The plant pathogenic fungus Leucostoma kunzei (formerly Valsa kunzei) is the causal agent of Leucostoma canker (also known as Cytospora canker or spruce canker), a disease of spruce trees found in the Northern Hemisphere, predominantly on Norway spruce (Picea abies) and Colorado blue spruce (Picea pungens). This disease is one of the most common and detrimental stem diseases of Picea species in the northeastern United States, yet it also affects other coniferous species. Rarely does it kill its host tree; however, the disease does disfigure by killing host branches and causing resin exudation from perennial lesions on branches or trunks.
Origins Leucostoma kunzei or “Valsa kunzei” (Fr.:Fr) Fr. (conidial state -Cytospora kunzei) was first described by Waterman in 1955, as the causal agent for the branch and stem cankers she observed on Douglas fir. Waterman cultured the fungus from cankered trees from Washington, Pennsylvania, Vermont, New Hampshire, and Massachusetts and provided the first descriptions of the canker and fungus. She also noted cankered trees usually occurred on unfavorable sites or had been weakened by other environmental factors.
Susceptible hosts
Balsam Fir Abies balsamea Fraser Fir Abies fraseri European Larch Larix decidua Japanese Larch Larix kaempferi American Larch Larix laricina Norway Spruce Picea abies Engelmann Spruce Picea engelmannii White Spruce Picea glauca Black Spruce Picea mariana Caucasian Spruce Picea orientalis Colorado blue spruce Picea pungens Red Spruce Picea rubens Jack Pine Pinus banksiana Red Pine Pinus resinosa Eastern white pine Pinus strobus Bhutan Pine Pinus wallichiana Eldar Pine Pinus eldarica Douglas-fir Pseudotsuga menziesii Western Redcedar Thuja plicata Eastern Hemlock Tsuga canadensis
Symptoms Symptoms of this disease in spruce hosts include dead and dying branches and perennial lesions on the branches and trunk, which exude resins. Older branches (lower on the trees) sustain more damage than younger ones. In spring and early summer the foliage of an infected branches on host trees fade and turns brown, which is an indication of girdling occurring within a branch or along the mainstem caused by this pathogen. These brown needles will remain attached during the growing season and then fall off during the winter, leaving behind bare twigs and branches. This entire process can occur annually, moving from low branches to higher branches, thus destroying the symmetry of the host tree. Twigs and branches killed by this disease may remain on the diseased host tree for several years. All of this damage caused by this fungal pathogen does not typically begin until the host trees are at least 10–15 years old. However, in landscape nurseries small branches of young blue spruce or occasionally white spruce may be killed. Lesions typically begin at the bases of small twigs and develop into elliptic or occasionally diamond-shaped cankers. Lesions that originate on branches close to the main stem may actually spread into the main stem. Cambium that has been killed by this disease has a brown to reddish brown internal coloration and is saturated with resins. The underlying sapwood, which has been killed and colonized by the pathogen, is rarely discolored. Amber colored resin profusely exudes from the edges of cankers, runs down the bark, or drips onto lower branches or the ground, and then hardens into a white crust. The lesions associated with this pathogen typically go visibly unnoticed on infected host trees for several years, due to the diseased tissue being held in place by resin. The formation of callus at canker margins is subtle or nearly absent, usually resin is the only indication of an underlying bark lesion. Cankers formed on the trunk will eventually appear as sunken due to living tissues (callus) expanding and growing around the wounded tissues. Complete girdling of the host trunk or large limbs may occur; however, this event may take several years to even decades to accomplish. In other susceptible conifers, symptoms are similar to those of spruce except resin exudation is usually less prominent. Regarding pines the key symptom to note is the inconspicuous branch cankers caused by this pathogen.
Signs Signs of Leucostoma kunzei include the fungal stromata of its Cytospora stage which form annually in recently killed bark of cankers and more abundantly, outside of girdling cankers. Pycnidial stromata are shaped like short cones, 1-2 mm in diameter, with fertile chambers radiating from the center and opening through a common pore at the top. During moist conditions, they will produce yellow tendrils of conidia. An individual stroma however, only does this once. Conidia are unicellular, allantoid (sausage shaped), and 4-6 x 0.5-1 μm in size. Perithecial stromata which mature in spring are also short conic and 1–2 mm in diameter. The interior tissue is pale yellow to grayish brown with 5-30 black perithecia embedded within. Perithecia are 200-600 μm in diameter, and their necks converge at the disc-like top of the stroma. The disc is usually 0.2-1.0 mm in diameter and gray to black on the surface. The ascospores are hyaline (colorless), unicellular, curved, and measure 5-8 x 1-2 μm.
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