Many frequently encountered twig and branch killing disease of ornamental, forest, and fruit trees are caused by fungi of the genera Cytospora and Leucocytospora. Many species of these fungi cause cankers on scaffold branches or young tree trunks in addition to twig dieback. The appearance, spread, and control of these twig diebacks and cankers are similar.
The host range of Cytospora fungi is broad: maple, spruce, willow, hemlock, poplar, mountain ash, cherry (ornamental, forest, and orchard types), Douglas firs, fir, pear, mulberry, walnut, sassafras, Japanese pagoda tree and peach. However, cross-infection by these fungi from one plant to another does not occur. Cytospora kunzei (syn. Leucocytospora kunzei) causes the well-known ‘spruce canker’. On many other hosts, larch and sycamore for example, Cytospora and Leucocytospora fungi are weak pathogens or secondary invaders of branches weakened by other causes.
The characteristic Spruce canker is a dead, slightly flattened area in the bark of a large branch. The bark tends to remain on the branch during the first year or more of infection. In many cases, the dead bark will cling to the branch indefinitely while the canker (area of dead bark) expands. The bark eventually breaks. Underneath it an area of dead wood surrounded by a roll of healthy tissue can be seen. This roll is callous that the tree produces while attempting to delimit the canker.
When twigs are killed, symptoms of dieback occur (Figure 1). Small branches may not produce leaves in the spring or leaves on infected twigs wilt and turn brown.
Conifers (spruce, hemlocks, etc.) often respond to twig infection or branch cankers by oozing resin (pitch) that builds up on the infected area as a whitish deposit (Figure 2), and in severe cases, amy drip onto lower branches. Cherries and peaches often respond by exuding gum at the base of dying twigs or at the margins of cankers. this gumming often first appears in early spring.
Cytospora and Leucocytospora species produce millions of microscopic spores on their respective host plants. The spores are produced in tiny, pimple-like structures that can be found in the surface layer of bark on a canker or killed twig. These structures can often be detected by shaving off a thin layer of bark. Sometimes they protrude through the surface of thin-barked species and can be seen without the use of a knife. During wet weather, masses of spores ooze from the fru iting structures and are carried by contaminated insects. These spores cause new infections.
Cytospora species are less likely to invade healthy plants than weakened or wounded plants. Winter injury, improper pruning cuts or stubs, insect injury, mechanical injury, or previous disease increase the chance of infection by Cytospora.