Armillaria fungi can live on dead woody material as well as living host tissue.

Diseases of the Fremont Cottonwood Tree

by Lori Norris

The Fremont cottonwood (Populus fremontii), hardy in U.S. Department of Agriculture plant hardiness zones 7 through 10, was named for General John Fremont, an early explorer. It's native to the Southwestern states of the U.S., and is often found along waterways. Pioneers would search out the Fremont cottonwood because they indicated water was nearby, nicknaming them "water trees." The Fremont cottonwood grows quickly and can reach 90 to 110 feet tall, but has a somewhat short life span lasting 20 to 40 years. They need lots of room, and can be a challenge in residential situations due to their size and extensive root system.

Root Diseases

Diseases that attack the cottonwood's root system can be difficult to spot until it's too late. Texas root rot is caused by a soil-borne fungus and mimics desiccation, so when gardeners first see the symptoms they may simply water the tree. Symptoms include wilting and rapid decline. There is no test to detect the pathogen in the soil, nor is there a cure for the disease. The tree must be removed. Gandomera root rot, another fungal disease, is easier to detect because in addition to yellowing and wilting leaves, conks (fruiting bodies of the fungus) grow on the trunk's base. Trees are more susceptible if already stressed by injury or weather issues. There is no cure and the tree should be removed.

Heartwood Diseases

Heartwood diseases attack the wood in the central part of the trunk. Inonotus heart rot is found mostly in the Southwestern U.S., and symptoms include branch die-back and conks on the upper part of the trunk. The fungus compromises the stability of the tree, and it will eventually have to be removed. The fungus remains alive on the dead wood, so remaining wood should be burned. Bacterial wetwood also affects the heartwood, and exudes a foul-smelling slime out the trunk of the tree. The bacteria is common in soils and probably enters the tree through an injury in the root. There are no controls available.

Leaf Diseases

Sooty mold may be present on cottonwood leaves, but the infection isn't actually attacking the cottonwood, it's feeding on the honeydew left behind by insects such as aphids. It causes damage by blocking sunlight from reaching the leaves, inhibiting photosynthesis. It can be controlled by controlling the insect population. Anthracnose is caused by one of several fungi. Leaves may die along veins first, followed by the entire leaf. The tree may experience total defoliation. The tree's shoots may also be affected. Controls include keeping debris cleaned up and application of preventative fungicides.

Trunk and Branch Diseases

Various cankers may affect cottonwood trees. Cankers are sunken, dark areas on branches or trunks and are often caused by fungi, but not always. Cytospora canker, a fungal disease, causes orange stains on the bark, and may cause heavy sap flow at the site of infection. Septoria canker, also a fungal disease, looks very similar but leaves are affected before branches and trunks. Branch canker forms cracked, callused cankers on mostly young trees. Remove diseased branches and cut out trunk infections then allow the treated areas to air-dry.

About the Author

Lori Norris has been writing professionally since 1998, specializing in horticulture. She has written articles for the Oregon Landscape Contractors Association, chapters of the certification manual for the Oregon Association of Nurseries and translated master gardener materials into Spanish. Norris holds a Bachelor of Arts from Linfield College.

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