A wide variety of diseases and pests can damage our trees and shrubs. Here are some that often appear in our area:
Scale is an insect that congregates on twigs and limbs of trees and sucks sap. They are not readily recognized as insects because they congregate under leathery coverings, which can be of several different colors. Ants sometimes cluster around the scale infestations to live off their secretions, and can function as unwitting protectors for the scale by warding off predators. The fungus sooty mould can also be associated with scale outbreaks. Scale can be treated with horticultural oil or soapy water in combination with thinning of leaves and twigs to increase airflow through the plant.
Mildew is a black substance that accumulates on leaves and twigs. White flies secrete a sugary substance on the plant which promotes the growth of the mildew. The mildew is not dangerous to your plant, but it is unattractive. Repeated applications of horticultural oil or soapy water can eventually kill it. Thinning out the limbs a bit to promote airflow around the branches can also be beneficial.
Southern Pine Beetles periodically infest our area, with fatal effect for many pine species. Outward signs of infestation are whitish-yellow tunnels of resin on the tree and reddish sawdust on the ground, in spider webs on the tree, or in crevices of the bark where the beetles have drilled into the wood. Once they have infested the tree, the beetles' entire life cycle takes place there. The needles of the tree turn yellow, then red-brown, at which point the tree will die. In a forest setting, the most effective ways of controlling this pest are thinning the stands of pine and controlled burns, neither of which are applicable to city lawns. Insecticides are available for small-scale use, but are considered too expensive to treat entire forests.
Chlorosis results from an iron deficiency in the plant. If there is too little iron available to the plant, the formation of chlorophyll is hampered and the leaves turn yellow. This may be the result of there being too little iron or nitrogen in the soil itself, or in a shortage of the nutrients the plant needs to access it. Fertilization with a high nitrogen fertilizer may help reverse the effects.
Gall is the result of insect or bacterial damage to the plant. Parasitic organisms affect the growth of the plant, causing abnormal growths on various parts of the plant. Sometimes these appear to be fuzzy or hard balls, but they may also be a mass of them on twigs. They are not in themselves harmful to the plant, being essentially an abnormal-looking part of the plant, so it is unlikely that treating them will have any effect, even if you target the insects, fungi, or bacteria themselves.
Spider mites are tiny insects that feed on leaves, sucking nutrients from their cells. The leaves thus affected appear speckled and eventually drop off. If this goes on long enough, it can result in death. Ladybugs and other beetles are predators that will take care of the mites the way nature intended.
• Buffalo Bayou Partnership •
Hermann Park Conservancy • Houston Parks Board • Houston Arboretum and Nature Center • Keep Houston Beautiful • Mercer Arboretum • Scenic Houston
• Tree City USA • Trees for Houston • Texas Urban Forestry Council