Symptoms and Signs

Dwarf mistletoe shoots are macroscopic and symptoms of advanced infections are often obvious. Especially in early stages of infection, however, symptoms and signs may be subtle or absent.

Symptoms

The first symptom of infection is swelling of a branch or stem near the point of infection (Figure 2). The swelling is usually fusiform (tapered at the ends) and the degree of swelling varies. Some dwarf mistletoes cause cankers in older stem infections, which may serve as infection courts for stem-decay fungi (Figure 3).

The most obvious symptom of dwarf mistletoe infection is a witches’ broom (Figure 4) forming after a branch has been infected for several years or more. This is a dense mass of profuse, sometimes distorted, branches that form in response to infection by dwarf mistletoe. A few species of dwarf mistletoes do not induce witches’ broom formation.

Figure 2

Figure 3

Figure 4

Crown symptoms include the visual effects of dwarf mistletoe on tree vigor. Upper crowns may thin and eventually die in trees that are severely infected, particularly with one or more large witches’ brooms in the lower crown (Figure 5). The brooms are nutrient sinks, disrupting the physiology of the tree and robbing the top of nutrients. Eventually the entire crown may thin, leading to mortality (Figure 6). Witches’ brooms are often the last part of the tree to die.

Figure 5

Figure 6

Signs

Shoots are the best indication that a tree is infected by a dwarf mistletoe (Figure 7). Shoots usually develop 3-5 years after a branch is infected. Depending on the species of dwarf mistletoe, shoots may be brown, orange, green, yellow, olive, or yellow-green. The shortest dwarf mistletoe shoots in the Americas are those of eastern dwarf mistletoe, A. pusillum; with shoots about 1 cm tall (Figure 8); the most massive plants in the genus are those of A. globosum subsp. grandicaule in Guatemala, whose shoots can exceed 70 cm (28 inches) in length (Figure 9). At each node is a pair of small, scale-like leaves (Figure 10). Shoots typically live for 5 to 7 years before they die and abscise. The plant stays alive and typically produces new shoots, but there may be periods when shoots are hard to find, such as during a drought. In such cases, residual basal cups may be present where shoots have abscised (Figure 11).

Figure 7	Figure 8
Figure 9	Figure 10
Figure 11

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