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!
