University of Missouri Extension

G5200, Reviewed November 2017

Tree Decline: What Is It?

Hank Stelzer
Forestry State Specialist
School of Natural Resources

A natural forest typically begins as a large group of small trees, perhaps as many as several thousand to the acre. Over a period of many years, the numbers are reduced to only a few large trees per acre. Foresters, farmers and gardeners have long realized that an acre of land can produce only a certain amount of plant growth for either many small stems or for a few large ones.

In general, the plants that survive are those that are best adapted to local growing conditions and those best able to compete effectively for sunlight, moisture and plant nutrients. There is a tendency for the strong plants to get stronger and the weak plants to get weaker, eventually resulting in a natural thinning process. As the eventual losers decline in vigor, a few will pass out of the picture each year. Periods of drought, early and late frosts, wide fluctuations in temperature, insect and disease epidemics, wildfire and other forces may speed up the natural selection process.

Photo of an oak in decline.Figure 1
Various stresses cause reduced tree vigor and allow secondary diseases and insects to cause further decline or death of branches, roots or sometimes entire trees. (Photo by Robbie Doerhoff, Missouri Department of Conservation)

As with humans, a decline in vigor makes plants more susceptible to other problems (Figure 1). Insects and diseases, alone or in combination, may cause tree decline and eventual death. In unmanaged forests, natural forces determine which trees are removed. In managed forests, professional foresters make the decisions about which trees will be removed to achieve management objectives.

Various stresses cause reduced tree vigor and allow secondary diseases and insects to cause further decline or death of branches, roots, or sometimes entire trees. One of the major causes of stress in trees is planting them in locations to which they are not suited. Some examples:

Another major cause of stress in trees is allowing too many of them to occupy an area. Crowded trees compete with each other for nutrients, moisture and sunlight. Not all will survive when trees increase in size. Although some trees will eventually dominate, their growth is slowed and stress is created until the final survival decision is made by nature.

Many other factors also place trees in stressful situations and eventually lead to their decline:

Many stress situations such as those listed can be avoided by checking with a tree care professional or by acquiring a knowledge of the cultural requirements of different tree species and the characteristics of the area in which the trees are located or will be planted.

Symptoms of decline

Symptoms of decline may develop quickly or they may not be noticeable for years. Early symptoms include premature fall coloration, late spring leaf development, decreased twig and stem growth, leaf scorch, death of tissues between the leaf veins, and premature leaf drop. Later symptoms include dieback of larger limbs and branches; sprouting from the trunk of the tree; heavy seed crops; foliage noticeably smaller, lighter green and sometimes produced in tufts or clumps on sprout-origin tissues. The foliage over the entire tree may also look thinner in decline conditions.

Certain fungi that cause stem canker or root infections are often associated with the decline of many tree species. When trees are under stress, physiological changes occur in the roots, allowing the fungi to infect and kill them. In the autumn, clusters of mushrooms may form at the base of trees that have been infected. These growths indicate that root and trunk infections are present.

Many wood-boring insects are also associated with the decline of trees. The ash borer, bronze birch borer and the flatheaded apple tree borer are examples of insects that breed in various hardwood tree species that have been weakened by drought, defoliation or other adverse conditions. Scale insects are also commonly associated with tree stress.

Minimizing stress and tree decline

Tree decline can often be minimized by taking these preventive maintenance steps:

In caring for trees in a yard, more intensive control measures may be justified. Get an accurate identification of the problem by submitting samples to a plant diagnostic clinic. Very few foliar diseases on mature trees need to be treated with fungicides. Raking and removing fallen leaves may reduce a source of inoculum. Dead limbs may be pruned out, when warranted, to remove sources of disease or insects. Care should be given to avoid spreading disease on pruning tools by sterilizing the pruning tools with wood alcohol, household bleach or other approved disinfectants.

Nutrient deficiencies can be corrected by proper fertilization. Mulches to conserve moisture, prevent deep freezing and prevent soil crusting are usually beneficial. But do not apply mulches too deeply around stems. Properly timed watering will reduce moisture problems.

Watering to reduce drought stress

Even large, well-established trees can be damaged by a severe drought. A few well-timed irrigations can greatly reduce the stress encountered by trees during a drought and may prevent the onset of decline.

During a midsummer drought, it is common for about 1/4 inch of water to be lost from the soil per day. Under these conditions, most available water would be depleted from the root zone of a tree within 14 days. If the soil is a clay loam, to completely replenish soil moisture in the tree root zone (to a depth of 18 inches) would require about 3.5 inches of water.

Fortunately, it is not necessary to wet the entire tree root zone. Irrigation applied to turf will usually supply enough water to trees in a lawn to greatly reduce drought stress. However, where turf and tree roots exist together, a rapid depletion of soil moisture will occur. These areas require more water than areas where few tree roots are present. A large mulched area under each tree will allow fine tree roots to grow near the soil surface without competition from grass roots and will allow more efficient irrigation and fertilization of both trees and turf.

If valuable trees are located in a nonirrigated area, a deep soaking irrigation every two or three weeks during a drought will prevent excessive drought stress from developing. Application of 2 to 3 inches of water over half the surface area contained within the drip line will be of great benefit to a drought-stressed tree. It may be necessary to irrigate on two or three consecutive days to apply this amount of water without runoff.

After irrigation, the soil should be moist to a depth of at least 12 inches and preferably 18 inches. Keep track of the number of inches applied with a rain gauge. The depth of water penetration usually can be estimated by pushing a 3/8- or 1/2-inch diameter steel rod into the ground. The rod will stop when it reaches dry soil.

An effective way to water smaller trees (up to 3 inches in stem diameter) is with leaky buckets. To water a tree about 2 inches in stem diameter, place four 5-gallon buckets 2 feet from the trunk in a square pattern. Two or three holes should be punctured in the sides of each bucket near the bottom so that the water will drain out in about five hours. The buckets should be refilled and moved to dry areas under the same tree two or three times and then moved to another tree. To water smaller trees, use the same procedure but with two or three buckets. If the drought persists, the tree should receive a watering cycle every week or 10 days.

A permanently installed drip irrigation system will achieve the same results as the leaky bucket technique with much less effort.

Overwatering can also seriously damage trees. It takes 62 gallons to apply 1 inch of water to 100 square feet. Applying this quantity of water for several days to a soil that already has a good moisture supply can suffocate some of the tree roots. Inexpensive timers are available that can be attached to the outside faucet to ensure that the desired amount of water is applied.

Prevention of tree decline

Here are some ways to prevent the common causes of tree decline:

Original author
John P. Slusher
G5200 Tree Decline: What Is It? | University of Missouri Extension

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