Fungicides can be classified using several criteria depending on the information we are looking for. For instance, fungicides can be divided based on when they are applied, i.e. preventative, curative, or both. In this case the preventative products will not kill the fungus or its spores; they work by not allowing the germinating spores to enter the plant. Curative products will inhibit spore germination or in some other way control a pre-existing occurrence of a pathogen. A second way that fungicides can be divided is based on whether or not they are contact or systemic in nature. Contact fungicides need to be on the plant before the pathogen arrives in order to keep the pathogen from infecting the plant. Systemic products may be transported throughout the plant and many can control a pathogen that is already located within the plant.
These two classifications are important when making decisions on what products to use and what results can be expected, however they should not be the only consideration. The mode of action (MOA) of a fungicide refers to the way it physiologically affects the target pathogen. The MOA becomes extremely important when we consider that many pathogen populations have become resistant to various products in our pesticide storage shed.
Original fungicides were contact products and the first incidence of resistance was not recorded until the 1960’s. Once systemic fungicides became available they quickly became the products of choice in many situations due in part to their specific activity and longer re-treatment interval. As the use of systemic products increased so did the incidences of resistance.
In an attempt to slow down the spread of resistance, the Fungicide Resistance Action Committee (FRAC) was formed in the early 1980’s. The purpose of FRAC is to evaluate fungicides and the factors associated with the buildup of resistance to better understand the mechanisms of resistance. One of the actions of FRAC was to group the fungicides based on their respective MOA and determine their risk of resistance. The contact fungicides are grouped into the M category indicating their multi-site contact activity and are considered to have a low risk of resistance. Risk of resistance within the other groups depends on different factors and some groups are more prone to resistance than others. Products within these groups affect one or more physiological processes including nucleic acid synthesis, mitosis and cell division, respiration, amino acids and protein synthesis, signal transduction, lipids and membrane synthesis, sterol biosynthesis in membranes, glucan and cell wall synthesis, melanin synthesis in cell walls, and host plant defense induction.
MOA influences resistance and cross resistance. The repeated use of a particular active ingredient may encourage development of resistance in a given fungal population. Depending on the MOA group this resistance may be permanent or temporary. Also, critical to the build up of resistance is that many fungi can become cross resistant to products with similar MOA. In other words, the fungus does not differentiate between one product and another if these products affect the same fungus in the same way. Therefore, in order to avoid encouraging resistance build up it is recommended that products from the same MOA group are not tank mixed or used in alternation with one another. For example, fungal populations that have been treated with MOA Group 2 (Dicarboximide) product containing iprodione should not be treated with vinclozolin (same MOA group) to avoid build up of resistance. Once these populations become resistant to one of these active ingredients it has cross resistance to the other. In this case the iprodione should be tank mixed or alternated with a different MOA group, i.e. with thiophanate-methyl of MOA Group 1 (Benzimidazole). This is why Cleary’s 26/36 has gained popularity as a prepackaged mixture containing two different MOA that are highly effective against several significant turfgrass pathogens.
Cleary has developed the Cleary Technical Information Bulletin “Turf and Ornamental Fungicides Grouped by Mode of Action (MOA)” to assist you in reviewing the products in your spray program and to insure that you are using proper tank mix partners. Cleary also packages products with the FRAC Group displayed on the front of the package so that even if you do not have full detail of the specific MOA of your product you can still decide which combinations to avoid. Next month we will examine the fungicide MOA Groups in more detail.
© Cleary Chemical 2008