What is bacterial leaf scorch?
     Bacterial leaf scorch (BLS) is an infectious chronic disease caused by the fastidious, gram-negative, xylem-limited bacterium Xylella fastidiosa. This bacterium, which is transmitted by xylem-feeding insects, colonizes and physically "clogs" the tree's water conducting tissues or xylem. Water transport becomes disrupted in roots, branches, and leaves due to large amounts of multiplying bacteria and their by-products. The presence of the bacteria also triggers a reaction in the tree that plugs the xylem, further impeding water transport and eventually killing the tree.
     Bacterial leaf spreads systemically and causes slow decline and death of a tree. BLS is not new but is appearing more frequently in landscape trees in various parts of the country. It has been found in coastal US states from New York to Texas, in Washington, DC, as well as in California, Indiana, Kentucky, Nebraska and Ohio. This may simply be because more people recognize the symptoms.

"Leaf scorching" symptom	Leafhopper vector	Photo showing bacteria in the vessel element of an elm leaf vein

What are the symptoms?
     Trees infected with Xylella fastidiosa exhibit marginal leaf necrosis, or browning, bordered by a pale halo band separating the dead or scorched tissue from the green tissue. Leaf discoloration begins at the leaf margin and moves toward the midrib. Symptoms recur each year and spread over the tree's crown, thus, reduction in growth and dieback are common in affected trees.

Scorching symptom in Elm	Scorching symptom in Oak	Scorching symptom in Sycamore

What are the hosts?
     Reported hosts include sycamore, mulberry, red maple, sugar maple, sweetgum, American elm, and a number of oaks such as bur, pin, scarlet, red, laurel, water, turkey, bluejack, and shingle oak. Xylella fastidiosa has a very wide host plant range with over 30 families of plants reported as either natural or experimantal hosts, including many asymptomatic herbaceous and woody species such as goldenrod, blackberry, alfalfa, clover, and some grasses.
     We have recently discovered, for the first time to our knowledge, the presence of Xylella fastidiosa in a Japanese beech bonsai plant exhibiting leaf scorch symptoms.

Scorching symptom in newly discovered Xylella fastidiosa infection in a Japanese beech bonsai plant

What is the problem?
     Many high value specimen trees and extensive landscape plantings are being severely affected by Xylella fastidiosa, particularly in the mid-Atlantic and southeastern United States. Infected trees often become unsightly or unsafe, necessitating early removal. No cure or therapy for infected trees nor a strategy for preventing infection is presently available.

Scorching symptom in landscape trees	Scorching symptom in Oak	Scorched trees on the Mall in Washington, DC

How are trees infected?
      Xylella fastidiosa is transmitted through grafting or, most commonly, by insects of the families Cercopidae (spittlebugs) and Cicadellidae (leafhoppers). Because this bacterium occurs in the xylem of plants, it is not surprising that all currently known vectors are xylem-sap feeding insects. A recent finding has been the identification of treehoppers (Membracidae) as potential vectors.

What is known about insect transmission?
      Insects are able to acquire and transmit the bacterium during the immature and adult stages of their development. Immature insects retain their ability to transmit until they molt. This is because insects shed the foregut during molting and this is the site from which the bacteria are introduced by the feeding vector into the plant. After molting, the insect has to feed from an infected plant to acquire the bacterium again. Fewer than 100 bacteria in the vector can transmit the disease. Acquisition and inoculation of bacteria can be accomplished very quickly, thus, no latent period is required.

Which are the insect vectors?
      Several leafhopper species have been identified as vectors of this bacterium. However, vectors vary in their efficiencies in acquiring and transmitting the different strains of X. fastidiosa. Vector efficiency may be related to some extent to host affinities or feeding habits.

Graphocephala versuta	Graphocephala coccinea	Aulacizes irrorata	Oncometopia undata

Are there any Disease Management Strategies?
      There is no effective preventative treatment or cure for bacterial leaf scorch, so one should expect diseased trees to be gradually lost over the years. The eventual best remedy for bacterial leaf scorch is tree replacement. However, in the meantime, infected trees can be made to look somewhat presentable for a few more years if the dead wood is pruned out. Careful scouting combined with judicious pruning can help to rid the tree of symptomatic branches especially since there are no chemicals registered for treatment.

• Leafhoppers, which spread the disease, are active most of the growing season making it impractical to control this disease by insecticidal treatments. Transmission of this disease is so unpredictable that efforts to prevent it by preventing vector feeding are likely to be futile.
• Trunk injections with antibiotics have been shown to suppress symptoms. Treatments must be made annually in late May or early June. The antibiotic oxytetracycline has been tested as a treatment, but it only caused the remission of symptoms; it did not provide a cure.
• Pruning has been another possible treatment; however with only limited success in delay of scorch development. Pruning has been devoted to public safety in trees that have shown some natural resistance to the disease.
• Mulching and irrigating during periods of little rainfall will reduce moisture stress and possibly delay scorch development.
• The effects of fertilization are still unclear with this disease. Fertilizing should be performed when a soil or leaf analysis shows a nutrient deficiency. 
• Removing trees has been necessary to maintain safety and is considered when trees no longer add to the landscape.

How does one know their trees are really infected?
      The challenge is that the symptoms can be easily mistaken for physiological leaf scorch or early fall color. However, since a number of other diseases, as well as cultural problems, can mimic bacterial leaf scorch symptoms, it has been recommended that suspected infections be confirmed by sending samples to a diagnostic clinic before concluding the tree is infected with the bacterium. Bacterial leaf scorch can (should) be confirmed by one or more of the two common tests currently used to detect BLS infection. Samples should be taken from symptomatic branches (leaves and twigs included) for testing by:

1. ELISA - Enzyme linked immuno-sorbent assay - which can detect around 10⁴ bacteria/ml; and/or
2. PCR - Polymerase chain reaction test - which is more senstive (can generally detect 10² bacteria/ml) but is much more expensive and labor intensive.

Picture of Xylella-specific PCR fragments generated from various strains and separated by agarose gel electrophoresis	Picture of Xylella-specific PCR fragments generated from leafhoppers carrying Xylella

Suggested further reading:

• Bentz, J. 2000. Transmission of the xylem-limited bacterium Xylella fastidiosa to shade trees by insect vectors. Proceedings of the APS Wilt Diseases of Shade Trees: National Conference, MN.
• Hopkins, D.L. & Adlerz, W. C. 1988. Natural hosts of Xylella fastidiosa in Florida. Plant Disease 72:429-431.
• Hopkins, D.L. 1983. Gram-negative, xylem-limited bacteria in plant disease. Phytopathology 73:347-350.
• Hopkins, D.L. 1989. Xylella fastidiosa: xylem-limited bacterial pathogen of plants. Ann. Rev. Phytopathol. 27:271-290.
• Kostka, S.J., Tattar, T.A., Sherald, J.L., and Hurtt, S.S. 1986. Mulberry leaf scorch, new disease caused by a fastidious xylem-limited bacterium. Plant Disease 70:690-693.
• Lloyd, J. (Ed.) 1997. Plant Health Care for Woody Ornamentals: a professional's guide to preventing and managing environmental stresses and pests. Urbana, Ill., Coop. Ext. Ser. Univ. Ill. at Urbana-Champaign. 223 pp.
• McElrone, A.J., Sherald, J.L., & Pooler, M.R. 1999. Identification of alternative hosts of Xylella fastidiosa in the Washington, DC area using nested PCR. J. of Arboriculture 25(5): 258-263.
• Pooler, M.R. & J.S. Hartung. 1995. Genetic relationship among strains of Xylella fastidiosa from RAPD-PCR data. Current Microbiology 31: 134-137.
• Pooler, M.R. & J.S. Hartung. 1995. Specific PCR detection and identification of Xylella fastidiosa strains causing citrus variegated chlorosis. Current Microbiology 31: 377-381.
• Pooler, M.R., Hartung, J.S., and Fenton, R.G. 1997. Characterization of a 1.4 kb plasmid in some strains of Xylella fastidiosa. FEMS Letters 155: 217-222.
• Pooler, M. R., Myung, I. S., Bentz, J., Sherald, J. and Hartung, J. S. 1997. Detection of Xylella fastidiosa in potential insect vectors by immunomagnetic separation and nested polymerase chain reaction. Let. Appl. Microbiol. 25: 123-126.
• Purcell, A.H. & D.L. Hopkins. 1996. Fastidious xylem-limited bacterial plant pathogens. Annu. Rev. Phytopathol. 34: 131-151.
• Raju, B. C. & Wells, J. M. 1986. Diseases caused by fastidious xylem-limited bacteria and strategies for management. Plant Disease 70:182-186.
• Sherald, J.L., Hearon, S. S., Kostka, S.J., & Morgan, D. L. 1983. Sycamore leaf scorch: Culture and pathogenicity of fastidious xylem-limited bacteria from scorch-affected trees. Plant Disease 67:849-852.
• Sherald, J.L. & S.J. Kostka. 1992. Bacterial leaf scorch of landscape trees caused by Xylella fastidiosa. J. Arboric. 18(2):57-63.
• Sherald, J.L., Wells, J. M., Hurtt, S.S., & Kostka, S. J. 1987. Association of fastidious, xylem-inhabiting bacteria with leaf scorch in red maple. Plant Disease 71:930-933.
• Sinclair, W.A., H.H. Lyon, & W.T. Johnson. 1987. Diseases of Trees and Shrubs. Ithaca, NY, Cornell University Press. 574 pp.
• Wester, H.V. & Jylkka, E.W. 1959. Elm scorch, graft transmissible virus of American elm. Plant Disease Reporter 43: 519.

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Last Updated   January 18, 2005 4:29 PM
URL = http://www.usna.usda.gov/Research/BacterialLeafScorch.html