Resistance Management Strategies for Strobilurin Fungicides (FRAC Code 11)

Andy Wyenandt, Assistant Extension Specialist in Vegetable Pathology, Rutgers University; wyenandt@aesop.rutgers.edu

The strobilurin, or QoI, fungicides (FRAC code 11) are extremely useful in controlling a broad spectrum of common vegetable pathogens. You may know some of strobilurins as azoxystrobin (Quadris), pyraclostrobin (Cabrio), or Pristine (pyraclostrobin + boscalid, 11 + 7). 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 to 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 code 11, with a very specific MOA, are susceptible to fungicide resistance development by some fungi. For us, knowing the specifics on the technical jargon isn’t so important, its understanding what is at stake. So, if you read or hear someone speak about G143A resistance development to the strobilurin fungicides (where resistance is known in cucurbit powdery mildew and downy mildew, for example), you know what they are talking about and how important it is. So much so, if cucurbit powdery mildew develops resistance to one strobilurin fungicide it may develop what is known as cross resistance and become resistant to all other chemistries in FRAC code 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 codes). Proper fungicides rotations are necessary when fungicides with specific MOAs 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 tankmixing high-risk fungicides (i.e. FRAC code 11) with low-risk, protectant fungicides (FRAC codes 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 pyraclostrobin 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.

The publication”Fungicide Resistance Management Guidelines for Vegetable Crops in the Mid-Atlantic Region-2010” is available from the county extension offices or online at http://ag.udel.edu/extension/pdc/documents/FRACGuide_2010.pdf

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