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Cytospora Canker of Conifers

by Paige Thrush, Nancy J. Taylor & Francesca Peduto Hand

Introduction

Cytospora canker is a stem and branch disease, causing dieback on hundreds of different species of trees and shrubs. The disease is especially destructive to spruces and other conifers in the Midwest and eastern U.S., most often damaging species that are stressed and/or planted outside of their native ranges. Colorado blue, Norway, Engelmann, white, black, red, and Oriental spruce are common hosts. Picea pungens, the Colorado blue spruce, sustains the greatest damage when it is planted in the landscape east of its natural range in the Rocky Mountains. Other susceptible conifers include balsam fir, Fraser fir, Douglas-fir, eastern larch, European larch, Japanese larch, eastern white pine, red pine, jack pine, Himalayan white pine, western red cedar and eastern hemlock.

The disease is caused by several different fungal species in the genus Cytospora. These pathogens can reproduce both sexually and asexually. The genus Cytospora refers to the asexual form of the pathogen, which is more commonly observed in the landscape. Some common species that cause Cytospora canker of conifers in North America include Cytospora abietis (=Valsa abietis), C. pinastri (=V. friesii), C. pini (=V. pini), and C. kunzei (=Leucostoma kunzei), the latter causing the most notable damage to conifers, including spruce. Although the disease rarely kills its host, it greatly reduces the vigor and aesthetic quality of trees, which then reduces the market value of Christmas trees and the ornamental value of conifers grown for and planted in the landscape.

Identification

Symptoms associated with Cytospora species include discolored foliage, premature defoliation, branch dieback, and cankers. The disease usually affects trees older than 15 years that are or have been stressed by drought, other diseases, or cultural problems. From a distance, the most obvious symptom of the disease includes dead branches, often starting in the lower part of the tree (Figure 1). If conditions are conducive in the following years, the disease will begin to spread upward.

Although canker is in the disease name, this symptom may or may not be readily visible on the host. Cankers first appear as discolored, sunken areas of bark at the base of a stem or branch, usually growing to be elongate or diamond shaped. As the fungus colonizes the vascular tissue of the plant, cankers eventually girdle, or encircle, branches, killing the branch and foliage above the canker site (Figure 2). Small branches can be girdled and killed in one to two years, while it may take several years to decades for the fungus to girdle large branches or trunks. Cankers may go unnoticed for years until the growth of new tissue causes the lesion to appear sunken. Often, a canker cannot be observed until the diseased bark has fallen from the tree or has been cut off to reveal the dead wood underneath (Figure 3).

Another phenomenon that may occur is resin flow, which is caused by a defensive response in the host plant that is especially evident in spruce species (Figure 4). Resin flow is often observed at the perimeter of the diseased tissue, but may drip down onto lower branches where there are actually no cankers. The resin may be a clear or amber color and will eventually crystallize to form a white crust.

Small fungal fruiting bodies may appear on or near the canker. These fruiting bodies hold the spores, or reproductive structures, of the pathogen, which allow it to survive, spread, and cause new infections. To observe these tiny fruiting structures, use a hand lens and look on the diseased canker tissue and in adjacent areas on the bark. During wet periods, yellow to orange spore tendrils may be observed coming out of the bark (Figure 5).

Once a branch or stem has been girdled, symptoms of dieback occur. Needles on infected branches will turn yellow, brown, purplish-brown, or reddish-brown in spring or summer during the first year of infection. Discolored needles often drop off the tree during winter, leaving bare, brittle and dead branches that may remain on the tree for many years. Small branches that have been infected may not produce new needles in spring of the following growing season. The result of Cytospora infections is a disfigured host with a barren and asymmetrical appearance. If disease is suspected, samples can be sent to a diagnostic laboratory for confirmation. Information on The Ohio State University laboratory can be found on the C. Wayne Ellett Plant and Pest Diagnostic Clinic website.

 

From top left to bottom right: Figure 1. Cytospora canker on the lower branches of a spruce tree (Penn State Department of Plant Pathology & Environmental Microbiology Archives, Penn State University, Bugwood.org); Figure 2. Canker progressing from a branch into the stem (Robert L. James, USDA Forest Service, Bugwood.org); Figure 3. Canker on trunk ad stems of spruce and fir (USDA Forest Service, USDA Forest Service, Bugwood.org) ; Figure 4. Resin flow over branch canker on blue spruce (Minnesota Department of Natural Resources, Minnesota Department of Natural Resources, Bugwood.org); Figure 5. Spore tendrils emerging from the trunk of a tree (William Jacobi, Colorado State University, Bugwood.org).

Pathogen Biology

Cytospora species are usually opportunistic pathogens attacking bark on twigs, branches or mainstems of plants that are under stress. Common stressors that may weaken plant health include drought, over-watering, temperature extremes, poor nutrition, misuse of pesticides/herbicides, de-icing salts, mechanical injuries, insects, or other diseases. The disease especially affects trees with root damage caused by improper transplanting techniques or unsuitable planting sites. Cytospora species are also ubiquitous colonists of dying or dead bark and are capable of colonizing cankers caused by other diseases.

Cytospora species overwinter in cankered bark and release spores when conditions of warm and wet weather are met. Indeed, infection usually occurs during springtime, typically in dead areas of the host plant or fresh wounds, such as those created by environmental stresses, insects, or mechanical injury from tools. Spores will germinate at temperatures between 20-33°C (68-91°F), but can remain dormant until germination requirements are met. Optimal temperature for both spore germination and initial growth of the fungus is around 27°C (80°F). Once infection has occurred, the fungus grows in the xylem and phloem, which are the vascular tissues that transport water and nutrients through the host plant. Symptoms usually occur on the host quite rapidly, with canker and fruiting body formation and discolored foliage occurring in spring or summer of the same year of infection. The needles may be cast prematurely during fall or winter of the same year of infection, or they may remain on the tree for up to a year.

Stromata (singular=stroma), which are compact masses of fungal tissue that hold fruiting bodies, may begin developing in and around the canker when moisture requirements are met. These will contain either pycnidia or perithecia. Pycnidia are fruiting bodies that contain asexual spores called conidia, while perithecia are fruiting bodies that contain sexual spores called ascospores. Pycnidial stromata are much more abundant than perithecia on the host. Stromata develop in the cambium of the host, causing the bark surface to appear raised. In the beginning, the outer bark may need to be removed to observe the fruiting structures underneath. The tops of these stromata will eventually protrude through the bark surface and appear as black, gray, tan or white dots. These fruiting bodies can be observed most readily around the edge of cankers or on the bark just outside of the canker, which allows spores to be released near and infect adjacent healthy tissue.

These fruiting bodies often mature in spring, shortly after the initial infection has occurred, and both conidia and ascospores are released in response to moisture. Spores are released most abundantly throughout spring, but also in summer and sometimes autumn months. The gradual progression of disease up the crown of the host is most often attributed to spore dispersal from rain splash. Spores can be carried to healthy, uninfected trees, through wind, insects, or cultural activities, such as pruning.

Favorable Environmental Conditions

  • Abundant moisture
  • The optimal temperature for both spore germination and initial growth of the fungus is around 27°C (80°F).

Management Guidelines

Integrated pest management (IPM) is an approach to plant health care and disease and pest control. IPM incorporates a wide range of strategies to prevent, minimize, and/or control abiotic and biotic diseases and pests. These strategies involve monitoring and scouting, learning behavior and life cycle of pests and pathogens, accurately identifying the source of disease, developing threshold levels, employing preventive measures and integrating cultural, mechanical, biological and chemical controls. All management decisions should be carried out based upon the specific requirements of the plant. Analysis of each strategy involves considering the impact on host plants, pests/pathogens, microbial interactions, the environment and society. The costs and benefits of each strategy should be evaluated before making management decisions.

Prevention:

  • Purchase certified pest and pathogen-free stock from reputable sources.
  • Nurseries:
    • Quarantine incoming plants and inspect for signs and symptoms of disease. Remove and dispose of, or return, all infected plants to the original source.
  • Proper site selection:
    • Avoid drought prone sites and those that may become waterlogged.
    • When planting a young tree in the landscape, consider the future requirements of the mature tree, and select a site that will provide the specimen with adequate space, water, nutrients and sunlight.
  • Use proper transplanting techniques to prevent damage to roots.
  • Prevent and minimize stress to plant:
    • Conduct regular soil and plant tissue analysis tests to determine if plants require supplemental nutrients. Otherwise, do not fertilize woody plants.
    • Water tree(s) during periods of drought.
    • Weed regularly and remove plants from around the tree to increase air circulation in lower branches of trees.
    • Be careful around the tree when using a lawnmower, trimmers, shears, or pruners on the tree, as these tools may create wounds, giving the pathogen easy access.

Scouting:

  • Inspect trees, especially those 15 years and older, for dead branch tips on lower branches.
  • Look near the base of dead twigs or branches for resin flow, which is indicative of cankers.
  • Remove diseased bark to reveal the dead wood underneath the surface.
  • Look for resin exudation or white patches of dried resin around and near the canker site.
  • Use a hand lens to observe fruiting structures on and near the canker site.

Cultural control practices:

  • If disease has been observed, remove infected limbs using proper pruning techniques:
    • When removing branches, make a smooth cut at the base of the limb without damaging the branch collar. Jagged and rough cuts and branch stubs are open wounds subject to infection.
    • Prune trees during dry weather.
    • Disinfect tools in between cuts.
    • Pruned branches should be discarded or burned.
  • If you have a tree plantation/nursery and Cytospora has been a problem, harvest and sell trees on poor sites as quickly as possible to prevent disease development and spread.
  • Once the pathogen has become well established in/on the host, the specimen may eventually become so unsightly it will need to be removed from the landscape.

Chemical control:

  • Fungicides are ineffective for Cytospora canker.