Summary:
Apiosporina morbosa causes the destructive and very common disfiguring black knot of Prunus (wild cherry, plum, etc.), (Bessette et al. 1997). Individual branches are deformed and if infection is severe for several years, the infected tree may become stunted. During April and May ascospores (produced by overwintering fruitbodies) usually infect the youngest branches and occasionally the larger branches or the trunk. Internally the fruitbody consists of both fungal cells and host cells (increased amounts of xylem and phloem through hypertrophy). The conidial state develops on the fruitbody early in the spring and the asci develop later.

Microscopic:
spores 14-22 x 3-6 microns, narrowly elliptic, smooth, pale yellowish brown, 1-3 septate, (Bessette), pseudothecial ascomata developing on conspicuous black fungal stromata, (Kendrick), spores 13-18 x 4.5-7.5 microns, clavate, apex obtusely rounded, tapering to an acute base, pale green becoming olivaceous, 1-septate near base, upper cells broader and about 2.5-3 times length of lower cell; asci 50-75 x 10-15 microns, clavate, bitunicate; pseudoparaphyses persisting in mature ascostroma, colorless, septate; ascostromata black, gregarious, somewhat spherical or somewhat turbinate [top-shaped], 150-300 microns in diameter, otherwise the outside of fruitbody of textura prismatica; conidia 4-9 x 3.5-5.5 microns, ovoid or obovoid to irregular in shape, smooth, light brown, 1-celled or occasionally 1-septate, (Corlett)

Notes:
Apiosporina morbosa is common throughout Canada but by 1976 was still restricted to North America. The distribution in Canada includes BC, AB, MB, NF, NS, ON, PQ, and SK, (Corlett).

Synonyms and Alternate Names:
Cyphella arachnoidea Peck
Leptoglossum arachnoideum (Peck) W.B. Cooke