Strobilurin vs triazole fungicides: QoI and DMI modes of action, resistance and mixtures

QoI and DMI fungicides are two of the most used groups of site specific fungicides, and FRAC describes the sterol biosynthesis inhibitors, which include DMIs, as one of the most potent classes available to growers [3]. They differ in target, movement in the plant, resistance risk and the way FRAC asks them to be used. This guide compares the two groups and explains why products that combine a strobilurin with a triazole are so common, with examples from Huasheng's catalogue.

1. Two different targets

QoI fungicides act at the quinone outer (Qo) binding site of the cytochrome bc1 complex (complex III) in mitochondrial respiration [1][2]. JMPR describes picoxystrobin, for example, as acting by blocking electron transfer at the Qo centre of cytochrome bc1 [5]. FRAC divides the group into Code 11 and Code 11A; within each code all fungicides are cross resistant, but metyltetraprole (11A) is not cross resistant with Code 11 in pathogens carrying G143A [2]. In FRAC's list, azoxystrobin and picoxystrobin are methoxy-acrylates, pyraclostrobin is a methoxy-carbamate and trifloxystrobin is an oximino-acetate [1].

DMI fungicides inhibit C14-demethylase (CYP51) in sterol biosynthesis [1]; difenoconazole, for example, acts by inhibiting demethylation during ergosterol synthesis [6]. FRAC Group 3 includes imidazoles such as prochloraz, triazoles such as tebuconazole, difenoconazole, propiconazole, epoxiconazole and cyproconazole, and the triazolinthione prothioconazole [1]. Because QoIs and DMIs act on different targets, they are not cross resistant, which is what makes them usable together [1][2].

2. Movement in the plant and timing

Purdue Extension separates penetrant fungicides into xylem mobile products, which move upward from the point of entry, locally systemic products with limited movement, and translaminar products, which move through the leaf to the opposite surface but not throughout the plant [4]. It lists DMIs such as propiconazole and tebuconazole as xylem mobile, azoxystrobin as xylem mobile and trifloxystrobin as locally systemic and translaminar [4]. JMPR describes picoxystrobin as having systemic and translaminar movement [5].

QoIs are very effective at preventing spore germination, so FRAC advises using them at the early stages of disease development and not relying on their curative potential [2]. DMIs have some curative action, as JMPR notes for difenoconazole [6], but FRAC still advises avoiding curative DMI applications in cereals and making preventive use the first choice in fruit [3]. Purdue adds that systemic fungicides have limited curative activity and that no fungicide is effective after visible symptoms develop [4].

3. Resistance risk

FRAC rates QoIs as high risk. Resistance is known in many fungi, through target site mutations in the cyt b gene, mainly G143A and F129L, and cross resistance has been shown between all Code 11 fungicides in G143A mutants [1]. F129L gives much lower resistance factors than G143A, and products used according to recommendations still perform well where it occurs [2]. In wheat Septoria leaf blotch, 2025 monitoring found high frequencies of QoI resistance in all European countries monitored [2].

FRAC rates DMIs as medium risk. Resistance is known in various fungi through several mechanisms, including cyp51 target site mutations, changes in the cyp51 promoter and ABC transporters; activity spectra differ widely between DMIs, and it is wise to assume cross resistance between DMIs active against the same fungus [1]. Shifts tend to be gradual: in Europe, the sensitivity of Septoria populations in 2025 stayed in the range of previous years and DMI products performed well when used as recommended, but DMIs are at higher risk where growers have few fungicide classes or commonly cut rates [3].

4. What FRAC recommends

QoI fungicides

• Limit the number of QoI applications in the whole programme, whether solo or in mixtures [2].
• Use solo QoI products in single or block applications in alternation with another cross resistance group [2].
• Use a mixture partner with a different mode of action that controls the disease on its own; mixtures of QoIs only are not an anti resistance measure [2].
• Apply preventively or as early as possible; do not use split or reduced rate programmes [2].
• Crop limits apply, for example no more than two Code 11 sprays in cereals and no more than two QoI sprays against powdery mildew per vine crop [2].

DMI fungicides

• Do not use repeated solo DMI applications on the same crop in one season against a high risk pathogen under high disease pressure [3].
• Where many sprays are made, alternate or mix with an effective fungicide from another cross resistance group [3].
• Use recommended rates: reduced rates can accelerate the shift to less sensitive populations, and many reported cases of resistance were traced to cut rates or poor timing [3].
• Prefer preventive use, and follow crop limits such as a maximum of four applications per season per mode of action in grapevine [3].
• Fungicides do not replace resistant varieties, good agronomy and hygiene [3].

5. Why strobilurins and triazoles are often combined

FRAC's QoI guidelines require a mixture partner with a different mode of action, and note that the partner may also widen the spectrum of activity or provide needed curative activity [2]. A DMI meets both conditions: it is not cross resistant with QoIs and it has some curative action [1][6]. For cereals, FRAC names SBIs and multisites as the non cross resistant partners that remain essential for robust disease management [2]. For soybean diseases it recommends QoIs preferably in mixtures, as formulated mixtures or, where permitted, tank mixes, and makes mixtures mandatory where target site mutations are present; for Asian soybean rust it asks for DMIs always to be applied in mixtures [2][3].

A mixture only protects each partner if both work on the target disease: FRAC defines an effective partner as one that gives satisfactory control when used alone at the rate chosen [2]. Where one partner has lost sensitivity, the mixture leans on the other. In Brazilian soybean rust, where sensitivity has shifted to both groups, Embrapa advises always using commercial or tank mixtures that include multisite fungicides [7].

6. Regional notes

Latin America

In Brazil, reduced sensitivity of Phakopsora pachyrhizi to DMIs, QoIs and SDHIs has been reported, and these three groups make up all registered site specific fungicides for soybean rust [7]. FRAC found high frequencies of the QoI mutation F129L in 2024/25, present at significant levels since at least 2012/13, and no G143A in this pathogen [2]. DMI sensitivity was stable in 2024/2025 after a slight shift several years ago; FRAC links earlier stabilisation to the rule of using azoles only in mixtures and to a crop free period [3]. Embrapa reports newer DMI sensitivity changes affecting control with prothioconazole and tebuconazole, attributed to mutations such as V130A, and advises avoiding sequential and curative applications [7]. In target spot (Corynespora cassiicola), G143A is widespread across Brazil [2].

China

FRAC data from China are limited. Less sensitive isolates of Rhizoctonia solani to QoIs were found at low frequency on potato in Inner Mongolia, Hebei and Gansu in 2017 [2], while DMI monitoring of rice sheath blight (Rhizoctonia solani) from 2015 to 2017 showed a stable and sensitive situation [3].

Europe

QoI resistance in wheat Septoria is now at high frequency across the countries monitored, while DMI sensitivity has stayed within its recent range; FRAC continues to recommend QoIs in cereals only in mixtures with non cross resistant partners such as SBIs or multisites [2][3].

7. Catalogue examples

The table lists QoI and DMI active ingredients from Huasheng's catalogue with their FRAC group and chemical group from the 2026 FRAC Code List [1], and what the cited sources say about each.

The table describes FRAC groups and what the cited sources say. It is not a list of registered uses: which products may be used on which crops, at what rate and interval, is set by each country's registration and the product label.

8. A practical sequence

• List the target diseases and the FRAC groups registered locally for each, with any documented resistance [1][2][3].
• Use resistant varieties and good agronomy first; fungicides do not replace them [3].
• Plan QoI and DMI use preventively, at the early stages of disease development [2][3].
• Use QoIs within FRAC limits and only with an effective partner from another group, such as a DMI or a multisite [2].
• Use DMIs at full recommended rates, in mixture or alternation with non cross resistant fungicides [3].
• Where sensitivity to both groups has shifted, as in Brazilian soybean rust, add a multisite partner [7].
• Avoid curative and repeated solo applications, and follow the local label [3][7].

Frequently asked questions