Otthia spiraeae

Otthia spiraeae is a pathogenic fungus that causes a destructive disease known as bark necrosis or stem canker in various shrub species, particularly those in the Spiraea genus. As an ascomycete fungus, it primarily colonizes the cortical tissues of stems, disrupting the plant's vascular system and causing progressive dieback of branches and twigs.

The host range is largely centered on Spiraea, but the fungus can occasionally be found on other related shrubs within the Rosaceae family. It is typically considered a weak pathogen that exploits host plants weakened by environmental stress, mechanical damage, or poor growing conditions, although it can become quite aggressive in favorable ecological niches.

Symptoms of an Otthia spiraeae infection are characterized by the development of localized dark-colored lesions on the stems. As the infection progresses, these areas become depressed and necrotic. The bark may crack open, revealing the presence of tiny, pinhead-sized black fruit bodies (perithecia) that emerge through the periderm, which are the reproductive structures of the fungus.

The fungus thrives under conditions of high humidity and moderate temperatures, which are common during spring and autumn. The spores are typically dispersed by rain splash and wind, settling on bark crevices or pruning wounds. Once established, the mycelium persists in the woody tissues throughout the winter, allowing the infection to flare up again when growing conditions improve.

Effective management and control of this disease require a systematic approach focused on sanitation and plant vigor. Key strategies include:

• Pruning out and disposing of infected branches to reduce the inoculum level in the garden.
• Applying wound dressings to major cuts to prevent fungal entry points.
• Maintaining plant health through proper fertilization and watering to improve natural defenses.
• Using copper-based fungicides as a preventative measure during the dormant season to suppress spore germination.