DMI (demethylation inhibitor) management strategy

Powdery mildew of grape
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)


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.
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 groupCommon nameTrade name
Imidazoles imazalil Baytan Universal (with triadimenol and fuberidazole), Fungaflor (postharvest fungicide), Monceren IM (with pencycuron), Vincit (with flutriafol)
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.
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.


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.