Understanding Fungicide Resistance

In recent years a number of new fungicide chemistries have become commercially available for use in vegetable production to control important diseases. Many of these new chemistries have site-specific modes of action and target a single site in fungal growth and development. Because of this specificity, some chemistries have a high risk for fungicide resistance development.

How Resistance Develops

Understanding why and how a fungus develops resistance to a specific fungicide is important to understanding fungicide resistance management. For the fungus, it’s simply a matter of survival.

A fungal population, much like the human population, is made up of a number of individuals. One of these individuals may have a mutation that enables it to be completely resistant to a fungicide group or to tolerate a higher dose of a fungicide than the other “wildtype” individuals in the population. Because mutation is a chance event, this resistant individual could be present even before a product in this group is applied for the first time.

Initially the fungicide will be very effective because the population is almost entirely made up of sensitive individuals. However, the situation can change rapidly, especially if a fungicide resistance management program is not being used, because certain fungi are capable of reproducing asexually (clonally) and doing so several times during just one growing season. As those resistant individuals multiply, a much larger proportion of the pathogen population will be unaffected by that particular fungicide, and will eventually reach sufficient numbers to wreak havoc on your crop.

A few years ago industry personnel established the Fungicide Resistance Action Committee, or FRAC, to address fungicide resistance management. FRAC helps identify existing and potential resistance problems, develops resistance management guidelines, and in the process, stimulates collaboration among universities, government agencies, crop advisers, extension workers, distributors, and growers. Most importantly, this committee established what are known as FRAC codes for different fungicide chemistries. Fungicides with similar modes of action (MOA) are placed into the same FRAC code group.

Why is this so important? Fungicides with similar MOAs have the same affect on fungal growth and development, and overuse of high-risk fungicides with similar MOAs may lead to fungicide resistance development. One of the best ways to combat this problem is to alternate fungicide products with different modes of action. That’s where FRAC groups can help.

Let’s look at one FRAC group in particular, FRAC group 11, which includes the strobilurin (aka QoI) fungicides. Strobilurins control fungi by inhibiting a very specific part of fungal respiration. Fungicides in FRAC group 11 include Amistar or Quadris (azoxystrobin, Syngenta Crop Protection), Flint (trifloxystrobin, Bayer CropScience), and Cabrio (pyraclostrobin, BASF), among others. Each fungicide has a slightly different chemistry but similar modes of action. Thus, simply applying a FRAC group 11 fungicide one week and another FRAC group 11 fungicide the next to control an important disease such as cucurbit powdery mildew is not really different than applying the same fungicide both weeks. Ultimately, as part of the “sensitive” fungal population controlled by FRAC group 11 fungicides is killed off, a new “resistant” population to the fungicide chemistry may be developing. All of this may be taking place before you realize it, and you will have spent a lot of money applying a fungicide (or group of fungicides) that are no longer effective in controlling the pathogen (see How Resistance Develops sidebar).

Look For The Label

To date, there are 43 numbered and three lettered FRAC group codes. Fungicide manufacturers recently have started placing these codes on the front of labels. If not found on the front of the label, FRAC codes can be present in the resistance management section of the label or in your local commercial recommendations guide.

Associated with the FRAC fungicide grouping system are important manufacturer restrictions for applying fungicides in a particular group, which should always be followed to help manage resistance development. Additionally, the label is the law. These restrictions include alternations, and tank-mixtures as well as limits on the total amount to be applied and the number of allowable applications per season.

Why should vegetable growers take the time to learn about and understand FRAC groups? Unfortunately, several economically important pathogens of vegetable crops already have developed resistance to key fungicide groups in the U.S., and this has been associated with poor control. For the widely used FRAC group 11 fungicides, resistance has been reported in the pathogens causing powdery mildew, downy mildew, and gummy stem blight/black rot in cucurbit crops. Another example is resistance to mefenoxam (FRAC group 4) in the pathogen causing Phytophthora blight, which results in devastating losses in pepper and cucurbit production throughout the country.

Over the next few months we will be looking more into fungicide chemistry, FRAC groups, and fungicide resistance management for vegetable crops. Until then, the next time you are out in your pesticide storage facility, have a look at your labels and determine which FRAC groups your more commonly used fungicides belong to.

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