Fungi in the genus Dibotryon are microscopic members of the class Ascomycetes, spore-sac fungi.
They produce thread-like mycelia and two types of spores: conidia (asexual) and ascospores (sexual), grouped together in asci, small sacs inside which meiosis occurs.
The life cycle may be completed in one year or extend over two years.
The fungi overwinter in infected branches and old knots, in the form of mycelia or immature ascospores.
In early spring (March to June), the mature spores are released into the air and dispersed by wind and rain. Given favourable weather conditions, they germinate quickly and the mycelia proliferate in the tissues. This primary infection period lasts about two months and causes new infections on other branches or other trees. The first symptoms appear in late summer or the following spring, depending on the cultivar's resistance.
Over the summer, the knots grow, becoming olive-green and downy looking, indicating that the fungus is producing summer spores (conidia). This relatively brief stage causes other, more limited infections.
In fall and winter, the knots turn hard and coal black. They shelter the winter spores (ascospores), which will be released the following spring, to contaminate new branches or new trees.