The live oak (Quercus virginiana) is a highly desirable landscape tree for use in United States Department of Agriculture plant hardiness zones 7b through 10. Low-growing, with a mature height of only 50 feet, the horizontal branches of a live oak can spread more than 80 feet. Because live oak trees may take 200 years or more to mature, it is understandable that live oak leaves suddenly falling off are of great concern to a homeowner.
1. Seasonal Leaf Drop
The most common cause of live oak leaf drop is the normal transition from winter to summer that occurs in the spring. While live oaks are considered evergreen plants, every year, between February and May, the leaves from the previous season of growth are shed to make room for new leaf growth. Because it occurs within such a short period of time, the amount of defoliation can be startling. Look for new growth buds along the stem and where the old leaves were located. New bright-green growth is usually easily seen.
Ongoing drought causes all plants to decline, including live oaks, even though they are considered extremely drought tolerant. Leaf drop is common during an extreme drought because the tree does not have enough water to support all the leaves. A tree growing in an irrigated landscape is particularly vulnerable to leaf drop if the regular watering schedule is disrupted.
3. Oak Wilt
When leaf drop suddenly occurs during the summer or fall, and the tree is irrigated and well cared for, a disease such as oak wilt may be the cause. Oak wilt is almost always fatal, and if suspected, needs immediate professional attention to save the tree. It is caused by the Ceratocystis fagacearum fungus and is spread by the nitidulid (picnic) beetle attracted to the sap of a fresh wound, or from the roots of infected trees that share the root system of a healthy tree. Because live oaks spread far underground, they may share the same root system of other live oak trees up to 200 feet away. If oak wilt is suspected in a neighborhood, steps must be taken to save any non-infected trees, such as trenching, to separate the root systems, or inoculation of the tree with a fungicide.
Insect pests, such as the "leaf curl" aphid along with a variety of different worms, such as canker worms, may affect new growth of live oaks in the spring, causing leaf drop. Before the leaves fall, they become distorted or ragged. The tree may look in poor health overall, but new leaves will appear once the insects grow out of the larval stage and move along. Infestations occur in cycles and may be severe one year and non-existent the next. Occasionally, severe infestations compounded with drought cause complete defoliation and even death of the tree if steps are not taken to provide adequate irrigation and pest control.
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