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Symptoms and Signs

The physical appearance of Leucostoma canker depends on the part of the tree infected. Infections of small twigs appear as sunken discolored areas, often with light and dark concentric circles of dead tissue, usually around buds killed by winter cold or leaf scars from the previous year's foliage (Figure 1). Nodal infections are easily observed 2 to 4 weeks after bud break (Figure 2). Infected tissues darken with time and an amber gum may ooze from the infected tissue unless the twig is killed entirely (Figure 3). One-year-old shoots that develop in the center of the tree are especially susceptible to nodal infections and, if left untreated, rapid invasion of scaffold limbs and large branches to which these shoots are attached may follow. Branch cankers that result from such infections will have dead twigs or twig stubs at the center of the canker (Figure 4).

Figure 1	Figure 2
Figure 3	Figure 4

Cankers that form on the main trunk, branch crotches, scaffold limbs, and older branches are the most conspicuous expression of infection (Figure 5). Dead scaffold limbs comprise a symptom called “flagging” that are recognizable at a great distance from infested orchards. Cankers are elliptical, parallel to the long axis of the stem. Usually the first external symptom of such cankers is the copious production of amber-colored gum. Gum production is a natural host response to irritation, but gum production due to infection by Leucostoma spp. is excessive. As the canker ages, the gum becomes dark brown, and the infected bark dries and cracks open exposing blackened tissue beneath. Leucostoma canker can occur on weakened branches without the production of gum, especially if winter injury precedes infection.

Figure 5

Beginning in late spring and continuing through summer, the tree grows rapidly and resists further penetration of the fungus into healthy tissues. During this time, the tree may form a callus ring around the canker, but the fungus usually invades this tissue again in late fall or early spring when the tree is dormant and cannot actively resist penetration. The yearly alternation of callus production and canker extension produces a canker with concentric callus rings. When tree defenses are compromised by environmental stresses, callus production may be inhibited and cankers may appear more diffuse.

Branch or twig infections may result in foliar symptoms during the growing season. Leaves on an infected branch often turn yellow, droop, and may wilt and die. Dead twigs and branches are usually covered with a multitude of pin-head-sized black structures erupting through the dead bark, which contain the asexual reproductive structures of the pathogen (Figure 6).

Figure 6

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