluoxastrobin), Proline, Raxil Star, Vitalis, Prosaro, Delaro, Joust, Kestrel, Prothago, Quilt, Pilot, Thiozole
	tebuconazole	Axis Gold, Capri, Compass, Custodia, Prosaro, Folicur, Orius, Raxil, Hornet, et al
	triadimenol	Luna Devotion, Vandia, Citadel, Tribute, Jupiter, Whiteout

Current status of DMI resistance

DMIs have a site-specific mode of action and repeated use over a long period of time appears to be eroding the sensitivity of many plant pathogens to them. 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. Instances where a high degree of DMI resistance has decreased disease control have been found in New Zealand for some pathogens, e.g., Venturia inaequalis (apple black spot or scab), Erysiphe necator (grapevine powdery mildew), Monilinia fructicola (brown rot of stonefruit), Pyrenophora teres (net blotch of barley) and Tapesia yallundae (syn. Pseudocercosporella herpotrichoides (eye spot of wheat).

There is considered to be cross-resistance amongst all the active ingredients within the DMI fungicides (Table 2), although empirical evidence suggests resistance affects different members of the group differently and older DMIs tend to be more affected by resistance than newer ones. 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 (directional selection). The detection of strains with decreased sensitivity does not necessarily imply a loss of disease control in the field. Alternating or mixing fungicides within this group is not a suitable strategy to avoid resistance development.

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.

**Table 3:** DMI use guidelines for specific crops.
Crop	Disease	Recommendations
Apples and pears	Black spot, powdery mildew	Maximum of two DMI-containing products per season and apply only in mixture, with a broad-spectrum protectant fungicide that is not at risk from resistance, at full label rate, preferably confined to the period from tight cluster to second cover. Minimise use of penconazole and flusilazole in all regions except Otago and minimise use of difenoconazole in Hawke's Bay. In Otago only, a maximum of four DMI-containing applications may be made in extremely wet (high risk) seasons. No DMIs should be used in any orchards with suspected DMI resistance (e.g., where disease control has been poor or previous DMI usage has been high). DMI applications targeting powdery mildew control count towards the maximum number of DMIs for black spot resistance management. Use DMIs at full label rate before disease has become established. Do not use when severe black spot has developed.
Grapes	Powdery mildew	Maximum of one DMI application per 12 month period, plus one winter application for vine trunk disease control, if necessary. New Zealand Winegrowers’ recommends a maximum of one application per season in tank mix with sulphur.
Stonefruit	Brown rot	Maximum of two DMI applications, preferably mixed with a protectant fungicide. Restrict use to one part of the growing season, either during flowering or pre-harvest. Ensure full coverage of foliage.
Cucurbits	Powdery mildew	Maximum of two DMI applications per season, preferably mixed with a protectant fungicide. Use non-DMI protectant fungicides regularly to reduce the rate of development of powdery mildew.
Cereals	Various	Maximum of three DMI applications, including seed treatment, per crop, at full recommended label rates. Use of a non-DMI fungicidal seed treatment is preferred to control bunt, smuts, net and spot blotch and seedling rots/blights. Where powdery mildew is expected to be severe, use DMIs in conjunction with alternative products.

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.

Acknowledgements

This strategy includes information from The Fungicide Resistance Action Committee (FRAC) about mode of action groups, the occurrence of resistance worldwide and resistance management recommendations.