Flattened, Swirled Stems on Cotinus and Pyracantha: Phytoplasma or Borers?

The Symptoms

Shoots become flattened, thickened at tips, and swirl instead of growing straight. Cross‑sections show dark pith. No visible pests.

Possible Causes

• Phytoplasma (mycoplasma‑like organisms): Cause witches’‑broom, phyllody, stem flattening. Vectored by leafhoppers/psyllids. Lab test required.
• Pith‑boring insects: Larvae tunnel in stems, causing dark, hollow pith and distortion.

Diagnostic Path

1. Contact your county Extension office for the state plant diagnostic lab.
2. Submit stem samples with symptoms for PCR testing (phytoplasma) or insect rearing.
3. Management, if biotic, is usually limited to pruning affected shoots or removing the plant.

Interim Care

Prune out distorted stems, sanitize tools, and maintain plant vigor with proper water/fertilizer.