Fungicides for Vegetable Crops: Demethylation Inhibitors

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» Demethylation inhibitor (DMI) fungicides (FRAC group 3) are an important class of fungicides used to help manage a wide range of fungal diseases in vegetable crops.
» DMI fungicides help reduce fungal growth by interfering with cell membrane development.
» DMI fungicides can have curative action by inhibiting newly established fungal infections.

The mode of action (MOA) of a fungicide indicates how the fungicide works to eliminate or inhibit pathogens. From a biological perspective, most fungicides work by inhibiting the germination of fungal spores or by inhibiting the growth of fungal hyphae (thread-like structures that make up the body of a fungus). From a physiological (biochemical) perspective, fungicides work in several ways, including interfering with respiration and energy production, inhibiting nucleic acid and protein synthesis, and inhibiting enzyme functions.^1,2 The Fungicide Resistance Action Committee (FRAC) classifies fungicides into groups based on thirteen MOA classes. Subgroups within the MOA classes are given FRAC codes indicating specific modes of action that might be susceptible to the development of cross-resistant pathogens.^3,4

Demethylation inhibitor (DMI) fungicides (FRAC code 3) are a subgroup of the sterol biosynthesis (SBI) class of fungicides (MOA - G). DMIs are an important group of fungicides that are used worldwide for disease management in agricultural systems. First introduced in the mid-1970s, this group of broad-spectrum fungicides has been effective against a wide range of fungal pathogen groups, including powdery mildews, rusts, and other foliar pathogens. They are locally systemic to systemic, usually have curative activity, and work by disrupting membrane formation, resulting in the inhibition of hyphal growth.^3,4,5 Triazole, imidazole, pyridine, and pyrimidine fungicides are all in the DMI subgroup^.3,6,7

Many of the DMI fungicides have been used to help manage ascomycete fungi, such as powdery mildews, and basidiomycete fungi, such as rusts, for many years. Oomycete pathogens, such as species of Pythium, Phytophthora, and the downy mildew pathogens, are not sensitive to most DMI fungicides. Triadimefon (Figure 1) and triadimenol, introduced by Bayer, were the first DMI fungicides available for agricultural use. Over thirty DMI fungicides have since been introduced.⁶

Many of the DMI fungicides can be absorbed by plant tissues, especially leaf tissues. They are usually locally systemic, meaning that they are absorbed into and move within leaves but may not be able to move from one leaf into another, and they generally do not have the ability to move downward in the plant within the phloem.^5,6,8 Because they are absorbed into leaf tissues, they can act against fungi that have begun to colonize the tissue in the early stages of infection. This action is referred to as curative activity. Their effect against pathogens often declines later in the infection process as the fungi become established in the plant tissue. Many DMI fungicides can be applied as seed and foliar treatments.⁶

While DMI fungicides all inhibit ergosterol production, the different active ingredients do so in different ways. Therefore, there can be large differences in the activity spectra of the various DMI fungicides.⁸ Therefore, it is important to carefully select the appropriate DMI component of a fungicide product intended to be used against a given pathogen to help successfully manage the disease.

An important reason for knowing the MOA of a fungicide and the FRAC group to which it belongs is to help better manage the development of fungicide-resistant pathogen populations. Applying a fungicide puts selection pressure on the pathogens present. The individual organisms that are most sensitive to the fungicide are less likely to survive and reproduce. Individuals that are less sensitive may then have a reproductive advantage and proportionally increase in the population. Pathogens that have developed resistance to one fungicide are often cross-resistant to other fungicides in the same FRAC group. FRAC groups also vary in their relative risk of resistance development for target pathogen populations.

The Fungicide Resistance Action Committee rates most DMI fungicides at medium risk for the development of fungicide resistance. Resistance to DMI fungicides has been documented, and fungal sensitivity to these fungicides ranges from high, to moderate (partially sensitive), to low (mostly resistant).^4,7 In contrast to other fungicide groups like strobilurins, resistance to DMI fungicides usually develops slowly and gradually. The resistance to DMIs is quantitative, meaning that different levels of resistance have developed as a result of independent mutations in the gene for the target enzyme. Because of this, using application rates at the higher end of the allowed range may help better manage the target pathogens when lower rates are no longer effective. However, the use of this group of fungicides should be stopped if high rates of application are not effective. Repeated applications of these fungicides in situations where disease pressures are high also should be limited. It is better to rotate to other fungicides with different MOAs (different FRAC groups). It is usually recommended that DMI fungicides be tank-mixed with protectant fungicides (FRAC group M), such as chlorothalonil and mancozeb, to help prevent the development of fungicide resistant pathognes.⁷

Resistance management procedures may be mandated on the label of some fungicide products in the form of application restrictions. These restrictions can include the number of allowed sequential applications of a product before an alternate product is applied, the total amount of active ingredient that can be applied in a season, the prohibition against using both soil and foliar applications of the product in the same field, prohibitions against using the product on seedlings or in greenhouse settings, and the requirement to tank-mix the product with another fungicide. Always follow the rates, application instructions, and restrictions provided on the product label.

1 Matheron, M. 2001. Modes of action for plant disease management chemistries. University of Arizona Cooperative Extension.

https://cales.arizona.edu/crop/diseases/papers/dischemistry.html.

2 McGrath, M.T. 2004. What are fungicides? The Plant Health Instructor. DOI: 10.1094/ PHI-I-2004-0825-01. Updated 2016.

3 SBI Fungicides: Introduction and general information. FRAC.

https://www.frac.info/frac-teams/working-groups/sbi-fungicides/information.

4 2021. Fungicide modes of action. Bayer Crop Science Canada.

https://www.cropscience.bayer.ca/en/articles/2021/fungicide-modes-of-action.

5 Mueller, D., Wise, K., Dufault, N., Bradley, C., and Chilvers, M. (Eds.) 2013. Fungicides for field crops. The American Phytopathological Society.

6 Mehl, A., Schmitz, H., Stenzel, K., and Bloomberg, J. 2019. DMI fungicides (FRAC Code 3): Sensitivity stats of key target pathogens, field vs laboratory resistance, and resistance mechanisms. In K. Stevenson, M. McGrath, and C. Wyenandt (Eds.) Fungicide Resistance in

North America, Second Edition. The American Phytopathological Society.

7 Wyenandt, A. 2021. Grower’s guide: Understanding the DMI fungicides (FRAC code 3) in 2022. Rutgers Cooperative Extension Plant & Pest Advisory. March 14, 2021.

8 Robertson, A. and Mueller, D. 2019. Preventative and curative fungicides. Iowa State University, Integrated Crop Management.

https://crops.extension.iastate.edu/blog/alison-robertson-daren-s-mueller/preventative-and-curative-fungicides.

9 Dittmar, P., Agehara, S., and Dufault, N. (Eds.) 2023-2024. Vegetable production handbook of Florida. UF-IFSA Extension.

Websites verified 3/6/2024

For additional agronomic information, please contact your local seed representative.

Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on their growing environment. The recommendations in this article are based upon information obtained from the cited sources and should be used as a quick reference for information about greenhouse cucumber production. The content of this article should not be substituted for the professional opinion of a producer, grower, agronomist, pathologist and similar professional dealing with this specific crop.

BAYER GROUP DOES NOT WARRANT THE ACCURACY OF ANY INFORMATION OR TECHNICAL ADVICE PROVIDED HEREIN AND DISCLAIMS ALL LIABILITY FOR ANY CLAIM INVOLVING SUCH INFORMATION OR ADVICE.

5026_379938 Published 03/27/2024