Stem & Cane Cankers
Coniothyrium spp. (Brand Canker, Common Canker, Brown Canker) · Fungal
Sunken, discolored patches on canes that girdle and kill from the canker outward. The reason a perfect-looking cane suddenly wilts in midsummer.
Symptoms
Brown, purple, or black sunken patches on canes, often with concentric rings or a dark border. Bark may crack and slough off. Tiny black fruiting bodies (pycnidia) speckle the surface. The cane above the canker wilts when the canker girdles it. Brand canker shows as a reddish margin on a brown lesion; common canker is more brown-purple.
How it progresses
Cankers expand slowly through cool weather and explode through canes in warm humid summer. A girdled cane wilts and dies in a week. Severe infection moves down a cane to the bud union and can kill the whole plant.
Conditions that favor it
Pathogens enter through pruning wounds and natural injuries. Worst in plants stressed by winter cold, heat, or drought. Spores spread on splashing rain.
Organic & cultural treatment
Prune out cankers below the lesion into clean white pith — disinfect tools between every cut with alcohol. Burn or trash prunings. Improve plant vigor with mulch and consistent moisture.
Chemical treatment (when warranted)
Copper-based protectants on dormant canes. Thiophanate-methyl. Most cane canker programs are sanitation-first.
Prevention
Prune in dry, sunny weather. Make clean cuts at a 45° angle just above an outward-facing bud. Avoid pruning during freezing or wet weather. Seal large cuts on canes thicker than a pencil with white glue.