Deter canker and dieback diseases
Canker diseases are common, widespread and destructive. They attack all woody plants, especially those low in vigor. Cankers may cause wilting and dieback of twigs and branches and structurally weaken a plant so much that it breaks in a wind, snow or ice storm. Cankers also slow the normal healing of wounds thus providing easy entry for wood-decay, wilt-producing and other disease-causing organisms.
Canker and dieback diseases are most common and easily seen on trees and shrubs growing under one or more of a variety of stresses: transplant shock, an excess or deficiency of water, prolonged exposure to temperature extremes, sudden hard freezes in mid- to late fall or spring, summer or winter sunscald, frost cracks, imbalances of essential nutrients, extensive defoliation by diseases or insects, soil compaction, changes in the soil grade, mechanical injuries (hail, wind, a heavy ice or snow load, lawn mowers, construction equipment, borers, dogs, livestock or deer), pruning wounds, root rot, nematodes, or improper digging, storage and shipping of nursery stock.
What is a canker? A canker is a dead area or lesion, usually in the bark of a woody plant, that often results in an open wound. Starting as a small, sharply delimited dead spot, usually round or oval to elongated in shape, a canker may enlarge and girdle the cane, twig (shoot), branch (limb), trunk or root.
Fungi cause practically all cankers. Bacteria (fire blight), viruses, other types of pathogens, and non-pathogenic or abiotic agents can cause cankers as well. Most canker-causing fungi are not host-specific. They have wide host ranges and may grow on fallen branches or other dead wood. Many are only weak or secondary pathogens with both sexual and asexual states (for example, Nectria and Tubercularia, Cytospora (Leucocytospora) and Leucostoma Valsa), Fusicoccum and Botryosphaeria). These states may or may not be present at the same time on the same plant.
Most canker-forming fungi overseason as mycelium or fruiting bodies in diseased and dead wood. They often grow in the bark of living woody plants on dead tissue both on the standing plant and on the ground. In general, cankers deform but do not kill their host plants.
Wound parasites account for nearly all canker pathogens. The death of the bark and underlying cambium tissue usually is associated with a dead bud, branch stub or twig, or with some type of mechanical injury, and extends radially from the wound.
When the water-conducting tissue (xylem) is killed or blocked, the most prominent symptom is a gradual or rapid wilting, withering and dieback of the entire shoot. This process begins at the tip and progresses downward to a girdling canker some distance away. Depending on the size of the canker and the organism involved, the leaves above the cankers are often smaller than normal, pale green at first, then usually turning yellow or brown, curled and sparse. The color change is often more pronounced at first between the veins and along the leaf margins.
Large or multiple trunk cankers may result in severe girdling and then death of the affected tree or shrub. Crown canker or collar (foot) rot refers to a canker that develops on the trunk near the soil line.
Cankers commonly develop as conspicuous, slightly sunken or flattened areas of bark or as inconspicuous diseased areas that you cannot easily detect by examining the bark surface. Diseased bark of conspicuous sunken cankers often crack within or at the margins, exposing the wood underneath, which may be discolored brown or black. In other cankers (such as Nectria), swollen ridges of callus form a target-like area. On woody plants with light-colored bark, diseased bark that does not become somewhat sunken may turn some shade of tan to brown or black. Such cankers are obvious due to their abnormal color.
Inconspicuous cankers usually appear as brown to black areas in the sapwood when you cut away the outer bark. Raised--speck-sized or larger--fruiting bodies in the diseased bark (usually brown, black, orange or red) indicate many fungus-induced canker diseases. Many fruiting bodies erupt through the bark appearing as dark spots or bumps. Oozing of gum, resin or sap (for example, Sphaeropsis on evergreens) is a common sign of certain cankers. Typically, you can only find the mycelium of the causal fungus in discolored tissue and 1 to 2 millimeters into healthy-appearing tissue.
Cankers are common in nurseries and plantations where they grow plants close together and air movement is poor. Canker dieback diseases usually occur at a low, indigenous level and may act as "selective agents" in eliminating the less vigorous members of a stand. Although abiotic injuries (such as hard impact) can induce cankers, they generally heal quickly on healthy, vigorous plants.
Perennial canker-producing fungi enter through mechanical wounds in the bark (such as a branch stub or mowing bruise). They invade and kill the bark, commonly during the dormant period. The host plant tries to limit invasion by producing a layer of callus cells around and over the edge of the invaded tissue; the fungus invades the callus tissue during the next dormant season. The host then forms new callus tissue. This cycle of fungal invasion and formation of concentric layers of callus tissue by the plant may repeat for many years in cankers caused by bacteria, Urnula (Strumella) and Eutypella (in maples). The growth of the pathogen and the amount of callus the host forms determines the extent and shape of the resulting "holdover" canker. If the fungus grows more rapidly than the host, callus layers do not form resulting in a diffuse-type canker.
Annual cankers contain little or no callus and increase rapidly during a single growing season. Branches or even entire woody plants, especially young ones, may be girdled and killed in a few weeks to one growing season. Fungi (Fusarium and Fusicoccum, for example) that infect stressed or weakened plants cause most annual cankers.
Most canker fungi are restricted to invasion of bark tissues. Some, however, colonize both the bark and the underlying xylem tissue. Canker rots (Fomes or Phellinus, for example) are stem diseases that result from a simultaneous canker and wood decay.
Canker fungi usually produce their fruiting bodies in recently killed bark. The spores these fruiting bodies produce serve as inoculum for new infections, which occur mostly during wet or damp weather. Air currents, splashing rain, and contaminated pruning and shearing tools spread spores; insects, mites and birds transmit a few as well. The spores may ooze from the fruiting bodies in long tendrils during wet weather.
Controlling canker-dieback diseases You can protect your trees and shrubs from cankers if you follow the proper procedures listed below. Prevention begins when you select your plant.
* Choose your plants. Purchase and plant only species and varieties or cultivars of trees and shrubs that are native or well-adapted to the area and planting site. The nursery stock should be vigorous, disease- and insect-free. In addition, it should be balled and burlapped (B&B) or container-grown from a reputable nursery that follows strict sanitation-indexing production procedures. Be sure the plants have white, healthy roots and good foliage color. Resistant species and cultivars are available for a few canker diseases. Check with an experienced nurseryman or your extension plant pathologist. Grow somewhat tender species in sheltered sites. Plant at the proper time and depth in a large hole--spaced well apart based on the expected diameter of the plant at maturity--in fertile, well-drained soil of the proper pH. Avoid planting in shallow or excessively drained soils--for example, in light, sandy or gravelly soils--or where drainage is poor.
* Remove bad wood. Carefully remove cankered twigs and branches, including all dead, dying and diseased wood. Cut back several inches into surrounding live, healthy tissue. Remove and burn, or haul away, all infected parts when you first notice them. Avoid leaving branch stubs. Make pruning cuts as flush with the branch or trunk collar as possible. Avoid pruning when the foliage, twigs and branches are wet, as this helps to spread canker-forming organisms. Where feasible, clean and disinfect pruning tools between cuts on a cankered plant (and always before pruning a healthy one). Swab or immerse tools in a solution of 70 percent rubbing alcohol or freshly prepared liquid household bleach (1 part bleach to 9 parts clean water). Coat pruning cuts larger than 1.5 to 2.0 inches in diameter with shellac, which acts both as a disinfectant and protectant against canker organisms.
* Remove trunk cankers. You can successfully remove some trunk cankers, if less than halfway around the tree or shrub stem, by surgically removing all diseased bark and the underlying discolored wood. Also include some apparently healthy tissue around and under the canker. You cannot restore severely cankered trees and shrubs to good health. Therefore, because they are a source of infection for other plants, you should cut them down and either burn or haul them away. Surgically remove serious girdling roots above and below the ground line.
* Promptly treat all bark and wood injuries. Cut away all loose or discolored bark and remove splintered or discolored wood. Clean, smooth and shape the wood into a vertical, oval or ellipse with rounded tips. Then paint the clean wound surface liberally with shellac. Most nurserymen do not recommend using commercial tree paints and dressings, as their effect is largely cosmetic. Surgery may prolong the lives of severely affected trees.
* Keep plants growing vigorously by: * Applying balanced fertilizer in mid- to late autumn or early spring based on a soil test. Use slow-release forms of nitrogen fertilizer. Excessive nitrogen fertilization prevents normal maturation, increasing the susceptibility to injury from sudden, extreme temperature drops in early winter.
* Slow-soaking the soil, extending to the outer drip line and beyond. Moisten the soil to an 8- to 12-inch depth every 10 to 14 days during extended hot, dry periods. Avoid overhead irrigations and syringing the foliage when watering, especially in late afternoon or evening.
* Pruning during the dormant season to eliminate all dead, diseased and insect-infested wood, crossing and interfering branches, and to open up the centers for good air circulation.
* Protecting young tree trunks in winter to prevent sunscald and bark injury (see below).
* Providing support for young trees with a stake or trellis. Loosen compacted soil and any pavement or soil fill and apply a thick (4 to 6 inches), loose organic mulch where feasible. Wrap young, thin-barked and sun-exposed trees with sisalkraft paper, special tree-wrapping paper or other appropriate material before winter. Contact local experienced nurserymen, county extension personnel or horticulturists for other cultural practices to follow.
* Avoid all unnecessary bark wounds. Keep the trunk base as dry as possible and free of grass, weeds or other debris that might attract rodents.
* Make timely applications of suggested insecticides to keep the feeding of defoliating insects, borers and other wood-attacking pests to a minimum.
* Avoid chemical injuries. Apply herbicides, other pesticides, salt, fertilizers and other chemicals strictly according to label directions.
No chemical treatments are available that effectively prevent or arrest the development of practically all canker diseases, although you may find some exceptions to roses, junipers, evergreens and Phytophthora cankers on fruits and rhododendrons.
Dr. Malcolm C. Shurtleff is professor emeritus of plant pathology at the University of Illinois at Champaign/Urbana.
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