When people
see a sick tree, they often think that some sort of disease is causing the
illness. Actually, a majority of the problems causing trees and shrubs to look
sick stem from stress or physical injury rather than disease.
A common
symptom of stress or injury is marginal leaf burn, or leaves fringed by dead
tissue. This has been a common problem with numerous species of trees and
shrubs this summer. Marginal leaf burns are seldom caused by leaf disease,
which usually shows up as random lesions (dead areas) scattered about the leaf?
Leaf burn occurs at the leaf tip or along the leaf margin because salts (plant
nutrients) accumulated along leaf margins. Anything that causes the plant to
pump insufficient water (stress) can result in a toxic burn of this tissue
because it contains the highest level of salt.
Stress
symptoms ranging from leaf burns to limb dieback or tree death can result from
numerous causes. Drought is the most obvious cause of stress. This year we have had both extremely wet
spring and in some cases excessive and the last most extremely dry drought
conditions. Large trees show responses
to stress more slowly, some of the marginal burns now being observed relate to
last summer. High temperatures cause plants to pump more water and simply
compound drought problems. As temperatures exceed 100° F, water loss by some trees and shrubs can equal or
exceed the ability of the roots to supply water, even when the soil moisture is
not deficient. I expect we will continue to see some problems with trees and
other landscape plants until we receive some significant rainfall.
Because
of extreme Texas temperatures each summer, freeze injury is often overlooked,
yet it is one of the most common and damaging causes of stress. Direct injury
to twigs and limbs is usually fairly evident, and the damaged wood can be
pruned. Often the injury is more subtle, occurring on a portion of the trunk
with no immediate or noticeable effect on the entire tree or shrub.
Thick
bark sometimes remains intact, hiding trunk freeze injury for well more than a
year. Probing the bark on the lower 3 feet of the trunk with a screwdriver or
tapping with a mallet (listen for hollow sound) will usually reveal hidden
freeze injury if it is present.
Just
as drought causes trees to stress, so does excess water. Tree roots need oxygen
in order to function properly, so roots that are waterlogged lose their ability
to take up water. It can take several years for a seriously injured root system
to be regenerated.
In
recent years, numerous trees growing in poorly drained soil have been killed or
damaged following periods of heavy rainfall. Trees with damaged roots systems
are vulnerable to summer droughts and heat stress. Be sure to deeply water your
landscape trees as we continue into what are normally the driest months of the
year.
