Texas Root Rot - September 16, 2009
Jeff Schalau, Associate Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County

Texas root rot (Phymatotrichum omnivorum), also known as cotton root rot, occurs throughout the southwestern United States and Mexico. It is present in the Verde Valley where it has been confirmed on many sites over the last 10 years. Most often, I have found it on fruit and nut trees. More recently, I’ve seen it on grape vines. In all cases, it was fatal to the host plant. Nevertheless, recognizing the symptoms and understanding what little we know about the disease could save you some frustration in the future.

The symptoms of Texas root rot are somewhat distinctive. Plants infected with Texas root rot often wilt suddenly during the summer when temperatures are high. The dead or dying leaves usually remain attached to the plant. After the plant has died, the root system is decayed and brown. It is diagnosed by looking for fungal strands that will be visible on the outer surfaces of the roots and under the root bark. Fungal spore mats may or may not be present on the soil surface near the infected plant. The mats appear off-white or tan and are level with the soil surface. These spores are not fertile and will not spread the disease. Be sure to inspect suspected cases for other causes of similar symptoms such as gophers, chemicals, mechanical damage, or other root diseases.

The species name, "omnivorum", means omnivorous or "eats everything". This is not quite, but almost true. It has been a confirmed pathogen on over 2,000 plant species. Texas root rot most often shows a preference for fruit trees and broadleaf deciduous trees and shrubs. Infected fruit trees are most often noticed because we tend to monitor fruit crops during the growing season. It could also spread more easily in an orchard because plants are usually more closely spaced.

Texas root rot spreads by root contact. A diseased root comes into contact with a healthy root and if the healthy root is from a susceptible plant, that plant becomes infected. Current thoughts on long distance transmittance are that it is not carried to new sites by human activity or vehicles.

Many native trees have some tolerance to Texas root rot with the exception of cottonwood, willow, and ash. Somewhat tolerant species include mesquite, sycamore, desert willow, hackberry, as well as Gymnosperms, such as pine, spruce, cypress, and juniper. Interestingly enough, the only fruit that appears to have some tolerance is the pomegranate. Using the term “resistance” is relative – it can infect these plants, it is just not as common. Monocots (agaves, yuccas, grasses, palms, yuccas, bamboo, iris, lilies, gladiolus, onions, garlic, etc.) are completely resistant to Texas root rot.

Some control measures have been recommended in the past, but more recent research has proven these treatments ineffective. There is no test to determine the presence of Texas root rot other than a confirmed kill. In other words, you must plant susceptible species and wait for them to display symptoms. As stated above, other root rots can display similar symptoms, so it is important to have the disease diagnosed by an experienced person.

Sampling a plant for Texas root rot is relatively easy. Take several samples of rotting and discolored roots on which the outer or cortical tissue still remains attached. The samples should be pencil size or slightly larger and at least 6 inches long. Leave soil attached and keep the roots cool in a plastic bag (refrigeration is fine). Do not add water or wet paper towels. Submit the sample to the Cottonwood Cooperative Extension Office. Our hours of operation are M-Th 9 am to 3 pm.

Possibly the best course of action is to remove the infected plant(s) and wait a full growing season before replanting. Resistant species make the best choices, but just as humans repopulate known flood plains following the floods, we will continue to replant susceptible species where we have had recent bouts with Texas root rot.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: September 8, 2009
Content Questions/Comments: jschalau@ag.arizona.edu
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