The development of turfgrass pathogens that are resistant to fungicides is well documented. High risk turfgrass diseases for resistance include dollar spot (several fungicides); gray leaf spot (a few), anthracnose (a few) and Pythium blight (one fungicide ). The main problem revolves around managers continuously using fungicides of same modes of action to target the same disease. The simple solution to avoid resistance is to rotate fungicides having different modes of action (MOA) and/or tank-mixing unrelated compounds. All documented cases of fungicide resistance in turf have been confined to golf courses. I am unaware of any formal reports of fungicide resistance issues in lawns, sports turf or in sod production.
To understand the complexities of fungicide resistance management, I need to preface this discussion by describing the different grouping of fungicides based on modes of action, chemical groupings, and FRAC codes. The Fungicide Resistance Action Committee (FRAC) is an international industry-based organization. A code system was developed by FRAC to assist end users in choosing compounds having different chemical modes of action. Each fungicide is assigned a number and the user simply chooses fungicides that have different FRAC codes when using pre-package mixtures or when tank-mixing or rotating fungicides.
Chemical groupings are based on mode of action (i.e., biochemical sites of action that prohibit growth of targeted fungal pathogens) and each is assigned a FRAC Code number. It is important to note that todays’ fungicides don’t kill pathogens, but instead they operate by preventing their growth and/or reproduction. High risk for resistance fungicides usually target one specific biochemical function, such as respiration, mitosis, enzyme or sterol production, and other chemical and physiological processes. A smaller number of fungicides (usually contact materials) are multi-site –that is they may target two or more different biological functions affecting some aspect of growth or reproduction. Multi-site fungicides seldom develop resistance problems.
The MAJOR grouping are based on mode of action include DMI (demethylation inhibitors; FRAC 3; (e.g., Banner, Bayleton, Eagle; Tourney, Trinity, Tourqe/Mirage and others); SDHI (succinate dehydrogenase inhibitors; FRAC 7; e.g., Emerald, Kabuto Prostar, Posterity, Velista, and Xzemplar); and the strobilurin’s/QoI’s FRAC11; e.g., Heritage, Insignia, Fame, Pinpoint, others); and the phosphites FRAC 33; e.g., Signature and many other related compounds that can be labeled as fungicides or fertilizers). Other notable broad-spectrum fungicides that are unrelated include Daconil (many chlorothalonil generics, FRAC M5); 3336 (other thiophanate generics, FRAC 1); Secure (fluazinam, FRAC 29); Affirm/Endorse (polyoxin D, FRAC 19) ; and Fore (mancozeb, FRAC M3). Then there are those fungicides that only target Pythium-diseases including Banol (propamocarb, FRAC 28); Segway (cyazofamid, FRAC 21); Signature ( fosetyl-Al) and other phosphites, FRAC 33); and Subdue (mefenoxam/metalaxyl, FRAC 4). The below chart assembled by Genesis (adapted from North Carolina State University, Rutgers University and the University of Kentucky extension publications) provides a clear and easy to read summary of FRAC groupings and target diseases, which you should print and keep handy. Fungicide efficacy, based on target disease, also is rated for each disease and fungicide on a 0 (-) to 4 scale where 4= good to excellent level of control. Chart provided below.
Resistance occurs when a fungicide totally fails to control the target pathogen, but reduced efficacy is a prelude to resistance for some fungicides, especially the DMI’s. There are two types of resistance: cross resistance and multi-resistance. Cross resistance occurs when a pathogen is not controlled by fungicides having the same mode of action. For example, dollar spot biotypes resistant to one DMI fungicide theoretically will be resistant to all other DMI fungicides. Multiple resistance is a phenomenon that occurs when a pathogen develops resistance to fungicides with different modes of action. For example, dollar spot biotypes have been shown to be resistant to Chipco 26GT ( iprodione) , Banner MAXX (propiconazole) and 3336 (thiophanate). In the case of DMI fungicides, development of resistance is gradual and may take ten or more years to occur. Conversely, as few as 4-5 consecutive applications of 3336 (with no rotations) can result in dollar spot resistance.
The major source of resistance comes from a tiny sub-set of biotypes, that are “naturally” resistant (i.e., insensitive) to any particular fungicide. The consistent use of fungicides having the same mode of action overtime eliminates sensitive biotypes (on say a green), resulting in dominance of the once obscure insensitive biotypes. The dollar spot fungus is most likely to develop fungicide resistance. This is “genetically” amazing since the pathogen does not produce spores (i.e., there is no sexual or asexual reproduction via spores) and must rely on DNA exchanges by mycelial (tube like body of fungi) contact and fusion (anastomose) in soil, thatch or foliage. Fusion allows for exchange of DNA and thus the source of those genes or mutations responsible for resistance to develop.
Resistant biotypes of the dollar spot fungus first appeared in the late 1960’s and early 1970’s as a result of repeated usage of cadmium (e.g., Caddy and Kromad), mercury-based fungicides (e.g., Tersan OM and PMAS), and benzimidazoles (e.g., 3336 and Tersan 1991). Biotypes of the dollar spot fungus resistant to Chipco 26 (iprodione) and DMI fungicides were first reported in the 1980’s and 1990’s. In 2018, researcher’s at the University of Massachusetts reported on Emerald (boscalid) resistance toward dollar spot in New England and Japan. Other documented resistant problems in turfgrasses have been associated with anthracnose (Heritage [theoretical cross resistance to all strobilurin’s] and 3336 [a benzimidazole]); gray leaf spot (Heritage and theoretical cross resistance to all strobilurin’s , and possibly 3336 [ thiophanate]); and Pythium blight targeted by Subdue (metalaxyl and mefenoxam).
SDHI resistance in dollar spot populations has become a concern in our region, given anecdotal reports of Emerald failures. It should be noted that most resistance problems, when targeting dollar spot, have occurred in New England and the Upper Mid-west. Resistance in dollar spot populations is less common in the mid-Atlantic; conversely, resistance to Pythium blight (via metalaxyl) and gray leaf spot (via azoxystrobin) are more common in the mid-Atlantic..
Emerald was the first SDHI fungicide marketed, has a very narrow spectrum (i.e., mainly targets dollar spot and dead spot), and has historically been highly effective. Technically, if there is resistance to one SDHI (e.g., Emerald) there will be cross resistance to other SDHI’s like Kabuto, Velista, and Xzemplar (note ProStar has no activity on dollar spot and is used mainly to target brown patch and fairy ring). The unusual aspect of newer SDHI’s is that they have different chemical structures and most (other than Emerald and ProStar) target several different diseases. Despite potential dollar spot resistance problems with Emerald, currently I am not aware of field reports of cross resistance to Kabuto, Velista or Xzemplar. Laboratory studies, however, have demonstrated that cross resistance among SDHI’s can occur. Hence, superintendents should take the potential threat for dollar spot-resistance among SDHI’s seriously, and implement sound resistance management strategies.
Resistance Management: The key to resistant management programs are to minimize the use of high risk compounds, and rotate and/or tank-mix fungicides of varying modes of action. Hence, a simple rule is to minimize the use of, or in the case of confirmed resistance, to avoid high risk fungicides and their mode of action relatives, especially at times when high risk diseases are active. Imposing a combination of resistant strategies uniformly is necessary to achieve their full biological effect including : 1) tank-mix and/or rotate fungicides or applying pre-pack mixtures with diverse modes of action; 2) restrict the number of applications of a high risk fungicides per season; 3) use manufacturers’ recommended dose; 4) employ integrated pest management techniques; and 5) intermittently apply low risk and appropriately labelled fungicides like Daconil (chlorothalonil), Secure (fluazinam), and Fore (mancozeb). Fore may be a “weak sister”, but it has its place in resistance management programs targeting dollar spot, gray leaf spot and Pythium blight . Simply stated: change the batting order often.
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Summary of Key Points:
Fungicide resistance problems are restricted to golf courses where fungicides are used routinely. There are no formal reports of fungicide resistance in lawns, sports turf or sod production.
Disease pathogens most likely to develop a resistance problem include dollar spot (most common), anthracnose, gray leaf spot and Pythium blight.
Fungicides most likely to develop resistance include 3336 (and other thiophanate generics); DMI’s; strobilurins/QoI’s; Subdue (mefenoxam/metalaxyl); Chipco 26GT (theoretically Curalan [vinclozolin]; and SDHI’s.
Preventive applications of fungicides using different FRAC groups (i.e., modes of action) are most effective in dealing with high risk diseases such as dollar spot, gray leaf spot and anthracnose.
Tank-mixings, rotating and use of pre-pack mixtures having varying modes of action (using FRAC Codes) vastly reduces the potential for resistance. Occasional rotation or tank-mixing low risk fungicides like Daconil, Fore and Secure further minimizes resistance.
Avoid high risk fungicides when dollar spot, gray leaf spot or anthracnose are active.
Use of high risk fungicides curatively can accelerate development of resistance.
Curative sprays should include two different modes of action (see FRAC Codes); preferably in combination with a low risk fungicides like Daconil, Secure and Fore (except anthracnose).
