Fungicide Resistance Management: Protectants, Systemics and Rotation

Fungicides divide into multi-site protectants and single-site systemics. Protectants — mancozeb (FRAC M03), chlorothalonil (M05), copper (M01) and sulphur (M02) — attack many enzymes at once, so pathogens almost never evolve resistance; but they only protect tissue they cover and must be applied before infection.

Systemics — triazoles like propiconazole (FRAC 3) and strobilurins like azoxystrobin (FRAC 11) — move inside the plant and can cure early infections, but each blocks a single biochemical step, so resistance develops quickly when they are used solo. QoI (strobilurin) resistance in powdery mildew can appear within a few seasons of exclusive use.

The standard defence: open with a protectant, bring in a systemic only at high-risk growth stages, never exceed label limits on consecutive single-site applications, and alternate FRAC groups. Azoxystrobin is also highly phytotoxic to apples — never let it drift onto apple orchards.

Sulphur and copper remain dual conventional/organic tools and stay effective precisely because they are multi-site. For post-harvest fruit, thiabendazole-class treatments (Tenderfrut) extend shelf life under export cold-chain requirements.