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Sooty canker: What it is, what to do

November 05, 2001|By THOMAS TURINI

University of California-Imperial County Cooperative Extension

A tree is a valuable commodity in the low desert. There are many kinds of trees in this area that are growing poorly. Some drop their leaves, others show branch dieback and in extreme cases the entire tree dies.

The causes of these problems include that the tree is not suited to our climate or poor management or a disease may be involved.

In some neighborhoods there are more dead trees than healthy ones, especially where ash or mulberry trees are popular. The most common cause of this tree decline and death is sooty canker, a disease caused by a fungus called Hendersonula toruloidea. This disease can also affect ash, mulberry, carob, all species of citrus, elm, fig and other Ficus species (including Indian laurel), grapevine, natal plum, oleander, pine, poplar, silk oak, sycamore, umbrella tree, willow, walnut and wisteria.

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In mid-summer, sooty canker will cause small branches to die and the leaves remain attached. This is the first obvious symptom of this disease. Upon closer inspection, brown, water-soaked areas may be seen on the bark.

With time, the fungus will move further into the tree canopy killing off larger and larger branches.

As the disease progresses, the outer layer of bark peels back, exposing masses of black sooty spores. The spores become obvious at later stages of disease development and are commonly first noticed at this time of the year. The black sooty mass of spores under paper-like bark distinguishes this disease from other maladies.

The spores are spread by wind, insects, birds and pruning tools and infect injured tissue. The fungus can't get into a tree through healthy, intact bark. To enter, it needs a pruning wound, sunburned bark, or other injury. If bark damage can be prevented, the disease will not occur. However, this can be difficult since even the damage caused by the branches rubbing together during high winds may be sufficient to create a wound through which the fungus may enter.

A vigorously growing tree that has a dense canopy is less likely to sunburn, and therefore may be less likely to develop sooty canker than a weak tree. In addition, the disease progresses more rapidly on weak trees than on trees that are vigorous. Maintaining plant vigor by regular fertilization and watering will reduce the likelihood that the tree will contract this disease and reduce disease severity or the rate of decline if it does become infected.

Avoid pruning unnecessarily. Pruning exposes branches to sunburn, produces wounds and stresses the tree making it more vulnerable to attack.

However, the best chance of saving a diseased tree is to cut out the affected part while the symptoms are confined to the upper portion of the canopy. The limb should be cut several inches below the diseased area. This is recommended because the fungus may be present in tissue that appears to be healthy. The pruning tool should be cleaned with a solution of one part household bleach to nine parts of water. Sanitize between cuts to avoid spreading the disease. This way, in the event a cut is made through an infected but symptomless portion of the tree, the disease will not be spread to other parts of the tree with subsequent cuts.

Remember that the black spores that make up the sooty substance are capable of being carried to other trees and causing disease if the conditions are correct. Therefore, diseased trees and branches that are carrying the spores should be taken out of the area as soon as possible.

Neither avoiding this disease nor curing a tree that has sooty canker are easy tasks. In some cases the only option is to pull out the diseased tree and start over. However, maintaining good plant health, avoiding unnecessary pruning and reducing the amount of spores in the area by removing diseased tissue are the most successful methods of controlling sooty canker. Removing diseased branches by pruning out infections is most likely to be successful if the disease is caught early. Control becomes more difficult as the disease moves into the larger branches and is virtually impossible once the trunk is affected.

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