The rains have returned, so why are all my trees dying?

Oak with hypoxylon canker

What a difference a year makes. In September 2011, most of Texas was in a drought — the most severe drought in Texas history. Across the state, trees — particularly mature oaks, elms, pines and junipers — were dying simply from the lack of water. 

With the return of seasonal rains to most parts of Texas in 2012, pastures and agricultural fields turned green once again. Trees that weren’t killed directly by the drought began to leaf out or put on new needles.

Why, then, are some trees continuing to die months after the drought has ended?

The best answer is that severe drought may have a long-term impact on trees, and the stress may linger for years, long after soil moisture returns. In many cases, drought kills the fine feeder roots that trees use to take up water and nutrients. Until these feeder roots can be fully replaced, the tree remains under stress and cannot support a full complement of leaves.

Prolonged stress renders the tree vulnerable to primary and secondary pests. Of course, trees also may be stressed by human activities, such as construction or other actions that disturb soil or damage roots.

Hypoxylon Canker 
In the case of hardwoods such as oaks and elms, stressed trees may be colonized and killed by hypoxylon canker even after the drought has ended. Symptoms of hypoxylon canker include the browning of leaves and sloughing of bark, which exposes a gray, brown or black fungus on the sapwood of the stem or major limbs. The dead leaves may remain on trees killed by hypoxylon long after the tree has died.  

There is no cure for hypoxylon canker. Once a tree is infected, it will die. Read more about hypoxylon canker: http://tfsweb.tamu.edu/main/article.aspx?id=1262. 

Oak Wilt 
In Central Texas, live oaks and red oaks may die from oak wilt. This fungal disease infects and kills even healthy oaks regardless of soil moisture levels. Once established, oak wilt infection centers may spread tree-to-tree through connected root systems in both rural and urban sites. Digging a 4-foot deep trench all the way around and 100 feet in front of an expanding oak wilt center is the recommended method of treatment.  Oak trees threatened by oak wilt can be protected for a year or two with injections of fungicide containing the active ingredient propiconazole (Alamo). For more information on identifying and managing oak wilt in Central Texas, visit www.texasoakwilt.org. 

Bark Beetles 
With pine trees, prolonged stress may be caused not only by drought but also by scorching from wildfires and other factors. This stress often results in attacks by bark beetles (also called engraver beetles or Ips beetles) even during the year following the drought or fire. For example, many pine trees that survived scorching by the Bastrop fire in September 2011 now are dying from engraver beetle attacks.    

Symptoms of pine bark beetle infestation are fading (yellow) or red foliage in the top portion of the tree or throughout the crown, masses of resin or pitch tubes resembling popcorn kernels in bark crevices, brown or white sawdust accumulated at the base of the tree and Y- or H-shaped trails made by the attacking beetles between the bark and wood surface. Bark beetle attacks can be prevented by applications of topical or systemic insecticides registered for this purpose.

Usually, however, the first symptom noticed by a landowner is discolored needles. By then, it usually is too late to save the tree. High value pines in yards can be treated with insecticide sprays containing bifenthrin (sold under the trade name Onyx) or permethrin (Astro or Dragnet). They work best when applied prior to the beetle attack. 

The Texas A&M Forest Service has developed a systemic insecticide that now is registered by the Environmental Protection Agency to protect pines and hardwood trees against a variety of forest pests, including engraver beetles and wood boring insects. The active ingredient is emamectin benzoate —  sold under the trade name of Tree-äge. This insecticide is for restricted use only and must be purchased and applied by a certified pesticide applicator. Insecticides — both systemic and spray applications — are best applied prior to the first beetle attack.

In general, healthy pine trees can defend themselves from engraver beetle attacks with a copious flow of resin without need for insecticides except during and immediately following periods of drought or fire.

Although scattered trees are likely to die for several years following a drought, the good news is that the number of dying trees on the Texas landscape is far less now in 2012 than in the previous year. The healthier trees are regrowing their fine root systems and recovering from last year’s drought. Trees also will fare better as temperatures drop to lower levels with the arrival of fall and winter.

 Landowners should keep a close watch on their valuable shade trees and take measures to maintain tree health throughout the year. Supplemental watering of trees near residences may be needed if several weeks pass without significant rainfall. Adding a 2- to 4-inch thick layer of mulch around mature trees also reduces soil moisture loss and helps prevent damage from low winter temperatures. Be sure to keep the mulch away from direct contact with the tree’s trunk.

Dead trees should be felled and removed as soon as possible if they pose a hazard. Bare spaces in the landscape can be restored by planting additional trees and properly caring for them with adequate mulch and water. Tree planting should take place during winter months when temperatures are cool and the saplings’ root systems have stopped growing.

Learn more about selecting trees native to your region at http://texastreeplanting.tamu.edu.

Written by: Ron Billings, Forest Health Manager, Texas A&M Forest Service