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Symptoms and signs

The disease is characterized by the presence of thick, black, irregular swellings on the twigs (figure 1). The presence of these symptoms is often first noticed in the winter season when they are unobscured by leaves. However, the fungal disease-causing agent has been present for quite some time. The pathogen's presence disrupts the normal growth of the twigs and a tumor-like growth forms at the infection site. Infections may take place as much as a year or more prior to the development of these characteristic "knots," therefore, the swellings are normally not noticed until the winter of the second season of infection. It takes a keen observer to notice the subtle, initial symptoms present during the first season of infection.

The first symptoms appear as small, light brown swellings of the current or previous season's growth (figure 2). By the next season the swellings turn olive-green in color with a velvety texture. Over this growing season the knots darken and appear to have a hard, brittle texture. The hard, black knots are the typical symptoms associated with the disease.

Characteristic thick, black, irregular swellings of the twig.

Figure 1

First symptoms appear as minor, light brown swellings and may be hard to observe.

Figure 2

Knots vary in size from approximately 1-30 cm (0.5 to 12 in.) in length and from minute measurements to 5 cm (2 in.) in circumference (figure 3). The infected twigs often appear bent at the tips because of extra cellular growth on one side (figure 4).

Knots can vary in size and shape and may completely take over a twig.

Figure 3

Extra cellular growth causes tips to curl.

Figure 4

Trees with heavy infections may contain numerous knots. Some of the older knots may appear white or pink in color. This discoloration is often seen in late summer and is caused by a fungal parasite, Trichothecium roseum (figure 5).

The fungal parasite, Trichothecium roseum, colonizing a knot.

Figure 5

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