Bacterial leaf scorch: What tree species might be susceptible?

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Bacterial leaf scorch is caused by a submicroscopic bacterium that survives only in the xylem fluids of plants. Most literature will say that the bacteria plug up the xylem and therefore cause the water stress and scorching symptom on leaves. That conclusion is now in question. New research results hint that the bacterium forms a toxin that causes the scorching pattern and the browning of the leaves. While the glassy-winged sharpshooter is the main vector in California, east of that state, we have little understanding of the insect vectors that spread the disease to shade trees.

Bacterial leaf scorch seems to be spreading across certain states in recent years. For example, regions of New Jersey apparently free of the disease after a thorough survey, showed extensive infection about five years later in a second survey. Also, new hosts are being discovered in Kentucky and elsewhere.

Our region of the nation is relatively unexplored in the host range and incidence of this disease, so we are pursuing that task as part of the comprehensive understanding of stress and decline in forest and shade trees.

Below is a summary of our first year of MSU research on detection of the bacterial leaf scorch pathogen in trees from the North Central states and the Plains states.

We found some hosts that were new records such as White ash and Swamp oak in last year’s work. Additionally, we found infection in states where the pathogen had never before been detected by diagnostic clinics of plant pathology labs, such as Wisconsin.

The lack of discovery of the pathogen in specific plant species or in specific states in this list does not imply exhaustive or conclusive results. We are certain, for example, that we may find the pathogen in many tree species in Michigan once we succeed in collecting many samples. One reason to present last year’s results are to show how thoroughly we are trying to approach the survey and also what trees have been found to be positive and what states are positive for the disease. We did have an unusually thorough coverage of well-distributed samples and broad host range of samples from North Dakota and have yet to detect the pathogen. So in the northern most states, we may run into a limit beyond which the climate restricts the disease. We have reports though of the pathogen along our border with Canada.

Hosts represented in the 2008 assayed leaf samples

Bacterial leaf spot positive hosts are in bold fonts:
Acer ginnala (Amur maple) 2, Acer negundo (Box elder) 2, Acer platanoides (Norway maple) 3, Acer rubrum (Red maple) 1, Acer saccharinum (Silver maple) 1, Acer saccharum (Sugar maple) 1, Acer tataricum (Tatarian maple) 1, Acer sp. 2, Aesculus glabra (Ohio Buckeye) 3, Aesculus hippocastanum (Buckeye Horsechestnut) 4, Cercis occidentalis (Western redbud) 1, Fraxinus americana (White ash) 1, Fraxinus pennsylvanica (Green ash) 9, Juglans nigra (Black walnut) 1, Malus spp. (Flowering crabapple) 1, Morus rubra (Mulberry) 3, Parthenocissus quinquefolia (Virginia creeper) 1, Populus tremula ‘Erecta’ (Columnar poplar) 2, Populus tremuloides (Quaking aspen) 2, Prunus virginiana (Chokecherry) 1, Pyrus ussuriensis (Prairie gem) 3, Quercus alba (White oak) 1, Quercus acutissima (Sawtooth oak) 1, Quercus bicolor (Swamp oak) 2, Quercus imbricaria (Shingle oak) 1, Quercus macrocarpa (Bur oak) 14, Quercus palustris (Pin oak) 10, hybrid Quercus rober ‘Fastigiata’ x Q. bicolor (‘Regal Prince’) 1, Quercus rubra (Red oak) 77, Quercus velutina (Black oak) 3, Quercus sp. 58, Syringa villosa (Late or Villous lilac) 1, Syringa vulgaris (Common lilac) 1, Tilia americana (American linden) 8, Ulmus americana (American elm) 5, Ulmus davidiana var. japonica (Japanese elm) 1, Vitis sp. 1

States with positive samples of bacterial leaf spot are: Illinois, Indiana, Kansas, Missouri, and Wisconsin. Oklahoma and south are generally known to have the pathogen.

States of uncertain status for bacterial leaf spot presence, to date are: Iowa, Michigan, Minnesota, Nebraska, North Dakota, and South Dakota.

Dr. Adams’s work is funded in part by MSU‘s AgBioResearch.

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