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 tissue to 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. 2). 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 floral inflorescence 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.
Hundreds of plant genera, including both angiosperms (flowering plants) and conifers, are susceptible to Botryosphaeria cankers and dieback. The following list includes some common 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).
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 online and other horticultural references. 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 that are compacted by construction or foot traffic and soils that are excessively wet or suffer from poor drainage. 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 in order 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 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 an area on the branch that is healthy (white to cream-colored when cut). Disinfecting pruning tools with rubbing alcohol between cuts may prevent the spread of Botryosphaeria inoculum on pruning tools.
Dirr, M.A. 1990. Manual of Woody Landscape Plants: Their Identification, Ornamental Characteristics, Culture, Propagation and Uses, 4th 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.
|Table 1. How to avoid some common plant problems that may predispose plants to attack by opportunistic fungi, such as Botryosphaeria spp.|
Virginia Cooperative Extension materials are available for public use, re-print, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Alan L. Grant, Dean, College of Agriculture and Life Sciences; Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
June 18, 2009