Xylella And Oaks: What Causes Oak Bacterial Leaf Scorch

By: Darcy Larum, Landscape Designer

Plant diseases in trees can be tricky things. In many cases, symptoms can go unnoticed for years, then seem to cause sudden death. In other cases, the disease may show obvious symptoms on certain plants in the area but then can affect other plants in the same location in completely different ways. What is xylella leaf scorch? Continue reading to learn more about oak bacterial leaf scorch.

What is Xylella?

Xylella leaf scorch is a bacterial disease caused by the pathogen Xylella fastidiosa. This bacteria is believed to spread by insect vectors, such as leafhoppers. It can also be spread from grafting with infected plant tissues or tools. Xylella fastidiosa can infect hundreds of host plants, including:

  • Oak
  • Elm
  • Mulberry
  • Sweetgum
  • Cherry
  • Sycamore
  • Maple
  • Dogwood

In different species, it causes different symptoms, earning it different common names.

When xylella infects oak trees, for instance, it is called oak bacterial leaf scorch because the disease causes the leaves to look as if they’ve been burned or scorched. Xylella infects the vascular system of its oak host plants, inhibiting the flow of xylem and causing the foliage to dry out and decline.

Olive green to brown colored necrotic patches will first form on the tips and margins of oak leaves. The spots may have light green to reddish brown halos encircling them. The foliage will turn brown, dry out, look crunchy and burnt, and drop prematurely.

Treating an Oak Tree with Xylella Leaf Scorch

Symptoms of xylella leaf scorch on oak trees can appear on just one limb of the tree or be present throughout the canopy. Excessive water sprouts or weepy black lesions may also form on infected limbs.

Oak bacterial leaf scorch can kill a healthy tree in just five years. Red and black oaks are specifically at risk. In its advanced stages, oak trees with xylella leaf scorch will decline in vigor, develop stunted foliage and limbs or have delayed bud break in the spring. Infected trees are usually just removed because they look so terrible.

Oak trees with xylella leaf scorch have been found throughout the eastern United States, in Taiwan, Italy, France and other European countries. At this point, there is no cure for the worrisome disease. Annual treatments with the antibiotic Tetracycline alleviate the symptoms and slows down progress of the disease, but it does not cure it. However, the United Kingdom has launched an extensive research project to study xylella and oaks infected by it to protect their nation’s beloved oak trees.

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Here's why your plant or tree leaves look sunburned.

There are three kinds of leaf scorch: nutrient-related, bacterial and weather-related, which is sometimes caused environmental leaf scorch.

1. Environmental leaf scorch occurs when tree leaves have literally been burned by the sun, hot temperatures or a general lack of rain.

  • In these conditions, tree roots do their best to keep water flowing, but sometimes not all leaves get enough water.
  • Those dehydrated leaves are the ones with brown, sometimes shriveled, tips that look burned.
  • The symptoms of this are usually at the tips of the branches and on the most exposed leaves.

2. Bacterial leaf scorch looks about the same as environmental at first glance, but has a few distinctions:

  • The disease starts toward the back of branches and gradually makes its way to branch tips.
  • Bacterial leaf scorch comes back year after year, browning more leaves each time. It also usually pops up right around late July or early August.
  • Leaf scorch caused by, say, drought will pop up shortly after the stressful weather.

3. Nutrient scorch occurs when the tree is deficient of a nutrient vital to its health–usually iron or manganese. Look for brown in-between the veins of the leaves at the tips of the branches.

Is my tree in danger?

The first two types typically aren’t a threat to trees that are established and healthy. You can find quick tips on treating these below.

Bacterial leaf scorch, on the other hand, becomes more dangerous each year. If you suspect it, talk to your arborist. If you catch it early, you can treat your tree each year to prolong its life. Or, if it’s too late, you may need to remove your tree entirely. Again, an arborist will tell you the next best step.

Environmental and Nutritional Leaf Scorch Treatment

Healthy trees, free of pests, diseases, and other stressors, are best at fighting this off.

Help your tree maintain vigor with these steps:

  • During stretches of sunny, hot, and dry days, water your tree deeply.
  • Lock in soil moisture by mulching your tree.
  • Fertilize trees regularly to provide needed nutrients.
  • Test the soil and provide adequate micro-nutrients (iron or manganese) if the tree is nutrient deficient.
  • Do a little research on your tree’s ideal growing conditions. For example, if it’s planted in a spot with full sunlight, but likes partial shade, you might consider moving it to a more suitable place.

Click here to learn what “deep watering” really means.

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Oak leaf blister on water oak.
Andrew J. Boone, South Carolina Forestry Commission, www.forestryimages.org

Oak Leaf Blister: Oak leaf blister is a disease caused by the fungus Taphrina caerulescens. Most oak species are susceptible, but the red and black oak groups are especially so. Minor infections cause little harm, but midsummer defoliation can occur when infections are severe. Blister-like patches appear on the leaves. They are often lighter green than the surrounding tissue and later turn brown. The blister-like patches result from an overgrowth of infected leaf tissue caused by substances secreted by the fungus.

The fungus survives the winter as spores in leaf buds. The spores germinate in the spring to infect the leaves. Infected leaf tissue grows much faster than uninfected tissue, resulting in distorted blisters. The fungus grows and produces spores within the leaf until the cuticle (surface wax layer) is ruptured by the mass of fungal tissue. Spore dispersal occurs in the fall.

Prevention & Treatment: Leaf blister is rarely severe enough to require control measures. Once an infection has occurred, fungicide treatments usually are ineffective. If the infection is very heavy and if the tree is small enough to obtain good coverage, a fungicide could be applied in the spring just before bud break. Rake up and dispose of fallen leaves in the fall.

Browning of edges of leaves caused by bacterial leaf scorch.
Edward L. Barnard, Florida Department of Agriculture and Consumer Services, www.forestryimages.org

Bacterial Leaf Scorch: This disease is caused by the xylem-limited bacterium Xylella fastidiosa and is characterized by a disruption of water movement, decline of vigor, marginal reddening or yellowing, followed by margins of leaves browning, leaf drop, dieback, and eventual death. The symptoms usually first appear in early fall, following summer drought. Because trees infected with bacterial leaf scorch will decline gradually, it may take five to ten years before a tree may need to be removed. This disease more commonly affects pin oak, red oak, white oak, bur oak, shingle oak, and sycamore. It damages red, silver, and sugar maples, hackberry, elm, and sweetgum.

Prevention & Treatment: Remove infected trees and replant the area with resistant tree species. Avoid planting all the same species close together. Leafhoppers and spittlebugs spread the bacterium, so start by controlling the weeds and wild plants that support these insects. Extra care to fertilize and irrigate may prolong the infected tree’s life, but trees with extensive leaf scorch and dieback should be removed. Currently, spray treatments are not available. However, certified arborists can perform annual root flare injections of antibiotic treatments using oxytetracycline (such as Bacastat), which can reduce symptoms by suppressing the pathogen.

Actinopelte Leaf Spot: This fungal disease may be a serious problem in wet weather. It is caused by the fungus Tubakia dryina (formerly called Actinopelte dryina). The symptoms are circular, dark to reddish-brown leaf spots with a diameter of ¼ to ½ inch. Spots may run together to form irregular blotches. Tiny black specks in rings are visible in the spots and blotches. Severe infections cause the trees to lose their leaves prematurely. Trees of low vigor that are repeatedly defoliated may die.

Prevention & Treatment: Destruction of all infected plant material will reduce the spread of the fungus. Rake up and dispose of fallen leaves. Small trees defoliated several years in a row may need spraying. Apply mancozeb, chlorothalonil, thiophanate-methyl, or a copper fungicide according to the instructions on the labels. Complete coverage is necessary for control. See Table 1 for examples of specific products.

White coating on leaf caused by powdery mildew.
Petr Kapitola, State Phytosanitary Administration, www.forestryimages.org

Powdery Mildew: This disease is caused by several fungi (Erysiphe trina, Microsphaera alni, Phyllactinia corylea, and or Sphaerotheca lanestris). The symptoms consist of white, powdery growth on both leaf surfaces. The foliage may be malformed, dropping prematurely, or drying out and shriveling. Sometimes the grayish-white fungal growth changes to tan and then to brown with age. Tiny black dots (fungal fruiting bodies) may be seen in the brown felt abundant in some years but rarely in others.

Prevention & Treatment: Control is usually not practical nor warranted. Myclobutanil or thiophanate-methyl sprays may be used to control powdery mildew in severe cases and on small trees. Follow the directions on the label. See Table 1 for examples of specific products.

Armillaria root rot mushrooms near infected oak tree.
Joey Williamson, ©2013 HGIC, Clemson Extension.

Armillaria Root Rot: This root rot disease is caused by the fungus Armillaria mellea and is widespread on oak. The symptoms are a slow decline of the tree. Mushroom fruiting structures on or near the root collar are prevalent in late summer and fall, especially during wet weather. Clusters of these honey-colored mushrooms may appear outward from the tree as they follow the major roots.

Prevention & Treatment:Remove diseased trees and as much of the root system as possible. Do not replant the site with a susceptible host. Trees that are resistant or tolerant to Armillaria root rot include bald-cypress (Taxodium distichum), Chinese elm (Ulmus parvifolia), dawn redwood (Metasequoia glyptostroboides), eucalyptus, ginkgo (Ginkgo biloba), Japanese cedar (Cryptomeria japonica), Leyland cypress (Cupressocyparis leylandii), maple (Acer species), sourwood (Oxydendrum arboreum), and sweetgum (Liquidambar styraciflua).

Asexual brown fungal stroma and black sexual stage of Hypoxylon on oak.
Meg Williamson, Plant Problem Clinic, Clemson University

Hypoxylon canker: This is a white-rot fungal disease primarily of oaks, hickories, and pecans in South Carolina and is caused by Biscogniauxia atropunctata var. atropunctata (formerly called Hypoxylon atropunctatum). Many species of oaks are susceptible to this disease, but post oak, water oak, southern red oak, white oak, and blackjack oak are most often affected. This opportunistic pathogen is a common inhabitant of the bark of hardwood trees, but it is only of consequence when the trees are under severe stress. Environmental stress caused by drought, as well as by root injury during construction, utility trenching in the root zone, soil grade changes, soil compaction, and root diseases, all can play a role in weakening the trees and the subsequent infection by B. atropunctata var. atropunctata. Any root injury will reduce water uptake by trees, and drought stress appears to be the most significant factor in infection.

As the fungus spreads and forms cankers, the first symptom that may be observed is the dying back of the crown (top) of the infected tree. However, other tree problems may also result in dieback. Subsequently, the outer bark begins to slough off in areas of infection, and pieces of bark can be seen at the base of the tree. This bark loss exposes the first sign of the fungus, which is a brownish fungal stroma where conidia (or asexual spores) of the pathogen are produced. This area may be several inches to several feet long on limbs and trunks. These conidia are wind-disseminated and can cause new infections on other trees.

As the infection continues to develop, the exposed area of fungal stroma changes to a gray or silver color and finally to black as a second type of spore is produced. This is the sexual stage of the fungus, and these spores, which are also infectious, are spread by splashing rain or insects to nearby trees.

Prevention & Treatment:There are no controls for Hypoxylon canker on these hardwood trees once the infection has begun on the trunk. If the infection is observed on branches, these may be removed and burned, but there may be other infection sites that are not yet apparent on the tree.

Stress reduction is the key to prevent infection. Keep the trees as healthy as possible.

  • Protect trees from damage during home construction and utility repairs. For more information, see HGIC 1002, Protecting Trees During Construction.
  • Water trees during periods of summer drought with 1 inch of irrigation water per week. For more information on proper irrigation, see HGIC 1056, Watering Shrubs & Trees.
  • Trees should be mulched with a 3-inch layer of organic mulch from the trunk to the dripline, but don’t pile mulch against the trunk. For more information on mulching trees, see HGIC 1604, Mulch.
  • Do not apply weed killers near the tree, especially beneath the limb canopy.
  • Fertilize trees with slow-release tree and shrub fertilizer during early April. For more information on proper fertilization, see HGIC 1000, Fertilizing Trees & Shrubs.
  • Remove and burn or dispose of any infected trees, and cut the remaining stumps flush with the soil.

Pine-oak gall rust spore-containing pustules on an oak leaf.
Robert L. Anderson, USDA Forest Service, www.forestryimages.org

Pine-Oak Gall Rust (Eastern Gall Rust): The fungus Cronartium quercuum causes gall rust on approximately 25 to 30 species of pine and oak. Infection of oak causes small brown or yellowing areas on the leaves. On the underside of the leaves, pustules with yellow to orange powder (spores) are visible.

Prevention & Treatment: All fungal spores, which infect both pine and oak, are primarily windborne. High humidity increases the incidence of infection. Chemical control is usually not warranted. The fungicide myclobutanil may be applied to oaks according to the directions on the label. Follow the directions on the label. See Table 1 for examples of specific products.

Disease cycle and causal conditions

Disease cycle and causal conditions The disease cycle of this bacterium in grape, peach, and plum is well known, and it is likely the same in blueberry. Infected hosts serve as reservoirs and overwintering sites of the bacterium. In the spring and early summer, insect vectors (sharpshooters and spittle bugs) transmit the bacterium by feeding on infected plant tissues and subsequently feeding on healthy plants. In other systems, the glassy-winged sharpshooter, Homalodisca vitripennis, is the most important vector, and 97 percent of the sharpshooters found in southern highbush blueberry plantings are glassy-winged sharpshooters (M. Tertuliano personal communication). The glassy-winged sharpshooter can be found abundantly in south Georgia and Florida, where it is known to be the major vector of Xylella in peach and also prevents production of European wine grapes. Once the insect has acquired the bacterium, it is transmitted to a new plant as the insect injects the bacterium into the xylem (the conductive tissues that transmit water and nutrients from the roots to the other plant tissues) during feeding.

This bacterial species is unique in that it is limited to life in the plant xylem. Movement of the bacterium occurs throughout the plant xylem system, and movement to the roots is relatively rapid in blueberry—rendering pruning of symptomatic tissues ineffective. At some point, bacteria form colonies, and through a combination of tyloses, gumming, and bacterial exudate production, the xylem is clogged. In time, clogging of vessels reaches a point at which individual stems or whole plants will no longer be able to carry sufficient water and nutrients to support life. At this point symptoms develop, and eventually the plant will die. Plant death can be relatively rapid, but in general, symptom development starts in one year and continues through at least a second season before plant death.

This bacterium can also be transmitted through propagation from infected plants. Propagation studies have shown that apparently healthy cuttings taken from infected plants will sometimes produce diseased transplants, and a massive number of plants can be disseminated rapidly, expanding the epidemic. The combination of propagation and insect vectoring could cause rapid spread throughout the entire region. However, it is unlikely that symptomatic plants would be used for propagation.

Root grafting may also serve as a potential transmission mechanism. In high-density beds, this may be particularly important as a means of spread, but there has been no research conducted to date to support this premise.

Treating Bacterial Leaf Scorch

Bacterial leaf scorch (BLS) is caused by the bacterium Xylella fastidiosa. This disease impacts certain shade trees resulting in uneven ‘scorching’ of leaf margins in late summer and early fall. The bacteria themselves live in the xylem tissue and gather in clusters called biofilms. These ‘bunches’ of bacteria clog the xylem and block water transport, which leads to the scorch symptoms.

Scorching of leaf margins caused by bacteria.

The disease typically progresses throughout the tree for up to a decade causing so much dieback that the tree eventually dies. Symptoms and damage are usually most visible on pin and red oaks, but shingle, bur and white oak can be affected as well. Additionally, BLS can infect elm, sycamore, mulberry, sweetgum, sugar maple, and red maple. There are also numerous asymptomatic hosts.

Xylem-feeding leafhoppers and spittlebugs spread the bacterium from one tree to another. BLS is sometimes mistaken for other leaf diseases, or for drought and heat stress. The unique late summer development of BLS symptoms is the defining characteristic.
Thanks to recent advances in DNA analysis, BLS can now be detected at any time of the year, not only when symptoms appear late in the growing season. This allows an arborist to determine well in advance if a tree requires treatment. Experience at Bartlett Tree Experts has shown that BLS can often be effectively suppressed for many years if the tree is treated and provided with good cultural care, especially if the disease is detected early.

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