By: Bonnie L. Grant, Certified Urban Agriculturist
Cotton root rot in plants is a devastating fungal disease. What is cotton root rot? This disease is caused by the fungus Phymatotrichum omnivorum. “Omnivarium” indeed. The fungus colonizes the roots of a plant, gradually killing them off and reducing its health. This voracious fungus is one of the most destructive diseases of cotton and over 2,000 other plants. Read on to learn more about it.
Ornamental, fruit and nut trees, cotton, and alfalfa are among the plants susceptible to cotton root rot. Fortunately for northern gardeners, the fungus that causes the disease is limited to the southwestern regions of the United States. Sadly for these gardeners, the fungus lives for years in soil and has the ability to kill even tall trees. It is important to recognize cotton root rot symptoms since correct identification of the disease is key to control.
Cotton root rot in plants is most prevalent in the summer months, from June until September. The fungus needs high summer temperatures and calcareous clay soil. The affected plant wilts and experiences foliar color changes, from green to yellow or bronze. Death is very sudden in warm weather once the fungus colonizes and fully invades the roots. Cooler weather can slow the decline of the tree, but once the warm season hits, it will invariably die.
Identification of the disease can be done by removing the dead plant. The roots will have wooly strands of fungus and a defined decayed appearance.
Treatment for cotton root rot after infection has been achieved on occasion with good cultural care. Prune the tree or plant back, working ammonium sulfate into a trench built around the tree and water thoroughly. Only 2 treatments may be applied per season and it isn’t a cure; only some plants will come out of the wilt and survive.
The acidification of soil creates an unfavorable environment for the fungus. Fertilizers high in nitrogen can minimize the spread of the disease. Chemical sprays do not exist for control of cotton root rot.
Because there are no sprays or formulas to kill the fungus, planning ahead in zones prone to the disease is essential. The only way to control cotton root rot is to purchase resistant plants or use plants that have resistance to the disease as barriers. Use monocotyledonous plants such as grasses and wheat, oats and other cereal crops as organic amendments.
Once the fungus is in the soil, it can survive for years and lives at the level at which most plants have a concentration of roots. This is why it is important to avoid plants susceptible to cotton root rot. These include:
Choose instead plants with some natural resistance as landscape ornamentals. Plant that seem to tolerate soils penetrated with the fungus with no ill effects include:
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Read more about Plant Diseases
Less than a century ago, the Dallas-Fort Worth area, was primarily agricultural land. This land was used to grow crops, hay, and raising cattle. The native tree species were much more limited. Now the land is used much differently. The old family farms are being developed into new shopping centers, homes, and other urban developments. The land use has changed and so has the landscape.
The DFW has added many new plant species to the pallet. As a landscape industry, we have integrated many plant species that have adapted to our area and climate. Some species were more successful than others. After many years of trial and error, plant diseases have found many suitable host species.
Photo 1: Fungal Mat ‘Phymatotrichopsis omnivora’
Photo 2: First Signs of Wilting
As new plant species were brought into our area, plant pathogens, whether native or not, have found their way in to our landscape as well. One disease that has over 2,300 host species (1,800 dicots), is known as, ‘Phymatotrichopsis omnivora’ (also referred to as, Cotton Root Rot, Texas Root Rot and Ozonia Root Rot). This is a soilborne fungus that lay dormant in the soil for many years.
As you may guess, cotton, a common crop that is grown in North Texas is very susceptible to this disease. Trees that are infected with Cotton Root Rot should be removed and only planted with tolerant or resistant plant species. Here’s a link to an online publication from Texas A&M University of Tolerant Plant Species: https://aggiehorticulture.tamu.edu/archives/parsons/publications/cottonrootrot/cotton.html
Diagnosis is very easy during the early and mid-summer. As the soil temperature exceeds 82 degrees, Fahrenheit, which is usually in the spring and early summer, the disease will develop in the plant. The first symptoms are wilting, followed by death. Often smaller plants are quickly killed by this disease. While larger trees may require more time for the disease to terminate the tree.
As a diagnostician, we are looking for key symptoms and signs out in the field. Common species, like Lace Bark Elms are very commonly killed by this disease. During the early summer, we look for fungal mats that develop on top of the soil as a key indicator that the pathogen is present. It has been demonstrated in research that the fungal mat does not spread the spores, so don’t worry about spreading this pathogen if you walk through a fungal mat or two.
Symptoms of diseased grapes caused by Phymatotrichopsis omnivora.
Dr. David Appel, Extension Plant Pathologist with the Texas A&M AgriLife Extension Service contributed this article that describes a new treatment for one of the most devastating diseases of Texas winegrapes. This was research conducted by Dr. Appel and our own TPDDL diagnostician, Sheila McBride. -KO
This disease, cotton root rot, occurs on winegrapes in the calcareous soils of Central, South, and West Texas where soil pH is well above 7. The most common symptom of CRR in grapes is a sudden wilt and death of the infected vine. All of the leaves on the vine usually turn yellow and quickly become brown and dry. Vines often die in groups as the fungus advances underground along trellises to adjacent vines. Taproot, crown, and lower stem tissues appear dark brown and decayed after the dead bark is removed with a pocket knife. The symptoms become most obvious during the mid-to-late summer months, or when soil temperatures exceed 82 o .
McBride evaluating experimental grape plots in 2014
More Cotton root rot damage on grapes.
As a result of four years of research on control of CRR in winegrapes, a fungicide has been found to reduce losses caused by this deadly disease. Cotton root rot, aka Texas root rot, is caused by the fungus Phymatotrichopsis omnivora, the same fungus that kills cotton, peaches, apples, and other specialty crops and ornamentals in Texas. This fungus survives for years in infested soils. The fungicide is called TOPGUARD ® Terra and is now available for winegrape growers to use in their vineyards. TOPGUARD ® Terra was labeled for use on cotton root rot in cotton two years ago, prompting us to test the same treatment on the disease as it occurs in vineyards. Research plots were established at three locations in Central Texas where there was perceived to be a high risk of infection by the fungus. The root zones of vines were soaked with different rates of the fungicide and then the vines were observed for disease development. By comparing the disease in treated vines with untreated control vines, it was apparent that the fungicide was effective in preventing losses. The effective rates and application methods were developed from the studies and are available in the materials referenced below.
The Texas Department of Agriculture requested a Section 24c registration for use of the fungicide on winegrapes based on current research results, called a “special local need registration”. A 24c registration is reserved for pest situations in a state for which there are no existing pesticide products. This applies to the case of TOPGUARD ® Terra on cotton , where there is a federally registered pesticide already shown to be effective on additional pests. However, winegrapes are not yet on the same label as cotton. In order to use the fungicide on winegrapes, a special copy of the TOPGUARD ® Terra label must be in hand in order to apply it in vineyards. A copy of the label for downloading can be found below. Remember – you MUST have this additional label on hand when applying TOPGUARD ® Terra on winegrapes in order to comply with pesticide regulations. Also, a bulletin prepared by FMC Corporation can be found. This bulletin contains some background information on how to use the product as well as some expectations for control when properly applied. The fungicide may be purchased from most local agricultural retail outlets.
Cotton root rot (CRR), aka Texas root rot, occurs on wine grapes in the calcareous soils of Central, South, and West Texas where soil pH is well above 7. The most common symptom of CRR in grapes is a sudden wilt and death of the infected vine. All of the leaves on the vine usually turn yellow and quickly become brown and dry. Vines often die in groups as the fungus advances underground along trellises to adjacent vines. Taproot, crown, and lower stem tissues appear dark brown and decayed after the dead bark is removed with a pocket knife. The symptoms become most obvious during the mid-to-late summer months, or when soil temperatures exceed 82 degrees Fahrenheit.
TOPGUARD® Terra Fungicide
As a result of four years of research on control of CRR in wine grapes, the TOPGUARD® Terra Fungicide has been found to reduce losses caused by this deadly disease. It is now available for wine grape growers to use in their vineyards.
In order to use the fungicide on wine grapes, a special copy of the TOPGUARD ® Terra label must be in hand in order to apply it in vineyards. A copy of the label for downloading can be found below. Remember – you MUST have this additional label on hand when applying TOPGUARD ® Terra on wine grapes in order to comply with pesticide regulations. Also, a bulletin prepared by FMC Corporation can be found. This bulletin contains some background information on how to use the product as well as some expectations for control when properly applied. The fungicide may be purchased from most local agricultural retail outlets.
Cotton root rot is difficult to treat or control. Some types of chemical controls have offered moderate success, but a licensed applicator has to apply the chemicals to the area every year or every other year. If the soil is contaminated, avoid planting redbud trees or other susceptible species in that area. If your tree is only showing mild symptoms, cut approximately half of the top growth back so the roots do not have to work as hard to transport water through the plant, and water the soil regularly.