Phytophthora Canker

Phytophthora canker attacks the lower stems and tree bases of young pears, ash, and ornamental trees, especially in nursery stock and recently planted landscape trees. Look for necrotic bark that turns brown and releases an alcoholic smell; the lesion edges are irregularly mottled and merge gradually into healthy tissue. In young stock, cankers appear slightly sunken and darker than normal bark. Wetting the canker makes it more visible, while dry conditions may hide it entirely.

This soilborne pathogen is a winter disease of nursery stock, thriving during the cold, wet periods typical of temperate climates. Trees harvested late in the season and stored in wet sawdust or heeled-in under rainy conditions are especially vulnerable since the pathogen moves through water in saturated soils. Prevention centers on avoiding mechanical injury during handling and harvest, maintaining good drainage, and never heeling trees in above the collar; removing harvested stock from the field during wet weather is your simplest defense against establishing this problem.