US National Arboretum

 

Floral and Nursery Plants Research Unit

BACTERIAL LEAF SCORCH OF SHADE TREES
Jo-Ann Bentz, Qi Huang & Ramon Jordan
Floral Nursery Plants Research Unit, US National Arboretum, Beltsville, MD 20705

 

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.

Click for larger Image of Scorch symptom on leaves image Click for larger Image of Leafhooper vector Click for larger Image of bacteria in sieve element
"Leaf scorching" symptom Leafhopper vector Photo showing bacteria in the
vessel element of an elm leaf vein

{You can click on any image on this page to see a larger version in a pop-up window}

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.

Click for larger Image of Scorch symptom on Elm Click for larger Image of Scorch symptom on Oak Click for larger Image of Scorch symptom on Sycamore
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.

Click for larger Image of Scorch symptom on a bonsai
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.

Click for larger Image Click for larger Image Click for larger Image
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.

Click for larger Leafhopper Image Click for larger Leafhopper Image Click for larger Leafhopper Image Click for larger Leafhopper Image
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.

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.

Click for larger PCR gel Image Click for larger PCR gel Image
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

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

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