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Mario Villarino
If a post oak had a “mission statement,” it would be to grow, reproduce and defend itself from a hostile environment, including pests and diseases.
If successful, it would become a slow-growing, medium-sized tree in the Southwestern and South Central United States.
Post oaks can flourish and do well where most other trees would not survive. In Texas, they grow best in the Cross Timbers, a prairie transition region where soils are dry with rocky outcrops and slopes. These soil types are well drained, sandy, coarse, deficient in nutrients, and low in organic material.
Because they are drought-resistant, post oaks can dominate sites that have little soil moisture.
Understanding the ideal conditions for healthy post oaks helps explain why they have recently experienced widespread decline.
Post oaks are not susceptible to any major virulent pathogens. Highly resistant to the oak wilt fungus Ceratocystis fagacearum, they rarely succumb to the disease.
But there are several less lethal, common diseases and pests, such as blights, blisters, cankers, blotches, foliar spots, root rots, and defoliating insects, that can disfigure a tree and cause some die back of branches, but rarely will kill it.
Post oaks are notoriously susceptible to various environmental problems associated with urban development, such as root disturbances, compaction, slow drainage, and poor irrigation practices.
The rapid decline of post oaks is the result of varying extremes (drought and drowning) that make the trees susceptible to attack by the cankers, root rots, soil moisture, and wood-boring insects.
All of these problems were made worse by the historic drought in Texas in 2011 that crippled tree health to such an extent that trees are still suffering today.
Extreme drought induces stress so that the trees must cope by expending storage carbohydrates (starches) to compensate for the strain.
During years of normal temperatures and rainfall, these storage carbohydrates accumulate in the roots as a result of photosynthesis. During stressful years, the tree responds and survives by mobilizing and depleting these stored reserves. One response is through osmotic adjustment of cell contents.
Osmotic adjustment, or changes in the levels of sugars in the sap, leads to regulation of tiny pores in the foliage called stomata, preventing water loss from the leaves.
Leaves may drop from the tree prematurely to prevent water loss. These adaptations allow trees to avoid wilting and continue metabolic functions with less water.
Another response during drought conditions is enhanced root growth with modifications such as increased suberization (waxy thickening of cell walls), which also helps the tree prevent water loss.
These alterations come with a cost by exhausting the starches normally used by a post oak to achieve its “mission statement.”
In other words, the result is a weakened, starchdepleted tree with poor growth, making it predisposed to diseases and pests that normally do them no harm.
In similar types of oak decline worldwide, research verifies the same patterns and causes as those occurring with the Texas post oak mortality problem.
The key to controlling rapid decline is to keep the trees healthy so they are not susceptible to diseases and pests that do not plague them during normal years. There are more options for trees in an urban setting than for those in rangelands and woodlands: improve aeration and soil drainage, Correct irrigation practices that may lead to overwatering and use mulches selectively to prevent continual watering around the base of the tree.
For more information on this or any agricultural/ natural resources topic please contact the Hopkins County Extension Office at 903-885-3443 or email me at m-villarino@tamu.edu.