Fungicide Resistance Management

Fungicides are important tools for managing diseases in many crops. Unlike insecticides and some herbicides which kill established insects or weeds, fungicides are most commonly applied to protect healthy plants from infection. To be effective, fungicides must be applied before infections become established and in a sufficient spray volume to achieve thorough coverage of the plant or treated area. Protection from fungicides is temporary because they are subject to weathering from light, irrigation, and rainfall. They also must be reapplied to protect new growth when disease threatens. This is particularly important in turfgrass following mowing which removes the treated grass. Poor disease control with fungicides can result from several causes, including insufficient rate, inherently low fungicide effectiveness, improper timing or application method, and excessive rainfall. Resistance to fungicides is another cause of poor disease control, but it is more complex and difficult to identify. Understanding what resistance is, how it develops, and how it can be managed is crucial for insuring sustainable disease control with fungicides. The problem of fungicide resistance became apparent following the registration and widespread use of the systemic fungicide benomyl (Benlate) in the early 1970’s. Prior to the registration of benomyl, growers routinely applied protectant fungicides such as maneb and mancozeb (dithiocarbamates), or coppers to control diseases without experiencing resistance problems. A distinct advantage of benomyl was its systemic activity, which in addition to protecting plants from infection, also provided disease control when applied after the early stages of infection. Superior disease control was often achieved with benomyl compared to the protective dithiocarbamates. However, benomyl differed from the dithiocarbamates in its site-specific mode of action (see “Fungicide Groups and Mode of Action” below), which was readily overcome by several fungal pathogens. Resistance problems appeared within a few years where the fungicide was used intensively. Sudden control failures occurred with diseases such as powdery mildew, peanut leaf spot, apple scab, and dollar spot in turf. Many of the fungicides developed and registered since the introduction of benomyl also have site-specific mode of actions and are at risk for resistance problems. While resistance risks with many of these fungicides may not be as great as with benomyl, strategies to manage the resistance risk should be developed and implemented to avoid unexpected control failures and sustain the usefulness of new products. The purpose of this fact sheet is to describe the resistance phenomenon, identify resistance risks in the different fungicide groups, and to provide guidelines for managing resistance.