Issue 11, July 12, 2013

Why BLS is nothing to LOL about! – FYI on testing for BLS at the U of I Plant Clinic How can I diagnose Bacterial Leaf Scorch (BLS)?

How can I diagnose Bacterial Leaf Scorch (BLS)?
The problem is that BLS can be easily mistaken for other environmental, disease, or cultural problems, so diagnosis cannot be based on symptoms alone.  Some examples that might limit the water uptake of trees and cause similar symptoms to BLS are as follows: drought stress, unfavorable site, construction damage, improper planting, girdling roots, root and butt rot, and canker fungi.  Late summer or early fall is the best time to test for BLS, because this is when Xylella fastidious is most active and bacterial populations are high in the tree’s water conducting tissues.  It has been found that testing for this disease too early in the growing season can result in false negative results.  We suggest that you obtain an accurate BLS diagnosis, because this disease is affecting many high value trees across the US and infected trees can be unsightly and unsafe.

Testing for BLS at the U of I Plant Clinic:   We will be testing for this bacterium at the U of I Plant Clinic. The fee will be $25 per sample. It is suggested that you call ahead to be certain you have prepared the correct sample and avoid resampling at your expense.  Petiole tissue from symptomatic leaves is preferred for this test, so leaves showing symptoms with green petioles are the usual request. Please send your samples in the next several weeks. We will collect samples, store them, and then run ONE serological test on all the submitted samples in August, 2013.

What is Bacterial leaf scorch (BLS)?  
Bacterial leaf scorch (BLS) is an infectious disease caused by bacterium Xylella fastidiosa that spreads systemically and causes a slow decline and death of the tree. The bacterial pathogen is found only in xylem tissue. Xylem-feeding leafhoppers, treehoppers, and spittlebugs are thought to spread the bacterium in landscape trees. It can also be transmitted between trees through root grafts. The transmission methods must not be very effective, though, because we do not see rapid spread of the disease from tree to tree.  This disease is not new, but is beginning to appear more frequently in the Midwest. Possibly this is a function of more people recognizing the symptoms.  It has been noted that drought conditions can cause greater disease severity and may make symptoms more noticeable.

What trees are affected by BLS?
The most frequent hosts of this disease include oak, elm, sycamore, mulberry, sweet gum, London plane, hackberry, ginkgo, sugar maple, and red maple.  Be aware, the many other landscape trees can be susceptible to this disease.  Most of the samples submitted to the U of I Plant Clinic are oaks.  Last year, we tested 23 trees from around the Midwest (not all were from Illinois) and found that 11 were positive for BLS. Thus far, in Illinois, BLS commonly infects pin and red oaks.  In 2013, we tested a ginkgo leaf and it was suspected to be positive for BLS; however, the tissue that was tested was old and I would like to confirm results with a resample.  Trees are not the only plant hosts that have been found to be infected with Xylella fastidiosa.  The bacterium has been found to have a wide host range including grasses and broad-leaved plants in some 30 families.  It has also been found that some plant hosts may be “asymptomatic” and not exhibit symptoms.  Some of the plants that have been found to harbor the bacterial pathogen that causes BLS are listed in

What are the symptoms of BLS?
Look for scorch symptoms that occur in early summer to midsummer and then intensify in late summer. The scorched leaf edges or tissue between veins may be bordered by a yellow or reddish-brown color.   Symptoms will often show on the oldest leaves first, distinguishing this disease from environmental scorch that first appears on the newest leaves.  The symptoms occur first on one branch or section of branches and slowly spread in the tree from year to year over the crown of the tree. It is one of those situations that you hope will be better next year but the decline of the tree only gets worse. The gradual decline or dead limbs may not occur until 5 to 10 years after infection.

Shingle oak infected with (Xylella fastidiosa), the pathogen that causes BLS.

What are the symptoms of BLS on oaks?
Most references say that oaks show symptoms on an entire branch at once. Bacterial scorch often allows infected leaves to remain on the tree until the fall. Oaks are again the exception. They will drop leaves early. If you have seen a slow decline in your oak, leaf scorch symptoms showing each July to August, and fall leaf drop about a month ahead of healthy oaks, BLS may be present.

Oak tree in Illinois diagnosed with BLS in 2012

What is the management of BLS?
There is no effective cure for the tree, once it is infected with BLS.

  • Insecticide treatment of the insect vector is considered impractical and is not recommended.
  • Trunk injections with antibiotics have been found to suppress symptoms and prolong the life of the tree, but will not eradicate the disease.  Treatments can be done each year into the root flare at the base of the tree in late May or early June.  Research has shown that injections are not an effective long-term solution for this disease.  In addition, research has also shown that injection holes can be an entry way for wood rotting organisms and frequent retreatment can cause considerable damage over the life of the tree.
  • Pruning can be done to help the aesthetics of the tree for a few years but has not been shown to slow the disease development.
  • Mulching and watering during times of drought may help to prolong the life of the tree.
  • The effects of fertilization are still not clear with this disease.
  • Tree replacement with a non-susceptible host to BLS before the tree infected with BLS is removed is recommended.  Some trees that have not been found to be affected by BLS can be found at the following Removing the tree infected with BLS may be necessary for safety.

Other BLS Fact sheets:

Stephanie Porter

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