How to Manage Pests

Stem cankers and dieback—Various causes

There are many causes of cankers and dieback, including adverse environmental or growing conditions, inadequate cultural care, mechanical (physical) impact, and plant pathogenic bacteria, fungi, and oomycetes (water molds).

Identification

A canker is a localized dead (necrotic) area on branches, stems, trunks, or roots. Cankers vary greatly in appearance but are often a circular or oblong lesion that may be discolored, oozing, sunken, or all of these. When the canker cause is a plant pathogen, cutting under cankered bark usually reveals brown to yellowish discolored vascular tissue and sometimes dark discolored wood. This discolored, unhealthy tissue commonly has well-defined margins separating it from healthy tissue (e.g., green or red cambial tissue or white wood). When cankers entirely circle (girdle) stems or trunks, foliage turns yellow or brown and wilts as plant parts die outward or upward from the canker. If the main trunk becomes girdled the plant dies.

Adverse environmental or growing conditions, mechanical (physical) impact, and cultural practices that can cause cankers include:

• Boron toxicity. Can cause cankering and dieback of young twigs.
• Hail. Causes elliptical wounds in bark. If wounds are large, branches develop cankers and can become girdled and die.
• Mechanical injuries. Many causes including impact from equipment (e.g., mowers and string trimmers) or vehicles (e.g., bumpers), pruning (especially when large limbs are removed or cuts are make improperly), vertebrate pest chewing, and other sources of mechanical injury can cause the development of cankers.
• Oil application. When sprayed with oil, trees exposed to extreme temperatures, low soil moisture (drought stress), prolonged high winds, or other stressful conditions can develop darkened bark and cankered, dead shoots and twigs.
• Staking. Improperly tying the trunk of young trees and leaving ties on for too long can cause cankered bark and wood.
• Sunburn. Defoliated, severely pruned, underwatered, and young trees can become cankered due to exposure to excessive heat and light. Sunburn cankers generally occur on the unshaded, south and west sides of limbs and trunks of most any woody species including almond, apple and pear, stone fruits, and walnut.
• Sunscald. Winter injury to bark, occurs in mountainous areas and high valleys of eastern California. Damage develops after bark warms from exposure to sunlight then cools as the sun drops below the horizon.
• Too little soil moisture. Plants are most susceptible to cankers caused by sunburn if roots lack sufficient water.

Bacterial, fungal, and oomycete causes of cankers include

• Annulohypoxylon canker, or Hypoxylon canker. Annulohypoxylon =Hypoxylon species hosts (and the causal pathogen) include alder, birch, maple, and hornbeam (Annulohypoxylon mammatum), podocarpus (A. mediterraneum), poplar and cottonwood (A. mediterraneum), honeysuckle (A. rubiginosum), mountain mahogany (A. mori, A. rubiginosum), and oak (A. thouarsianum).
• Anthracnose. Various fungi affect numerous hosts and commonly also cause distorted terminals, leaf spots, and premature defoliation.
• Bacterial blast, blight, and canker. Pseudomonas syringae causes cankers and otherwise damages many hosts including California bay, cotoneaster, forsythia, hazelnut, magnolia, maple, mulberry, oleander, pear, pittosporum, plum, podocarpus, poplar, cottonwood, rose, syringa, viburnum, and willow.
• Bacterial canker of avocado. Xanthomonas campestris is a bacterium that causes cankers on the limbs and trunk of avocado.
• Blackline of walnut. Cherry leafroll virus causes cankers around the graft union of English walnut cultivars grown on rootstocks of Northern California black walnut and Paradox.
• Botryosphaeria canker and dieback. Hosts of Botryosphaeria and Fusicoccum species, most commonly B. dothidea, include alder, birch, California bay, ceanothus, cedar, incense cedar, chestnut, Chinese pistache, coast redwood, crabapple, dogwood, elderberry, eucalyptus, hazelnut, giant sequoia, liquidambar, madrone, manzanita, maple, oak, rose, strawberry tree, walnuts, willow, and wisteria.
• Branch canker and dieback of avocado. Formerly called Dothiorella canker, Phomopsis =Diaporthe species and various other Botryosphaeriaceae and Diaporthaceae species, cause limb cankers and branch dieback in avocado.
• Brown rot. Monilinia species kill blossoms and cause cankers on stems of almonds, apricots, cherries, nectarines, peaches, plums, prunes, and quince.
• Brown rot of citrus. Phytophthora species can cause cankers on bark and cause various other disease symptoms such as fruit rot.
• Cane blight. In caneberries Paraconiothyrium (=Coniothyrium =Microsphaeropsis) fuckelii =Kalmusia (=Leptosphaeria) coniothyrium cankers and kills stems.
• Chinese elm anthracnose. Stegophora ulmea damages only Chinese elm (Ulmus parvifolia). In California, damage occurs mostly in coastal areas.
• Cypress canker. Cytospora (=Coryneum) cardinale is most damaging to Leyland cypress and Monterey cypress when they are planted away from the coast. Other hosts include arborvitae, cypress, incense cedar, juniper, Port Orford cedar, and western red cedar.
• Cytospora canker. Cytospora species can cause cankers on various trees and shrubs. Host include birch, ceanothus, coast redwood, cottonwood (C. chrysosperma), cypress (C. cenisia), dogwood, fir, maple, mountain ash, podocarpus, poplar (C. chrysosperma), Port Orford cedar, redbud, and willow.
• Deep bark canker of walnut. Erwinia rubrifaciens is a bacterium that cankers walnut limbs and trunks and causes trees to decline but does not kill them.
• Dematophora root rot, or Rosellinia root rot. Rosellinia (=Dematophora) necatrix can cause dark cankers in the lower trunk and main roots of infected hosts. Dematophora root rot is not a common disease in landscapes.
• Diplodia canker and shoot dieback. Diplodia species hosts include almond, apple, citrus, palms, pine, oak, rhamnus, and walnut.
• European canker, or Nectria canker. Nectria galligena in California infects at least apple, California bay, dogwood, maple, and pear.
• Eutypa dieback. Eutypa lata commonly causes canker disease in apricot, cherry, and grape.
• Ficus canker. Neofusicoccum (=Nattrassia) mangiferae and Botryosphaeria species cause cankers and limb dieback in Indian laurel fig, or Chinese banyan, and can kill the trees.
• Fire blight. Erwinia amylovora most severely affects pear and quince and also causes cankering and shoot dieback of some other plants in Rosaceae family.
• Foamy bark canker. Geosmithia pallida infecting coast live oak causes branch and trunk cankers and dieback and sometimes kills infected trees.
• Foamy canker, or alcoholic flux. White, frothy material sometimes exudes from cracks or holes in bark, commonly on elm, liquidambar (sweet gum), oak, and pittosporum. Other hosts can also be affected.
• Nectria canker. Coral spot fungus (Nectria cinnabarina), European canker (N. galligena), and other Nectria species affect numerous woody species. Hosts (and some causal fungi) include alder, apple, ash, birch, California bay (N. cinnabarina, N. coccinea, N. galligena, N. peziza), crabapple (N. galligena), dogwood, elderberry, elm (N. cinnabarina), eucalyptus (N. eucalypti), honey locust (N. cinnabarina), Japanese pagoda tree, locust, magnolia, maple, mountain ash, mulberry, oak (N. peziza), pear (N. cinnabarina, N. galligena), rhus (N. cinnabarina), rose, walnut (N. cinnabarina), willow, and zelkova (N. cinnabarina).
• Oak branch canker and dieback. Diplodia corticola and Diplodia quercina hosts include California black oak, coast live oak, English oak, and valley oak.
• Pestalotiopsis tip blight and dieback, or cryptomeria blight. Pestalotiopsis funerea most commonly cankers cryptomeria. Other hosts in California include coast redwood, common juniper, monkey puzzle tree, Monterey cypress, Monterey pine, and Port Orford cedar.
• Phomopsis canker, or Diaporthe stem canker and dieback. Phomopsis =Diaporthe species affect many conifers and woody broadleaves. Hosts (and some causal fungi) include almond (Phomopsis amygdali),avocado, cedar, cypress, dogwood, Douglas-fir (P. lokoyae), elderberry (D. sociabilis), eucalyptus (D. eucalypti), fig, golden-chain tree (D. rudis), grape, honey locust, honeysuckle (D. eres), incense-cedar, madrone, pear, Port Orford cedar, strawberry, walnut, and wisteria (P. wistariae).
• Phytophthora gummosis of citrus. Phytophthora species infecting the lower trunk cause bark to crack and ooze.
• Phytophthora root and crown rots. Phytophthora species can infect numerous plants. Discolored and oozing bark, foliage discoloring and wilting, and premature leaf drop are common damage symptoms.
• Pitch canker. Fusarium circinatum infects pines and causes stem cankers that result in shoot dieback. Douglas-fir can also be infected but is less severely damaged.
• Purple blotch of caneberries. Septocyta ruborum =Rhabdospora ramealis in California is an uncommon disease of caneberries that causes cane lesions, girdling, and dieback.
• Rust of peach. Tranzschelia discolor infection causes elongate splits and lesions in bark of 1-year-old twigs mostly on peach and prune.
• Raywood ash canker and decline. Botryosphaeria stevensii infects Fraxinus oxycarpa 'Raywood' that are drought stressed, causing stem cankers and branch dieback.
• Rusts. Many hosts are infected by various species of rust fungi that commonly form orangish pustules on infected plant parts.
• Sappy bark. Trametes (=Coriolus) versicolor on apple and numerous other hosts causes bark and wood to decay and become discolored and spongy. Affected bark commonly peels away, exposing dark, decayed wood underneath.
• Septoria leaf spot. Populus species (e.g., cottonwood and poplar) severely infected with Septoria populicola develop leaf spots and branch cankers (Septoria canker).
• Shallow bark canker of walnut. Brennaria (=Erwinia) nigrifluens causes irregularly shaped to round lesions on outer bark, which turn brown and dry. These superficial cankers can become abundant, but usually cause little damage to the tree.
• Sooty canker. Neofusicoccum (=Nattrassia) mangiferae =Hendersonula toruloidea most commonly cankers citrus, ficus, mulberry, and walnut. It can affect many other hosts.
• Sudden oak death. Phytophthora ramorum cankers and kills coast live oak, canyon live oak, California black oak, Shreve’s oak, and tanoak.
• Sycamore canker stain. Ceratocystis fimbriata f. sp. platani causes Platanus species to develop elongate cankers on large limbs and trunks. The margins of cankers can appear dark, flattened, and sunken and may be covered with discolored or flaky bark. Cutting into cankered cambium, phloem, and sapwood reveals dark discoloration, typically bluish black. Infected sycamores usually die within 1 or 2 years after becoming infected.
• Volutella blight, or Pseudonectria canker. Pseudonectria rousseliana =Volutella buxi is one of the most common diseases of boxwood. Infected shoots become cankered and die back.
• Walnut blight. Xanthomonas arboricola (=X. campestris) pv. juglandis infects green shoots on walnut. The bacterium causes superficial lesions one to several inches long that can extend into the pith to form cankers and girdle and kill twigs.
• Walnut thousand cankers. Geosmithia morbida is a fungus spread by the walnut twig beetle. It kills infected walnut trees, primarily black walnuts.
• Western gall rust. Endocronartium (=Peridermium) harknessii cankers and galls pines that have needles in bundles of two or three. Hosts include Aleppo, Bishop, lodgepole, Monterey, ponderosa, Scots, and shore pines.
• White pine blister rust. Cronartium ribicola cankers and kills all species of pines that have needles in bundles of five. Hosts include foxtail, limber, singleleaf pinyon, sugar, western white, and whitebark pines.

Solutions

To manage canker diseases, do not plant species that are poorly adapted to the site conditions. Provide plants with a good growing environment and appropriate cultural care so they grow vigorously. Adequate soil moisture is especially important in reducing the presence and severity of many canker diseases. Protect aboveground plant parts and roots from injury.

Prune dead and dying branches when conditions and plants are dry. Dispose of cuttings away from valued plants because dead leaves and wood are common sources of spores of many canker pathogens. Use sharp tools and make the cuts in healthy wood below any apparent cankers. Make each cut just outside the branch collar and branch bark ridge. After working on a plant infected with a canker pathogen, clean tools and dip or spray them with a 10% bleach solution, 70% isopropyl rubbing alcohol, or a commercial disinfectant as directed on the product label. Consult the information at the links above for information and advice specific to the particular disorder or disease.

Adapted from publications cited in the links above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).

Shoots killed by Volutella blight, also called Pseudonectria canker.

Cankers of Chinese elm anthracnose.

A canker and oozing bark due to Eutypa dieback.

Bark cut off a bacterial canker revealing brown, diseased cambium.

A Nectria canker at the base of a dead twig killed by the fungus.

Pimplelike fruiting bodies of a Botryosphaeria canker fungus on bark.