Botryosphaeria Canker and Dieback of Trees and Shrubs in the Landscape
Most trees and shrubs are susceptible to dieback and cankers caused by several species of the fungal genus Botryosphaeria. Botryosphaeria fungi are typically opportunistic pathogens. Opportunistic pathogens only cause disease on plants that are stressed. Therefore, avoiding plant stress, which predisposes plant tissueto infection and colonization by this fungal group, is the best strategy to prevent Botryosphaeria disease problems.
Symptoms of Botryosphaeria cankers and dieback are most commonly seen as wilting or dieback of a branch or branches on a tree or shrub that, in other respects, appears healthy (figs. 1a, 1b). Cankered twigs and branches may not be noticeable until wilt and dieback occur. However, if bark is removed, the wood beneath the bark will be discolored brown to reddish- brown instead of white (fig. 2a). The discoloration in the wood may also be observed in a cross-section of a branch or main stem (fig. 2b). In some cases, cankers may appear sunken and/or darkened (fig. 3) or be surrounded and contained by callused wound wood, particularly on larger branches or trunks. In other cases, bark may peel and drop from cankered areas (fig. 4).
Cankered areas on tender stems of smooth-barked species, such as crab apple, may appear blistered (fig. 5). On gum-producing trees, such as sweetgum and Prunus species, gum may exude from cankered areas (fig. 6).Because of this symptom, Botryosphaeria canker and dieback on peach is called “gummosis.” Black fungal spore-producing structures (pycnidia) are sometimes present on diseased tissue and can be observed erupting through the bark. These fruiting bodies are white inside when sliced open (fig. 7).
Botryosphaeria fungi overwinter as fruiting bodies on dead tissue. They can also survive as endophytes on bark or evergreen leaves. (Endophytes do not cause disease, but live in a beneficial or nondetrimental manner on plant tissue.) Unlike most plant pathogens, Botryosphaeria fungi are not host-specific but can cause disease on many plant species. Individual isolates of this fungal group have been reported to differ in aggressiveness, with some being weak pathogens.
Botryosphaeria fungi colonize plant tissue through wounds, growth cracks, leaf scars, and lenticels (pores on stems, branches, and trunks that allow gas exchange). On rhododendron, Botryosphaeria spp. have been shown to initiate infections on the stem of the flower and colonize tissue progressively downward, causing branch dieback. Similarly, Botryosphaeria fungi may initially colonize dead branch tissue and move downward on the branch into healthy bark and sapwood. Spread of these fungi occurs through air movement or splash dispersal of spores, and can also occur through use of contaminated pruning tools.
Avoiding and Managing Botryosphaeria Disease
There are no effective fungicide controls for Botryosphaeria dieback. The best defense against this commonly occurring disease is to ensure plants are in optimal health by providing the appropriate cultural requirements for the particular plant species, avoiding plant stress and injury, and employing appropriate sanitation measures.
It is important to remember that under optimal growing conditions, trees and shrubs are typically able to resist infection and colonization by Botryosphaeria fungi. The remainder of this fact sheet outlines specific tactics homeowners can use to avoid infection by these opportunistic fungal pathogens on landscape shrubs and trees.
Proper plant selection and placement in the landscape are the first steps toward vigorous, healthy landscape plants that are able to resist attack by Botryosphaeria fungi. Growers should carefully inspect potential plant purchases for signs of stress or pest problems and avoid purchasing plants that are infested with pests or show evidence of stress or injury (e.g., severely pot-bound container plants; wounded branches, stems, or trunks; wilted or browning leaves). Growers should also avoid purchasing plants that exhibit signs of plant neglect (e.g., inadequate or excessive irrigation, poor sanitation, overcrowded or toppled container plants).
Growers should select plants with an appropriate hardiness zone rating for their region. Plants that are planted outside their recommended hardiness range will not thrive and are likely candidates for Botryosphaeria attack. Also, trees and shrubs should be placed in landscape locations appropriate for the particular plant species.
Different plant species have preferences and/or tolerances for certain cultural conditions (e.g., sun versus shade, moist versus dry soil conditions, protected versus exposed locations, etc.). For example, redbud and sweetgum trees located in shade have been reported to experience more numerous and more severe Botryosphaeria cankers and dieback compared to those located in full sun. Conversely, rhododendron planted in full sun is more susceptible to Botryosphaeria disease than rhododendron in part shade.
Cultural preferences and hardiness ratings are typically listed on the plant tag, but there are many other sources of information on plant cultural requirements, including local Virginia Cooperative Extension agents (www.ext.vt.edu/offices/), knowledgeable staff at nurseries and horticultural publications. Time taken to carefully examine potential plant purchases and ensure proper cultural conditions will go a long way toward avoiding Botryosphaeria dieback and cankers.
Many environmental stress factors, such as heat, drought, freeze injury, and compacted soil, can predispose trees and shrubs to infection and colonization by Botryosphaeria fungi. Some environmental stress factors are unavoidable, but many can be avoided.
Providing irrigation to trees and shrubs during dry periods is recommended. Likewise, susceptible trees and shrubs should be protected from a sudden and significant drop in temperature when possible.
Growers should avoid planting locations compacted by construction or foot traffic and soils that are excessively wet or drain poorly. If these conditions are present, they should be corrected before planting.
Poor cultural practices can also predispose plants to infection by Botryosphaeria spp. Some cultural problems commonly observed include: plant tissue injury during pruning and other activities, excessive mulch application, plants set at an improper soil depth, failure to irrigate plants during establishment, improper soil pH, and inadequate sanitation practices.
Wounded plant tissue is susceptible to infection by Botryosphaeria fungi. Growers should practice proper pruning techniques to avoid large pruning wounds. When working in the landscape, avoid injury to trees or shrubs through careless use of equipment, such as lawn mowers or trimmers. Also, handle plants carefully during planting to avoid injury to roots, trunks, stems, and branches of trees and shrubs. Monitor plants carefully for signs or symptoms of insects, voles, and other pests that can injure plant tissue and predispose tissue to infection by Botryosphaeria spp.
Proper nutrient availability is necessary for vigorous, healthy trees and shrubs. However, growers should avoid fertilizing woody shrubs and trees unless a soil test report or tissue analysis indicates a need for supplemental nutrients. Excessive fertilization encourages succulent tissue that is prone to pest problems. Additionally, most nutrient problems on landscape plants are not caused by inadequate soil nutrients, but by improper soil pH. A local Virginia Cooperative Extension agent (https://ext.vt.edu/offices.html) can assist homeowners in obtaining a soil test to check soil pH and determine if a fertilizer application is recommended. The soil test report will include information on adjusting soil pH to the optimal range for individual tree and shrub species if a pH problem is identified.
Sanitation measures are also important in reducing the risk of Botryosphaeria disease and spread. Pruning out dead wood is very important, because Botryosphaeria spp. colonize dead wood and can move from dead to healthy wood. Prune dead wood back to a healthy area on the branch (white to cream-colored inside when cut). Disinfecting pruning tools with rubbing alcohol between cuts may prevent the spread of Botryosphaeria inoculum on pruning tools.
Diagnosing the Disease
Branch or main stem/trunk samples that include the junction of healthy (white) and diseased (brown to reddish-brown) tissue are needed for diagnosis of Botryosphaeria dieback and canker (Fig. 2a). Cankered areas on branches may not be apparent until bark is removed and it may take some time to find such a junction. Do not remove all the bark when you locate a junction of healthy and diseased tissue, since it is best for diagnostic purposed to keep bark intact. Another option is to cut cross-sections through branches to locate cankered areas and a junction of healthy and diseased tissue (Fig. 2b). Completely dead or healthy branches are not useful for diagnosing Botryosphaeria dieback, so take time to find appropriate branch samples.
The Virginia Tech Plant Disease Clinic can diagnose this disease and other plant diseases. Refer to the Plant Disease Clinic website (https://bit.ly/VTplantclinic) for the current diagnostic form, fees, and instructions on collecting an appropriate diagnostic sample and submitting samples to the Plant Disease Clinic.
Hundreds of plant genera, including both angiosperms (flowering plants) and conifers, are susceptible to Botryosphaeria cankers and dieback. The following list includes some common ornamental landscape trees and shrubs that are reported hosts of Botryosphaeria species. Plants with an asterisk represent plant samples that have been diagnosed with Botryosphaeria disease in the Virginia Tech Plant Disease Clinic since electronic records have been kept (1999).
- black gum*
- crab apple*
- eastern red cedar*
- Euonymus species*
- flowering quince*
- fringe tree*
- giant sequoia*
- honey locust*
- Japanese pagodatree*
- Laurus species*
- Leyland cypress*
- mountain laurel*
- ornamental cherry*
- ornamental pear
- Photinia species*
- Pieris species*
- Pittosporum species
- Prunus species
- Pyracantha species
- Robinia species
- sweet gum*
- tulip tree
- umbrella pine*
- wax myrtle*
- witch hazel*
Table 1. How to avoid some common plant problems that may predispose plants to attack by opportunistic fungi, such as Botryosphaeria spp.
- Set woody shrubs and trees at the proper planting depth. The structural root nearest the soil line should be placed no deeper than 1 inch to 3 inches below the soil surface, measured 4 inches out from the trunk. (Structural roots are the large, woody roots that support the tree/shrub.) Note that structural roots are sometimes placed too deeply when potted or planted at the nursery. If this is the case, remove excess soil or potting medium so that plants can be set correctly in the landscape.
- Avoid purchasing severely pot-bound container plants. If circling roots are evident when the container is removed, try to tease the circling roots out to encourage growth of roots out of the root ball into the surrounding soil. Carefully cut away any circling roots at the top of the root ball that might eventually enlarge, girdle the stem and threaten the long-term health of the plant.
- Water adequately during establishment. (If rainfall is below 1 inch per week, provide enough water to bring the total to 1 inch.) Keep in mind that newly planted trees and shrubs require frequent irrigation during dry periods, because their root balls will dry out before the surrounding soil does. It is best to apply the irrigation directly to the container rootball to fully saturate the rootball during establishment. Less frequent, deeper irrigation is preferable to more frequent, shallow irrigation, because this will encourage root systems to grow deeper into the soil profile.
- Place mulch no more than 2 inches deep over the root zone of trees and shrubs. The mulch should not contact the bark of woody plants (i.e., place in a donut-shaped ring around the stem or trunk). Deep mulch and mulch in contact with bark reduces soil aeration and makes plants vulnerable to colonization by wood-decay organisms.
- Avoid damage to roots, stems and branches during and after planting. Wounds create entry points for opportunistic fungi. Damage to woody plants often results from lawn mowers and trimming equipment.
- Practice proper pruning techniques to avoid creating wounds that can serve as entry points for Botryosphaeria spp. Refer to the following Virginia Cooperative Extension publications:
- Pruning at the wrong time of year can predispose woody plants to infection by Botryosphaeria fungi. Recommended pruning times vary for different plant species. Refer to the following pruning calendars:
Dirr, M.A. 1990. Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses, 6th ed. Champaign, Ill.: Stipes Publishing Co.
Coyier, D.L., and M.K. Roane. 1986. Compendium of Rhododendron and Azalea Diseases. St. Paul, Minn.: APS Press.
Sinclair, W.A., and H.H. Lyon. 2005. Diseases of Trees and Shrubs, 2nd ed. Ithaca, N.Y.: Cornell University Press.
This publication benefited from the advice of Bonnie L. Appleton, Hampton Roads Agricultural Research and Extension Center; and the reviews of Chuan Hong, Hampton Roads AREC; Adria Bordas, Virginia Cooperative Extension Fairfax County Office; Regina Prunty, Virginia Cooperative Extension King George County Office; and Karen Rane, Maryland Cooperative Extension. The author appreciates the careful review and input from all of the reviewers.
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November 17, 2023