Pathogen biology

Black knot is caused by a fungus, Apiosporina morbosa (previously referred to as Dibotryon morbosum). The fungus belongs to the family Venturiaceae, which makes it closely related to the plant pathogen that causes apple scab, Venturia inaequalis. The fungus produces pseudothecia, fruiting structures that are embedded in the black stroma on the surface of the gall (figure 6). The mass of pseudothecia is often referred to as the ascostroma. (figure 7).

Figure 6	Figure 7
Figure 8

In the spring, two winters after initial infection, the fungus produces sexual spores called ascospores. The ascospores develop in asci (sacs) contained in the pseudothecia. Each clavate (club-shaped), bitunicate (double-walled) ascus contains eight ascospores. Each ascospore has two unequal-sized cells (figure 8). The ascospores mature during the early spring of the infection's second season and are forcibly discharged into the air during rain events. The spores are distributed short distances on wind currents and through rain splashing.

The anamorph, or asexual, stage of the black knot fungus (figure 8a) is a species of Fusicladium, that produces abundant olive-green conidia during the summer on the surfaces of one-year-old knots (figure 9). It appears that the infection capabilities of the conidia are quite limited. Therefore, management strategies are focused on ascospore development and infection processes.

Figure 8a

Figure 9

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by The American Phytopathological Society