Black knot

French disease name: Nodule noir
Pathogen name: Apiosporina morbosa (Schwein.:Fr.) Arx
Kingdom: Fungi
Phylum: Ascomycota
Class: Dothideomycetes
Order: Venturiales
Family: Venturiaceae
Partial list of synonyms:
- Dibotryon morbosum (Shwein.:Fr.) Theiss. & Syd.
- Plowrightia morbosa (Schwein.:Fr.) Sacc.

Distribution

Eastern Canada

Damage, symptoms and biology

The name "black knot" aptly describes the most obvious sign of this disease: a rough black gall that can encircle the entire branch. Infection occurs during the spring, but the symptoms do not appear until the following spring. At that time, the bark on affected branches ruptures and a green liquid is expelled, and an olive-green swelling is formed. Over the summer, the gall turns a darker colour, eventually becoming black and hard. While the older parts of the gall die subsequently, the fungus continues to grow both around and along the affected branch. Large branches can resist the disease a long time after the infection begins, thereby supplying spores that enable continued spread.