Dibotryon morbosum or Apiosporina morbosa is a plant pathogen, which is the causal agent of black knot.

[1][2] It affects members of the Prunus genus such as; cherry, plum, apricot, and chokecherry trees in North America.

The disease produces rough, black growths that encircle and kill the infested parts, and provide habitat for insects.

While it was one of the most destructive diseases of plum and cherry trees in the late 19th century, today it is relatively well controlled in many cultivated areas and seen primarily in poorly managed orchards, or where strongly established, including in the wild.

Black knot occurs only on the woody parts of trees, primarily on twigs and branches, but can spread to larger limbs and even the trunk.

Olive-green swellings from the disease are visible in the late spring; as it spreads and matures, typically by autumn, rough black knots circle and kill affected parts.

In the spring, after overwintering in a previous host, the fungus produces ascospores, which are stored in a fruiting structure known as the pseudothecia.

Symptoms are typically not noticeable in the season of initial infection, as the fungus grows inside the host.

Dibotryon morbosum produces pseudothecia, fruiting structures that are embedded in the black stroma on the surface of the gall.

The ascospores mature during the early spring of the infection's second season and are forcibly discharged into the air during rain events.

The anamorph, or asexual stage, produces abundant olive-green conidia during the summer on the surfaces of one-year-old knots.

[5] The fungus favors warm and wet weather with any temperature within 60–80 degrees Fahrenheit as it is the most ideal for dissemination, germination, and infection of new plant tissue.