The strobilurin, or QoI fungicides (FRAC group 11) are extremely useful in controlling a broad spectrum of common vegetable pathogens.
You may know some of older strobilurins as azoxystrobin (Quadris), trifloxystrobin (Flint), pyraclostrobin (Cabrio), or Pristine (pyraclostrobin + boscalid, 11 + 7). For example, FRAC group 11 active ingredients such as azoxystrobin are also now available in combination products as Quadris Top (azoxystrobin + difenoconazole, 11 + 3), Quilt (azoxystrobin + propiconazole, 11 + 3), or Quadris Opti (azoxystrobin + chlorothalonil, 11 + M5).
All strobilurin fungicides inhibit fungal respiration by binding to the cytochrome b complex III at the Q0 site in mitochondrial respiration. Simply said, the fungicide works by inhibiting the fungi’s ability undergo normal respiration. The strobilurin chemistries have a very specific target site, or mode-of-action (MOA).
Although highly effective, fungicide chemistries like those in FRAC group 11, with a very specific MOA, are susceptible to fungicide resistance development by some fungi. Why is that? In the strobilurin’s, a single nucleotide polymorphism of the cytochrome b gene leads to an amino acid substitution of glycine with alanine at position 143 of the cytochrome b protein.
For us, knowing the specifics on the technical jargon isn’t so important, it’s understanding what is at stake. So, if we hear someone speak about G143A resistance development to the QoI fungicides (where resistance is already known in cucurbit Powdery mildew and Downy mildew), we know what they are talking about and how important it is! So much so, if cucurbit powdery mildew develops resistance to one strobilurin compound it may develop what is known as cross resistance and become resistant to all chemistries in FRAC group 11, even if only one chemistry has been used!
How do we avoid the chances for fungicide resistance like this to develop? It’s simple, don’t let the fungus ‘figure out’ what it is being sprayed with and do this by rotating different fungicide chemistries (i.e., FRAC groups). Proper fungicides rotations are necessary when fungicides with specific MOA’s are used in fungicide programs for controlling important diseases. That’s why it is important to follow a fungicides label precisely and be certain that some fungicide chemistries aren’t overused.
All strobilurin fungicides should be tank mixed with a protectant fungicide, when possible. Remember tank-mixing high-risk fungicides (i.e., FRAC group 11) with low-risk, protectant fungicides (FRAC groups M1-M9) helps reduce (and/or delay) the chances for fungicide resistance development. Never tank mix strobilurins together and never apply any strobilurin fungicide (either the same chemistry or different chemistry) in consecutive applications if stated by the label. Remember, azoxystrobin acts against the fungus the same way as trifloxystrobin does and so on. Even though you are spraying two different fungicides, each has the similar MOA and is acting against the fungus in the same exact way.
Azoxystrobin (i.e., Quadris 2.08F) used heavily in vegetable production is coming off patent and should soon be available in generic forms from a number of companies looking to sell azoxystrobin. In the 2015 Vegetable Crops Recommendations Guide you will see that most references to Quadris have been replaced with azoxystrobin, much like we did the chlorothalonil (i.e., Bravo Weatherstik) a few years ago. Over the next couple years other FRAC code 11 fungicides will also be coming off patent and similar changes in the recommendations guide will occur. Resistance to FRAC group 11 fungicides has been reported in multiple pathogens in multiple crops worldwide. Resistance to FRAC group 11 fungicides in our region have been reported in cucurbit powdery mildew and gummy stem blight. Importantly, in recent years the lack of efficacy in strobilurin fungicides in crops, such as parsley, where azoxystrobin was one of the few fungicides available for septoria leaf spot control is showing up in the Vineland area.
The threat for resistance development in important pathogens to FRAC group 11 fungicides is always present, and it will become even more important to monitor as generic products with azoyxstrobin and other group 11 fungicides as active ingredients begin to emerge in the future.