Cause A fungus, Cephalosporium sp. It infects leaves through wounds such as those from rust pustules, insects, or machinery.
Symptoms Irregularly shaped black spots on leaves rapidly coalesce and kill them. Infection may move down the leaf petiole into the stem. It develops rapidly in cool, wet weather and causes severe leaf drop, especially if hot, dry weather follows cool, wet weather.
- Minimize mechanical wounds to the crop.
- Control rust (see Peppermint and Spearmint - Rust).
- Control insects when at or above threshold levels. See the PNW Insect Management Handbook for details.