Garden column for the week of Feb. 11, 2005
There is no photo for this week's column.
By Katrina Duttweiler
Iowa State University Extension
Have you noticed discolored patches on the bark of your trees this winter? If so, your trees may be victims of fungal cankers. Cankers, which are dead areas in the bark, can appear on the stems, branches and twigs of trees.
While tree cankers can be caused by environmental stresses (temperature extremes or hail, for example), chemicals, insects or bacteria; most cankers are caused by fungal infections. Canker fungi often invade trees at branch stubs or through wounds caused by insect, environmental or mechanical damage. The fungi grow under the bark, often discoloring the wood just beneath the bark. The outer bark becomes discolored in trees with thin bark, but this symptom is less apparent in trees with thick bark.
Cankered bark can appear sunken or flattened, with swollen and cracked edges. Resin or gum, which becomes white and sticky after drying, is often secreted from cankers on infected conifers. Tiny, pimple-like bumps may develop on the cankered region. These red, dark-brown or black bumps are the reproductive structures containing spores that spread the fungi.
Appearance of cankers is also impacted by the particular fungal species that invaded the tree. Certain fungi cause target-shaped cankers, which are slow to develop and non-fatal. The shape of these cankers is due to callus development. Callus, which is rapidly created woody tissue, is the tree's defense against fungal expansion. Years of alternately attacking fungi and defending the tree results in the formation of concentric rings of callus surrounding a focus of infection.
Other fungi cause diffuse cankers, which can be a serious concern due to their potential to cause tree dieback and death. Encirclement by a canker kills twigs, branches, or even entire trees by preventing the movement of nutrients and water above the cankered area. Diffuse cankers can be shallow with discolored margins but are often recognizable by their lack of callus development. The fungi establish so quickly that the tree is not able to develop a callus barrier.
All cankers can detrimentally impact a tree's growth. While cankers rarely kill trees, deformation and weakening of trees is common. Cankered branches can be killed and show wilted leaves that do not drop off. Cankers on the trunk are especially damaging since the whole tree is affected rather than individual branches or twigs.
Cankers are easier to prevent than treat. Fungicides are ineffective against cankers, and the benefits of wound dressing are questionable at best. Pruning cankered branches can delay the spread of the disease, but can be costly and disfigures the tree.
Prevention by maintaining tree health is the best way to manage fungal cankers. A stressed tree is less capable at defending itself against the ever-present canker fungi. Since an unfavorable growing environment is one of the main causes of tree stress, canker prevention means making the tree's environment as stress-free as possible.
Mulching, in particular, can moderate the impact of environmental stresses such as droughts and temperature extremes. Mulches conserve soil moisture, which can prevent water stress during dry periods. Mulch partnered with watering avoids moisture deficits even during droughts.
Soil temperature is also buffered by mulch, keeping the soil warmer on cold days and cooler on hot days. The result of these modifications is that tree roots grow more extensively and the tree is better able to absorb the water and minerals it needs. Mulch also protects the trunk from mechanical wounding, due to lawn mowers, by creating a grass-free zone underneath the tree.
So when it comes time to mulch again, think about spreading mulch at a depth of three to four inches around the base of your trees. The mulch ring should be as wide as the tree's canopy. While increasing the beauty of the landscape, decreasing the area you need to mow, and smothering weeds, mulch will also enhance the growing environment of the trees. Your trees will be healthier and less susceptible to many pests and diseases, especially fungal cankers.