Tag Archives: Walnut

Bacterial Blight

Death of lilac branch tips and leaves due to bacterial blight.
Death of lilac branch tips and leaves due to bacterial blight.

What is bacterial blight?  Bacterial blight, also known as blossom blight or shoot blight, is a common and often serious disease of Chinese, Japanese, Persian and common lilac, as well as walnut, apple, pear, plum and cherry.  White flowering varieties of common lilac are most susceptible to the disease.

What does bacterial blight look like?  Initial symptoms of bacterial blight may include dark brown necrotic (dead) leaf spots with yellow halos.  If leaf spots develop before leaves are fully expanded, leaf curling and twisting may result.  More advanced symptoms include necrotic blotches starting at the leaf margins and advancing inward, as well as black streaking on twigs.  In its most severe form, bacterial blight can result in the death of branch tips, leaves and blossoms.

Where does bacterial blight come from?  Bacterial blight is caused by the bacterium Pseudomonas syringae pv. syringae (Pss), which survives in diseased stem tissue (cankers), plant debris, and soil.  Pss can be spread by insects and on pruning tools but is more commonly spread by wind and rain.  Often Pss is found on the surface of healthy plants and does not cause disease.  Infections can occur when the bacterium enters tissue through natural openings or through wounds caused by insects, pruning, wind damage or hail.

How do I save a plant with bacterial blight?  Prune diseased twigs 10 to 12 inches below the point of visible symptoms, and dispose of the branches by burning (where allowed by local ordinance) or burying them.  Always prune in dry weather, and disinfest pruning tools after each cut by treating them for at least 30 seconds with a 10% bleach solution or preferably 70% alcohol (e.g., rubbing alcohol straight from the bottle or a spray disinfectant).  If you decide to use bleach, be sure to thoroughly rinse and oil your tools after pruning is complete to prevent rusting.

How do I avoid problems with bacterial blight in the future?  When planting lilacs, provide adequate spacing between shrubs.  Thin individual shrubs each winter to promote good air circulation (see University of Wisconsin Garden Facts XHT1015, Pruning Deciduous Shrubs, for pruning tips).  Properly water, fertilize and mulch shrubs to avoid stress that may predispose them to disease.  Avoid overhead watering that may keep leaves wet.  If you have had chronic problems with bacterial blight, you may want to use a combination of copper and mancozeb-containing fungicides for control.  Apply fungicides two to three times at seven to 10 day intervals as leaves emerge, but before symptoms develop.  Read and follow all label instructions of the fungicide that you select to ensure that you use the fungicide in the safest and most effective manner possible.

For more information on bacterial blight:  Contact your county Extension agent.

Anthracnose

Anthracnose is very common on many types of trees and shrubs. It often occurs on the leaves of ash (left) and maple (right) trees, causing blotchy-brown, dead areas.
Anthracnose is very common on many types of trees and shrubs. It often occurs on the leaves of ash (left) and maple (right) trees, causing blotchy-brown, dead areas.

What is anthracnose?  Anthracnose is the name of several common fungal diseases that affect the foliage of woody ornamentals in Wisconsin.  Trees that are most commonly and severely affected by anthracnose include ash, maple, white oak, sycamore, and walnut.  Anthracnose typically affects young leaf tissue.

What does anthracnose look like?  Symptoms of anthracnose vary from host to host, but in general include irregular spots, and dead areas on leaves that often follow the veins of the leaves.  Affected tissue can vary in color but is often tan or brown.  Severely affected leaves often curl and may fall off.  In some tree species, such as sycamore, twigs can also become infected leading to twig dieback.

Where does anthracnose come from?  Anthracnose is caused by several fungi (many historically classified in the genus Gloeosporium) that survive in leaf litter.  These fungi are host specific.  The anthracnose fungus that infects one type of tree (e.g., ash) is not the same one that infects another type of tree (e.g., maple).  However, when anthracnose occurs on one tree, then weather conditions (typically cool and moist conditions) are favorable for development of the disease on many types of trees.

Anthracnose can be severe on some hosts. On white oak (left), it can cause extensive leaf browning and curling. On sycamore (right), anthracnose can cause twig dieback.
Anthracnose can be severe on some hosts. On white oak (left), it can cause extensive leaf browning and curling. On sycamore (right), anthracnose can cause twig dieback.

Anthracnose can be severe on some hosts. On white oak (left), it can cause extensive leaf browning and curling. On sycamore (right), anthracnose can cause twig dieback.

How do I save a tree with anthracnose?  DO NOT panic.  For many trees, anthracnose is a cosmetic disease.  It may make a tree look a little ragged but will not kill the tree.  However, if a tree has been defoliated by anthracnose for several years, or it is a tree, such as a sycamore, where twig infections can occur, then you may want to use a fungicide for disease control.  Three treatments are typically needed for adequate control: one at bud break, one when leaves are half expanded, and one when leaves are fully expanded.  Fungicides containing copper, chlorothalonil, or mancozeb are registered for anthracnose control in Wisconsin.  DO NOT use the same active ingredient for all treatments.  Instead, alternate the use of at least two active ingredients to help minimize problems with fungicide-resistant strains of anthracnose fungi.  Be sure to read and follow all label instructions of the fungicide(s) that you select to ensure that you use the fungicide(s) in the safest and most effective manner possible.

How do I avoid problems with anthracnose in the future?  You can reduce the number of spores that cause anthracnose infections by removing and disposing of fallen, infected leaves in the autumn.  Leaves can be burned (where allowed), buried or composted.  When composting, make sure that your compost pile reaches high temperature (approximately 140°F).  Also, make sure that your compost pile is routinely turned so that leaves on the outside of the pile eventually end up in the center of the pile.  The combination of high temperature and decay of leaf tissue in a compost pile helps eliminate anthracnose fungi.  Also, maintain good tree vigor by watering and fertilizing trees appropriately.  Check with your local county Extension agent for details on how to properly care for trees.

For more information on anthracnose:  Contact your county Extension agent.