San Antonio Express News
Sunday, August 15, 2004
by Lynn Rawe
Each year, when the weather gets hot, the gardener’s old enemy returns. We never know where it will strike. Phymatortrichum omnivorum, or cotton root rot, does not discriminate and can attack more than 2,000 species of plants. This disease is one of the most destructive plant disease organisms. Outside of our state, the disease is known as Texas root rot.
What is this deadly disease? This is a fungal disease that is prevalent in calcareous clay soils with a high pH range of 7.0 – 8.5. Symptoms first appear when the temperatures get hot and the soil temperatures reach 82 degrees F. In really hot years, this can be as early as June, but usually occurs in July and August.
The fungus lives deep in the soil and grows as mycelia strands (threadlike structures)upwards towards the taproot.Once it infects a root it can move down and across rows killing additional plants.The symptoms of wilt start usually in midsummer.The plant dies soon after wilt symptoms start.The leaves of the plant generally remain attached to the plant.The roots are darkened with a reddish stain and mycelia strands (called rhizomorphs),which are tan in color,can be seen on the roots.Under very moist conditions,mats of white to tan mycelia can be found on the soil around the dead plants.This disease is worse in summers when there is good rainfall. pestdata.ncsu.edu
This is a fast-acting fungus, causing wilt at the upper portion of the plants first. Within 72 hours, the lower leaves wilt. By the third day, permanent wilt has occurred and is followed by death of the entire plant. The plantwill retain its leaves, but they will turn a bronze color, then dry up.
By the time you see wilting on the plant, the roots have already been invaded by the fungus. The roots will be brown and decayed, and wooly-like strands of the fungus will be on the root surface. For a confirmation of the disease, a root sample should to sent to a diagnostic lab.
The fungus grows slowly in the soil. You might have a row of plants, and the disease may attack every third plant, or perhaps only one in every seven or eight. There will be no pattern. The fungus can survive for many years before invading the roots. We can not tell where or when it will occur in the soil.
This disease has provided job security for plant pathologists since it was first discovered in the 1930’s. No control has been found. The best defense against cotton root rot is organic matter. Slowly building organic matter into the soil will help improve and slightly lower the pH of the soil. Keep in mind that trying to acidify the soil enough to lower the pH below 7.0 is not practical and far too costly.
What can you do? You can use plants that are resistant to cotton root rot. Decades ago, Drs. J. J. Taubenhaus and W. N. Ezekiel compiled a list of plants that are resistant to and tolerant of cotton root rot. The list consists of trees, shrubs and flowers.
Examples of resistant trees are anacua, live oak, pecan, date palms, sycamore, cedar elm, yaupon holly and most junipers. Shrubs include dwarf yaupon holly, rosemary, barbados cherry, oleander, and most yuccas.
The complete list can be found on our website at http://bexar-tx.tamu.edu/ or send a self addressed stamped envelope to 3355 Cherry Ridge, Suite 212 and, we will send you the Cotton Root Rot publication. We also have the forms should you wish to submit a sample for diagnosis to the Plant Pathology Lab at Texas A&M.
For a list of plants resistant to cotton root rot, click here:
This article was written by Lynn Rawe, County Extension Agent-Horticulture with Texas Cooperative Extension in Bexar County.