Ecosystem Health > Plant Diseases

Diseases occur on many of the root systems and woody and herbaceous tissues of the flora of Central Texas.

There is rarely just one factor leading to the decline or death of a plant. Many predisposing factors throughout the life history of the plant may create stress or favorable conditions for infection. These factors may be heat, drought, cold or flooding; direct damage by animals, livestock, construction and clearing activities, and storms; soil compaction by animals and livestock; grade or drainage changes; and wildfire, among others.

Abiotic pathogens:

  • Nutrient deficiencies
  • Poor water relations
  • Climatic extremes
  • Herbicides
  • Air pollution
  • Toxic chemicals

Biotic pathogens:

  • Fungi
  • Bacteria
  • Nematodes
  • Viruses
  • Parasitic plants

Some of the diseases that cause damage to the plants of Central Texas include:

Oak Wilt

This disease is responsible for the death of hundreds of thousands of oaks across 73 counties of Central and West Texas. All oaks are susceptible. An infection center starts when a sap feeding beetle visits a red oak that has died of the disease and feeds and breeds on the spore mats produced between the bark and the wood. The beetle, now covered with spores, flies off, sometimes long distances, seeking a new wound on a tree for its next meal. If the tree is an oak with an open wound from brush clearing operations, storms, pruning, livestock damage, etc. the tree may become infected with the disease. Oaks in Central Texas often vegetatively reproduce from roots growing through the shallow soils. Adjacent oaks are connected to each other in this way or from their roots physically lying over each other and growing together. Once an oak gets oak wilt, the disease can spread and kill every adjacent, connected oak tree.

For detailed information on oak wilt go to:   http://www.texasoakwilt.org/

Root Rots

These rots are caused by decay fungi and water molds that kill woody trunk and root tissues and fine, feeder roots of woody and herbaceous plants. They cause branch death, stunting, slow growth and often, plant death. Some of the structural root rots include Amallariella spp., Ganoderma spp., and Heterobasidion annosum. Some of the feeder root rots include Phytophthora spp. and Phymatotrichum omnivorum. Infection is often associated with previous wounding of the plant by animals, fire, lightning, and mechanical means or by environmental stress, especially drought and waterlogged soils.

Hypoxylon Canker of Oaks

This canker disease occurs in trees when they are stressed by environmental extremes and other damaging agents. As the tree is declining, Hypoxylon secondarily colonizes the wood just under the bark causing a distinctive necrotic lesion and spore mat. The signs of the fungus appear as small patches but will eventually merge to form large strips along the trunk and major limbs of the tree.

Fact Sheets may be found at: http://plantclinic.tamu.edu/page.asp?o=tamu&s=TPDDL&p=Factsheet

Bacterial Leaf Scorch

The bacterium Xylella fastidiosa causes a scorch to the leaf margins of many trees, shrubs, and economically important plants in Central Texas. Symptoms usually occur in early summer as plants come under heat stress. The bacteria lives in the water conducting system of the tree (xylem).  It is transmitted (vectored) by insects that feed on xylem fluid, such as leafhoppers called sharpshooters and to a lesser extent, spittlebugs. It causes a stress-related decline in the plant resulting in stunting and branch die-back and often, death to plants such as oleander and grape vines (Pierce's disease). The scorch on the leaves can be somewhat diagnostic to the trained eye due to a chlorotic, yellow halo between the green and scorched tissue of the leaf. Diagnostic tests are available at the Texas Plant Disease and Diagnostic Laboratory at Texas A&M University for BLS. BLS symptoms can be confused with oak wilt symptoms in red oaks, especially Texas red oak or Spanish oak, Quercus buckleyi. The difference is that BLS does not cause all of the leaves and tree to die in a matter of weeks as oak wilt does. BLS scorch symptoms remain for the growing season and the tree leafs out the next year.

An excellent article on BLS is at: http://www.apsnet.org/online/feature/bls/

For a comprehensive look at Plant Diseases in Texas go to: http://planthandbook.tamu.edu/texlab/index.htm



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