View source for Xylella fastidiosa (Bacterial leaf scorch of ornamentals)

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'''<big>Author: Elizabeth Bush, Virginia Tech</big>'''

'''Reviewed by: John R. Hartman, University of Kentucky'''

=Pathogen=
<i>Xylella fastidiosa</i> is a xylem-limited bacterium, vectored by certain insects. Various strains of this bacterium cause disease on a number of economically important crop plants (e.g. Pierce's disease of grape, almond leaf scorch, phony peach, citrus veinal chlorsis) and bacterial leaf scorch on a broad range of ornamental landscape trees. Common insect vectors of the disease include leafhoppers (Cicadellidae) and spittlebugs (Cercopidae).  <i>Xylella fastidiosa</i> colonizes the tree’s water-conducting tissue where it disrupts water movement causing reduced water availability to the tree.

=Symptoms and Signs=
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Symptoms of bacterial leaf scorch usually appear in the latter part of summer and progress through the fall. The most characteristic symptom of this disease is marginal leaf scorch. Often a yellow band is evident between brown and green tissue. This yellow band is characteristic of bacterial leaf scorch; however, this symptom is not always present on bacterial leaf scorch-diseased leaves. In contrast, when marginal leaf scorch is caused by other problems (e.g. drought, root disease, salt injury) no yellow band is typically observed between brown and green tissue.  Other symptoms, depending on the host species, include leaf yellowing, browning and premature leaf drop. On many oak species, brown leaves do not drop prematurely; however, on pin oak (Quercus palustris) marginal scorch symptoms may not occur, but leaves may drop prematurely. On trees with an indeterminate growth habit, such as elm and sycamore, symptoms on infected branches begin on older leaves and move to younger leaves; on trees with a determinate habit, such as oak, all leaves on a diseased branch are the same age and show symptoms at the same time. Initial symptoms of bacterial leaf scorch often appear on only one or two branches before gradually spreading to more and more branches in subsequent years.

=Ecology and Spread=
The pathogen that causes bacterial leaf scorch is spread by insect vectors. The primary vectors are  sharpshooter (Cicadellinae) leafhoppers, but other insects, such as spittlebugs (Cercopidae), also vector the bacterium. These insect vectors have piercing and sucking mouthparts that allow them to penetrate the tree’s water-conducting tissue (xylem) and extract nutrients. An insect acquires the bacterium by feeding on infected plant tissue; subsequently, the insect may spread the bacterium to non-infected hosts while feeding. Once a plant is infected with the bacterium, the disease becomes systemic, since the bacteria move throughout the tree in the tree’s water transport system.

Many oaks are susceptible to bacterial leaf scorch and oaks in the red oak group are most commonly afflicted with this disease. Elms and sycamore are also relatively common hosts of the disease in Virginia. The disease is also reported on many other tree species, such as maple, hackberry, mulberry, sweet gum, and gingko. Some other woody plants that are reported hosts of <i>X. fastidiosa include oleander</i> (<i>Nerium oleander</i>), peppervine (<i>Ampelopsis arborea</i>), beauty berry (<i>Callicarpa americana</i>, Virginia creeper (<i>Parthenocissus quinquefolia</i>), American elder (<i>Sambucus canadensis</i>), southern highbush blueberry (interspecific <i>Vaccinium corymbosum</i> hybrids), and grape (<i>Vitis species</i>). <i>X. fastidiosa</i> is also an economically significant pathogen of several important fruit and nut trees (e.g. almond, citrus, plum, peach, pecan) and many weeds and grasses are hosts to <i>X. fastidiosa</i>. Many plant species have also been identified as asymptomatic hosts of <i>X. fastidiosa</i>. The host list of this pathogen continues to grow.

=Geographic Distribution=
<i>Xylella fastidiosa</i> is found in North and South America on many plant hosts and has also been reported on grape in Kosovo and pear in Taiwan. In October 2013 it was reported in southern Italy on olive (<i>Olea europea</i>). This is the first confirmed report of <i>Xylella fastidiosa</i> in Europe and the presence of this pathogen in Europe is of serious concern; <i>Xylella fastidiosa</i> had been on the EPPO A1 List of pests recommended for regulation as quarantine pests since 1981.

=Management=
There are currently no curative controls for bacterial leaf scorch. Injection of antibiotics into diseased trees has been shown to slow the spread of the disease, but will not cure the tree. Antibiotic injection is an expensive option and trees must be treated on an ongoing basis. Injection holes are also prone to colonization by wood decay fungi that can negatively impact the health of the tree. Control of leafhoppers and other vectors of <i>X. fastidiosa</i> is not effective in preventing spread of bacterial leaf scorch to non-infected trees.

Avoiding stress to trees and maintaining them in optimal health can help trees better withstand the effects of bacterial leaf scorch, resulting in a longer period of time in which they are aesthetically acceptable and survive. Since bacterial leaf scorch progresses gradually over a period of years, this is a reasonable approach to controlling the disease. Providing water to trees during hot, dry periods of the summer and during drought are the most critical tactics to help trees avoid stress. This is particularly important if a tree has been already been diagnosed with bacterial leaf scorch. Mulching trees can also help avoid water stress. Ensuring adequate nutrient uptake by having soil tested is also recommended. Soil test results will give details on correcting the soil pH if pH is found to be a problem. Inspecting diseased trees on an annual basis to determine the extent of dieback is also recommended, so that pruning and/or removal options may be considered. Pruning out dead wood is recommended: remove branches back to healthy tissue (white or cream-colored in cross-section).  

Replacing diseased trees is another option for homeowners and urban landscapers. Trees with extensive dieback are best removed. Choose replacement trees that are not reported hosts of bacterial leaf scorch or other major pests and that are well-adapted to the location. (New hosts of bacterial leaf scorch continue to be identified, so it is prudent to avoid all species of tree genera on which the disease has been reported.) Some trees to consider as replacements include: European black alder (<i>Alnus glutinos</i>a), European beech (<i>Fagus sylvatica</i>), black gum (<i>Nyssa sylvatica</i>), yellow buckeye (<i>Aesculus flava</i>), northern catalpa (<i>Catalpa speciosa</i>), katsuratree (<i>Cercidophyllum japonicum</i>), Kentucky coffeetree (<i>Gymnocladus dioicus</i>), American linden (<i>Tilia americana</i>), littleleaf linden (<i>T. cordata</i>), silver linden (<i>T. tomentosa</i>), cucumbertree (<i>Magnolia acuminata</i>), Osage orange (<i>Maclura pomifera</i>), tulip poplar (<i>Liriodendron tulipifera</i>), and Japanese zelkova (<i>Zelkova serrata</i>).

=Diagnostic procedures=

Late summer or early fall are the best times to test for bacterial scorch, since this is when <i>X. fastidiosa</i> is most active and bacterial populations in the tree’s water conducting tissue are highest. Testing too early in the growing season may result in a false negative result. Since <i>Xylella fastidiosa</i> is very difficult to culture diagnostic labs have relied on ELISA (testing kits are available from Agdia, Inc.) and molecular methods, such as quantitative real-time polymerase chain reaction (e.g. Vincelli and Amsden, 2010) for detecting the bacterium in plant tissue. Samples consisting of a dozen symptomatic leaves with the petioles still attached are sufficient for submitting for testing for this disease.

=Resources and References=
*Anonymous. First report of <i>Xylella fastidiosa</i> in the EPPO region.  2014. European and Mediterranean Plant Protection Organization. Online: http://www.eppo.int/QUARANTINE/special_topics/Xylella_fastidiosa/Xylella_fastidiosa.htm
*Anonymous. 2013. First Report of <i>Xylella fastidiosa</i> in Italy. EPPO Reporting Service. Online: http://archives.eppo.int/EPPOReporting/2013/Rse-1309.pdf 
*Berisha, B., Chen, Y. D., Zhang, G. Y., Xu, B. Y., and Chen, T. A. 1998. Isolation of Pierce's disease bacteria from grapevines in Europe. European Journal of Plant Pathology 104:427-433.
*Doddapaneni, H., Yao, J. Q., Lin, H., Walker, M. A., and Civerolo, E. L. 2006. Analysis of the genome-wide variations among multiple strains of the plant pathogenic bacterium <i>Xylella fastidiosa</i>. BMC Genomics 7:(01 September 2006).
*Gould, A. B. and J. H. Lashomb. 2005. APS Feature Story: Bacterial Leaf Scorch of Shade Trees. Online: https://www.apsnet.org/edcenter/intropp/lessons/prokaryotes/Pages/BacterialLeafScorch.aspx
*Harper, S. J., Ward, L. I., and Clover, G. R. 2010. Development of LAMP and real-time PCR methods for the rapid detection of <i>Xylella fastidiosa</i> for quarantine and field applications. Phytopathology 
100:1282-1288.
*Hartman, J. 2007. Bacterial leaf scorch. University of Kentucky Cooperative Extension Service. Online: http://www2.ca.uky.edu/agcollege/plantpathology/ext_files/PPFShtml/PPFS-OR-W-12.pdf 
*Hopkins, D. L. 1989. <i>Xylella fastidiosa</i>: Xylem-limited bacterial pathogen of plants. Annual Review of Phytopathology 27:271-290.
*Schaad, N. W., D. Opgenorth, and P. Gaush. 2002. Real-time polymerase chain reaction for one-hour on-site diagnosis of Pierce’s disease of grape in early season asymptomatic vines. Phytopathology 92:721-728.
*Schaad N. W., J. B. Jones, W. Chun. 2001. Laboratory guide for Identification of Plant Pathogenic Bacteria. 3rd ed.  The American Phytopathological Society, St. Paul, MN.
*Sherald, J. L. 2007. Bacterial leaf scorch of landscape trees: what we know and what we do not know. Arboriculture and Urban Forestry 33(6): 376-385.
*Sinclair, W.A., and H.H. Lyon. 2005. Diseases of Trees and Shrubs, 2nd ed. Ithaca, N.Y.: Cornell University Press.
*Su, C.-C., Chang, C. J., Chang, C.-M., Shih, H.-T., Tzeng, K.-C., Jan, F.-J., Kao, C.-W., and Deng, W.-L. 2013. Pierce's Disease of Grapevines in Taiwan: Isolation, Cultivation and Pathogenicity of <i>Xylella fastidiosa</i>. Journal of Phytopathology 161:389-396.

=Acknowledgments=
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