General Guidelines for Managing Fungicide Resistance
Margaret Tuttle McGrath
Department of Plant Pathology, Cornell University
Long Island Horticultural Research & Extension Center
3059 Sound Avenue
Background Information. Fungicide resistance is a stable, heritable trait that results in a reduction in sensitivity to a fungicide by an individual fungus. This ability is obtained through evolutionary processes. Fungicides with single-site mode of action are at relatively high risk for resistance development compared to those with multi-side mode of action. Most fungicides being developed today have a single-site mode of action because this is associated with lower potential for negative impact on the environment, including non-target organisms. Presence of fungicide resistant strains has been associated with control failure, therefore managing resistance is critical to disease control.
When fungicide resistance results from modification of a single major gene, pathogen subpopulations are either sensitive or highly resistant to the pesticide. Resistance in this case is seen as complete loss of disease control that cannot be regained by using higher rates or more frequent fungicide applications. This type of resistance is commonly referred to as “qualitative resistance”.
When fungicide resistance results from modification of several interacting genes, pathogen isolates exhibit a range in sensitivity to the fungicide depending on the number of gene changes. Variation in sensitivity within the population is continuous. Resistance in this case is seen as an erosion of disease control that can be regained by using higher rates or more frequent applications. Long-term selection for resistance in the pathogen by repeated applications may eventually result in the highest labeled rates and/or shortest application intervals not being able to adequately control the disease. This type of fungicide resistance is commonly referred to as “quantitative resistance”.
Fungal isolates that are resistant to one fungicide are often also resistant to other closely-related fungicides, even when they have not been exposed to these other fungicides, because these fungicides all have similar mode of action. This is called cross resistance. Rarely negative cross resistance occurs between unrelated fungicides because the genetic change that confers resistance to one fungicide makes the resistant isolate more sensitive to another fungicide.
Fungicide Group Codes designating chemical groups were developed to facilitate managing resistance by the Fungicide Resistance Action Committee (FRAC). These codes are usually on the front of labels or in the resistance management section. Fungicides with the same Group Code have similar mode of action and therefore could exhibit cross resistance. Thus it is critically important for managing resistance to know the group code for the fungicides being used for a particular disease to avoid alternating among chemically similar fungicides. Presently there are 43 numbered and three lettered FRAC Group Codes. A list of the codes for all fungicide active ingredients (e.g. common names) can be downloaded from www.frac.info/frac/. Select ‘publications’ in the list on the left of the page, then one of the ‘FRAC Code Lists’ under ‘Fungicide Lists’. The resistance risk for each fungicide group is in the ‘comments’ column.
It is important to realize that resistance risk for a new fungicide can be difficult to predict. Risk cannot always be predicted solely from the mode of action. Additionally, resistance development in model systems with yeasts or non-obligate pathogens is not always similar to that in obligate pathogens. For example, the QoI fungicides were initially thought to have a low to medium resistance risk and resistance was predicted to be quantitative based on their mode of action (inhibition of respiration) and on results of research with yeast. However, resistance developed quickly and in a disruptive manner. Furthermore, it developed first in the US in a pathogen not considered highly prone to developing resistance (gummy stem blight fungus) rather than in the cucurbit powdery mildew fungus.
Managing fungicide resistance is critically important to extend the period of time that an at-risk fungicide is effective and these management practices can prevent control failure. The primary goal of resistance management is to delay its development rather than to manage resistant fungal strains after they have been selected. Therefore, resistance management programs need to be implemented when at-risk fungicides first become available for commercial use. The overall strategy to managing resistance is to minimize use of the at-risk fungicide without sacrificing disease control. This is accomplished by using the at-risk fungicide with other fungicides and with non-chemical control measures, such as disease resistant cultivars, in an integrated disease management program. It is critical to use an effective disease management program to delay the build-up of resistant strains. The larger the pathogen population exposed to an at-risk fungicide, the greater the chance a resistant strain will develop. When an integrated program is used to manage resistance and resistance develops to one of the fungicides, the other practices and fungicides used may provide enough control that the inefficacy of the one fungicide can be difficult to detect, especially in a commercial field.
Specific Guidelines. The first step in effectively managing fungicide resistance is to obtain information on current occurrence of resistance and on fungicide risk for the target disease as well as fungicides to be used. Just as some fungicides are more prone to resistance developing, some pathogens are more prone to developing resistance. Information about a fungicide’s risk can be found in the FRAC Code List (see paragraph on codes above). Fungi that cause powdery mildew diseases are the pathogens most prone to developing resistance. Cucurbit powdery mildew is a good example. The pathogen has developed resistance to almost every chemical class at risk for resistance following repeated use somewhere in the world. Other pathogens attacking vegetable crops that have developed resistance include those causing late blight of potato and tomato, downy mildew of cucurbits, early blight of potato and tomato, Phytophthora blight, and gummy stem blight of cucurbits. Tables with resistance risk for specific combinations of fungicide and vegetable pathogen are being developed and will eventually be posted on the web. While it is critical to use resistance management practices when the combination has high risk, it is prudent due to the challenge of predicting resistance to use these practices with most fungicides except those with multi-site contact activity.
Reduce the need for fungicides at-risk for resistance development by using disease-resistant varieties and other cultural management practices.
Start fungicide applications very early in disease development or before symptoms are seen. It is more difficult, often impossible, to control the pathogen in an established lesion, as opposed to a germinating spore, thus the potential is greater for resistance to develop.
Reduce the use of specific at-risk fungicides by using them only when needed most and by alternating among those with different FRAC codes. The most critical time to use at-risk fungicides for both disease control and resistance management is early in an epidemic when the pathogen population is small. Multi-site contact fungicides should be used alone late in the growing season, where they have been shown to provide sufficient disease control to protect yield. Alternating among fungicides in the same chemical group (FRAC code) is not really an alternation. It is not known whether a strict alternation (apply once and then switch) is better than a block alternation (apply twice in a row and then switch). At-risk fungicides should be used at the manufacturer’s recommended rate and application interval. Using highest label rates is expected to minimize selection of strains with intermediate fungicide sensitivity when resistance involves several genes (quantitative resistance).
When one crop could serve as a source of inoculum for a subsequent crop, the alternation scheme among at-risk fungicides should be continued between successive crops such that the first at-risk fungicide applied to a crop belongs to a different cross-resistance group than the last at-risk fungicide applied to the previous crop.
Tank-mix at-risk fungicides with multi-site fungicides because these have low resistance risk. Multi-site fungicides (those with a FRAC code that includes ‘M”) will control any resistant strains they contact. Check the label: some at-risk fungicides are formulated as premix products with other fungicides to manage resistance (e.g. Gavel).
Maximize spray coverage by adjusting application methods (nozzles, spray volume, pressure, ground speed) and spraying when calm. The better the coverage, especially on the underside of leaves, the greater the contribution of the multi-site fungicides to control and the lower the selection pressure for resistance development. Use water sensitive paper to assess coverage.
Follow any additional resistance management guidelines specified on the label. These will be in a section on resistance management and/or in the use directions for specific diseases. Remember that the label is a legal document. In addition to manufacturer restrictions pertaining to alternations and tank-mixtures, there are often limits on the total amount to be applied and the number of allowable applications per season.
Another important component of resistance management is assessing disease control and reporting any loss of efficacy potentially due to resistance to local extension specialists.