Oak diseases
  First step in sustaining tree life is identification

July 29, 2004
LORETTA JOHNSON
Victoria County
Master Gardener Intern

This article follows last week's discussion about recognizing various forms of tree stress - and how to help trees in stressed conditions. While frequently steps can be taken to alleviate such conditions, there are times when trees are so beleaguered that the next step toward tree loss - tree disease - sets in. Such is the case with long-lived and loved oak trees.

Oaks represent one of the major shade trees of Texas and are also important components of forests and rangelands. They are not only long-lived, but possess the ability to withstand adverse weather and have been considered insect and disease resistant. Despite susceptibility changes over the years, oaks are still considered prime specimens, and their weaknesses from disease may need to become better understood so that their lives can be sustained.

Since the drought of the mid-1950s, oak mortality has become a serious problem where oaks are the predominant native tree species. Research shows that much of this mortality is due to disease-causing organisms or disease complexes involving environmental stress and pathogens. Following is a brief synopsis of various types of problems that may be affecting oak trees in our area.

First are foliar diseases - and the most common is anthracnose. All oaks are susceptible to this fungus; however, post oaks appear to be the most vulnerable, and with high levels of rainfall this year, a serious problem has existed. Symptoms include small irregular spots formed along the leaf veins with premature defoliation during periods of severe infection. It rarely kills, but trees become weakened if severely defoliated annually. A protective fungicide may be required if damage persists.

Running a close second in prevalence is oak leaf blister. Water oak, post oak, red oak and live oak are most often infected with the oak leaf blister fungus. Of these, water oak is the most susceptible. Leaves infected with the fungus have a distinct bulge on the leaf's surface. The surface opposite the bulge has a light green-to-olive, velvety fungal growth. With age this growth becomes dark brown to black in color. Little defoliation occurs unless infection is extensive. The fungus overwinters on bud scales, and in the early spring, if rainfall and humidity are high as leaf buds begin to open, overwintering fungal spores germinate and infect developing leaves. Chemical control of oak leaf blister is not often suggested except for very susceptible and valuable trees.

Tying for second place is rust, a disease that primarily affects post oaks but has also caused live oaks to drop their leaves in early spring. Again, this disease requires moisture to be present and has been extensive this year on area oaks. It appears on the bottom of the leaf as orange-colored pustules that can be rubbed, leaving orange spores on your fingers. Leaves will yellow around the orange pustule and, if sufficient infection occurs, they will die. Chemical control is not usually recommended except for weak trees that need to hold their leaves for building food reserves.

Other foliar diseases of lesser importance on oaks include powdery mildew, which can affect all species, and actinopelte leafspot, which affects primarily red oaks. Powdery mildew on live oaks is first visible as faint yellow spots on the upper and lower surfaces of the leaves. Actinopelte leafspot causes small, distinct reddish brown spots to form on red oak leaves. Both occur during the latter part of summer, and leaf removal or good sanitation is important to reducing the fungus spread.

Hypoxylon canker is one of the worst diseases of limbs and trunks. All oak species are susceptible to it, with water oaks and post oaks being among the most likely to be affected. Trees with hypoxylon canker first appear chlorotic and develop thin foliage. In severe cases, leaves die quickly and turn a light brown, and may cling to the tree for a short time. Soon after foliar symptoms develop, fungal structures may be observed on limbs and trunks. The rough, outer bark separates from the limb and trunk, exposing either a reddish brown, gray or black color under the bark. Trees die from hypoxylon canker in a random pattern with no movement of the fungus from one tree to another through rootgrafts. Hypoxylon canker is most often associated with trees that are under environmental stress such as drought or mechanical damage. There is no control except to maintain tree health and minimize stress.

Another devastating disease of oaks is oak wilt. All species of oaks are susceptible; however, Texas red oak and live oaks in the Hill Country and South Central areas are the most commonly affected. It is a fungus disease and spreads through rootgrafts and common root systems as well as through man and insects transmitting the disease to fresh pruning cuts made on trees out of the dormant season. There have not been any diagnosed cases of oak wilt in Victoria County, but it has been found to be extensive in DeWitt, Lavaca and other counties. Although prevalent, there are cultural, mechanical and chemical control measures for oak wilt.

Everyone has heard of oak decline and has dreaded getting it in their live oaks. It has been touted as a major fungal disease for years but as reported over the past 10 years, it is not a disease nor any one single pathogen, but a disease complex caused by numerous factors on stressed trees. In Texas, drought and urban stress are factors most often associated with this occurrence. Trees having oak decline show a slow decline with a reduction in leaf size and foliage, which results in death. There is no treatment except to maintain tree health.

While oak diseases seem extensive and may make you wonder if you should even consider having one, remember that very few trees are spared some sort of problem. We need to choose the tree species for our landscape that fit the environment and that, with proper management and care, will provide many years of shade and enjoyment. For further types of tree disease and more information on those addressed in this article, contact the Victoria County Extension office at 575-4581.

And when you call, don't forget to ask for an application for the upcoming fall training class to become a Texas Master Gardener in Victoria County. Aug. 2 is the deadline to sign up for the class, which begins on Aug. 12 and runs through Dec. 9. Like mentioned previously by several of my classmates in another article, I am so proud to be a part of a group that "practices what they preach" with good gardening practices.