Yard and Garden column for the Week Beginning July 14
Tree Cankers: Nature's Scavengers
Canker diseases are among the most destructive and hard-to-manage disorders of nursery, landscape, shade, forest and fruit trees. Cankers -- localized dead areas in the bark of stems, branches or twigs -- cause limb dieback that mars the beauty of landscape trees. Cankers can create openings for invasion by wood decay fungi, which can make a bad situation even worse.
Viewed through the windshield, girdling cankers are eye-catching by their results -- dead branches, sometimes with the wilted leaves still attached, scattered among healthy-appearing foliage. Close up, cankers look different according to the particular canker fungus and the plant they attack. On shoots, cankers are sometimes easy to spot since the cankered area can be dark and discolored in contrast to nearby healthy bark. On older and thicker-barked hardwoods, discoloration may not be so evident, but the diseased area is frequently sunken, the branch can appear somewhat flattened and the bark at the margins of the canker can be swollen and cracked where a roll of callus has developed.
Especially on thin-barked trees, numerous tiny bumps, called fruiting bodies, may protrude from the bark in or around cankers. These fruiting bodies and the spores they contain provide very helpful clues in identifying the causal fungus. On conifers, cankers are often marked by leakage of resin, which crystallizes to a whitish, sticky mass on the bark. The sapwood beneath cankered bark is typically dark and discolored.
Many canker fungi survive and thrive on dead plant tissue, plant debris or in the soil. But most of them do not cause problems for healthy, vigorous trees. Instead, they are scavengers, part of Mother Nature's cleanup crew. So they pick on trees weakened by environmental stresses. A stress-weakened tree lacks the energy to erect chemical and physical barriers, so invading canker fungi can have a picnic at the tree's expense.
Among the most common sources of environmental stress are drought, flooding, freezing, extreme temperature fluctuations, nutrient deficiencies, defoliation, mechanical injury, chemical injury and transplant shock. Here in Iowa, with its harsh continental climate, our trees are often buffeted by multiple stresses: drought one month, flood the next, then freeze injury to top it off. With all this stress, it's a wonder we have any trees without cankers.
Trees are especially stress-prone during transplanting. Another stress-prone period comes after trees reach maturity and begin to decline in vigor. Because canker-causing fungi are already hanging around on or in healthy trees, cankers usually begin at a wound or a dead stub and expand in all directions from the entry point. Expansion is typically fastest along the main axis of the limb, so cankers tend to be elliptical in shape with their long axis parallel to the limb.
Other cankers are caused by mechanical injury. Many such cankers result from "mower blight," a too-close encounter between a lawn mower and an innocent tree.
Fungicide sprays are seldom useful against fungal cankers. Wound dressings have not shown much benefit either, and are even more laborious to apply than fungicide sprays. Many canker fungi are ubiquitous, colonizing all sorts of live and dead plant tissues, so chemical warfare is usually futile.
Instead, a good general recommendation is to treat the tree, not the fungus. Since most canker diseases single out severely stressed trees, practices that prevent or reduce tree stress can prevent cankers. If our patients were human, we'd call this strategy "wellness." In other words, the key to avoiding canker diseases is to keep our patients -- landscape trees -- in vigorous health.
Wellness practices for drought-prone trees include mulching, which reduces both water loss from the soil and competition by thirsty, water-hogging turfgrass. A 3- to 4-inch-thick layer of mulch, from just beyond the trunk out to the drip line (below the tips of the tree's branches) works well in Iowa. Timely watering and fertilizing (when justified by soil test results) helps to keep a tree in vigorous, canker-proof condition.
Because most canker-causing fungi enter the tree through wounds created by insect feeding, pruning or other damage, it's helpful to avoid wounding and severe pruning when possible. But pruning out cankers can delay the appearance of new cankers by removing a large proportion of the fungal spores from the tree. It's a good idea to burn, bury or otherwise destroy cankered prunings promptly. Try to avoid pruning out cankers during wet weather because infections occur readily during this time.
Tree wellness begins before planting. Probably the most important step to wellness is to match the tree with the site. The emphasis on planting native species is commendable, but practicing "safe selection" doesn't end there. If the site has special challenges -- say, for example, the building contractor bulldozed and sold off the topsoil right down to the clay before the trees were transplanted -- the trees selected should be adapted to these challenges. For this type of site, safe selection is more than a bit challenging. Naturally, proper transplanting technique and post-planting maintenance are also critical to wellness.
Some canker diseases can be deterred by selecting the right cultivar. Doing your homework, and contacting local experts, in advance can prevent cankered trees down the road by making sure you've selected tough, hardy trees.
When tree cankers appear, it's already late in the game. The tree, and you, are losing. For cankers, prevention -- wellness -- means doing everything feasible to keep trees in vigorous, canker-proof health. Wellness is the only anti-canker strategy that really works.
Extension programs are available to all without regard to race, color, national origin, religion, sex, age, or disability.