DMI (demethylation inhibitor) management strategy

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

R.M. Beresford
Plant & Food Research, Private Bag 92169, Auckland 1142, 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).

**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	imazalil	Baytan Universal (with triadimenol and fuberidazole), Fungaflor (postharvest fungicide), Monceren IM (with pencycuron), Vincit (with flutriafol)
Imidazoles	prochloraz	Mirage, Octave, Sportak
Piperazines	triforine	Saprol
Pyrimidines	fenarimol	Rubigan
Triazoles	bitertanol	Baycor
	cyproconazole	Alto
	difenoconazole	Score
	epoxiconazole	Allegro (with kresoxim methyl), Opus
	flutriafol	Vincit (with imazalil)
	flusilazol	Nustar
	myclobutanil	Super Shield, Systhane
	paclobutrazol	Cultar¹
	penconazole	Topas
	propiconazole	Tilt, bumber, propiphar, pro-P
	prothioconazole	Fandango (with fluoxastrobin), Proline
	tebuconazole	Folicur, Orius, Raxil, Axis
	triadimefon	Bayleton, Miltek
	triadimenol	Assure (with carbendazim), Baytan Universal (with imazalil and fuberidazole), Cereous, Tribute

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.

**Table 3:** DMI use guidelines for specific crops.
Crop	Disease	Recommendations
Apples and pears	Black spot, powdery mildew	Maximum of four applications, which include a DMI active ingredient per season, preferably confined to the period from tight cluster to second cover. DMIs should always be mixed with a protectant fungicide, such as captan, mancozeb, metiram or dodine. DMIs applied specifically against powdery mildew (e.g. Bayleton) count towards the maximum number of DMIs allowed per season.
Grapes	Powdery mildew	Maximum of four DMI applications per season, preferably mixed with a protectant fungicide. No more than two DMIs should be applied alone per season.
Stonefruit	Brown rot	Maximum of three 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.

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

Gaunt RE, Elmer PAG, Manktelow D, Moore M 1996. Demethylation inhibitor (DMI) resistance management strategy. In: Boudot GW, Suckling DM ed. Pesticide resistance: prevention and management. New Zealand Plant Protection Society, Rotorua, New Zealand. Pp. 152-158.