Cytospora or Perennial Canker of Fruit and Shade Trees
Sherman V. Thomson/Extension Plant Pathologist
Scott C. Ockey/Plant Disease Diagnostician
|Healthy tissue on the left with dead, cankered tissue on the right on a peach tree infected with cytospora.|
|Orange tendrils of spores produced by the spore bearing pycnidia of Cytospora.|
|Cytospora producing small (1 mm) pimple-like bumps in which black fungal structures called pycnidia are embedded.|
|Peach branch with a gumming canker and an advancing margin of necrotic tissue. Cytospora causes gumming sunken cankers in many stone fruits such as sweet cherry and peach.|
Cytospora or Perennial Canker
Cytospora Canker or Perennial Canker is one of the most common diseases of fruit and shade trees in Utah. This canker disease is caused by a fungus called Cytospora. There are several species of Cytospora that attack many different hosts, but the symptoms and control are essentially identical for all of them. The host range of Cytospora is broad, including peach, cherry, apricot, apple, poplar, willow, birch, aspen, and many other broadleaf trees. Stone fruits are more susceptible to perennial cankers than are pome fruits.
Cytospora is considered a weak parasite and invades only weakened or stressed trees. It gains entry through injuries in the bark caused by machinery, sun scald, frost, pruning wounds, broken branches, mechanical shakers, and insect injury. Cankers on trunks or branches are sunken and range in color from brown to gray depending on the host species. The canker expands slowly over a period of months or years and may eventually girdle the branch, causing it to die. The fungus may produce small (1 mm) pimple-like bumps in which black fungal structures called pycnidia are embedded. The presence of pycnidia can be confirmed by slicing the bark with a knife where raised areas are evident. Pycnidia are quite common and obvious on mountain ash, cherry, and birch. During warm, wet weather, brown to orange-colored masses of spores (analogous to seeds) are extruded from the pycnidia in a tendril-like mass. These spores are carried by rain or blown by wind to susceptible sites where they cause new infections. Optimum conditions for Cytospora infections occur in the spring when daytime temperatures are 60 to 80F. The fungus continue to grow and produces spores during the warm weather months.
Preventing infection is the best way to control Cytospora. There are no fungicides which are effective in controlling the pathogen once it is in the tree. No single method of control can be used to prevent this disease; therefore, it is necessary to use several of the methods described below to maintain healthy plants.
- Maintain high tree vigor. Trees should be watered deeply during dry summer months to prevent drought stress. Fertilize in the spring to keep trees vigorous. Avoid late summer applications of nitrogen because it stimulates growth in autumn which does not harden off before winter. Trees stressed with iron chlorosis are particularly susceptible to Cytospora infections. The roots of birch trees are quite shallow and are damaged or killed by high soil temperatures and drought on south or west facing slopes. Keep soil cool and moist by frequent irrigation.
- Prune out and destroy dead or diseased twigs and branches. Do not leave stubs or narrow crotches. Prune on a regular basis so that large cuts will not be necessary. Pruning wounds are susceptible to infections, so prune in the early spring and not when rain is imminent. Treat pruning cuts larger than one inch in diameter with a paint of 1% thiram or 3-10% Copper Naphthenate. Asphalt pruning paints are not effective. An application of benomyl as a spray immediately following the pruning of a fruit orchard may reduce new infections. Benomyl is no longer registered for use on ornamental trees.
- Prevent sunscald by painting the trunk of thin-barked trees with white latex paint. The trunks of newly planted trees should be wrapped with burlap or white-colored tree wraps to prevent sunburn. These techniques will also reduce winter damage which occurs on the southwest side of trunks.
- Control borers and other wood-attacking insects.
- Avoid mechanical injury to tree with lawn mowers, lawn trimmers, ladders, shakers, or other equipment.
- Woodpiles are an important source of inoculum for the disease. To prevent infections on nearby trees, destroy any wood that appears to have pycnidia.
Precautionary Statement: All pesticides have benefits and risks, however following the label will maximize the benefits and reduce risks. Pay attention to the directions for use and follow precautionary statements. Pesticide labels are considered legal documents containing instructions and limitations. Inconsistent use of the product or disregarding the label is a violation of both federal and state laws. The pesticide applicator is legally responsible for proper use. This publication is issued in furtherance of Cooperative Extension work. Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, Vice President for Extension and Agriculture, Utah State University.