Oak Tree Diagnostics Disease and Prevention
The Oak Tree
The following infographic provides tips to spotting common pests and diseases, mortality rates, and oak tree maintenance tips. Some diseases, such as Galls are more unattractive then harmful. However, spotting diseases such as oak wilt and root rot early can save the infected tree and prevent the disease from spreading to other nearby trees.
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Oak Tree Maintenance
Oaks are a very “hearty” species of tree. Rarely do oak trees need maintenance, but there are a few common pests and diseases that can harm them if not properly treated. Often the best cure is to maintain a healthy oak tree as many diseases and pests attack trees that are already in weak condition. To keep oaks healthy make sure they get plenty of water during droughts, prune dead or dying branches, and fertilize and mulch annually to provide plenty of nutrients for the tree.
Ceratocystis fagacearum (Oak Wilt)
Tubakia dryina (Tubakia Leaf Spot)
Phytophthora (Root Rot)
Wood Boring/Bark Beetles
Wood boring/bark beetles are small, cylindrical, hard bodied pests often black, dark red or brown in color. In the beginning of an attack it can be difficult to spot a bark or wood boring beetle infestation. Signs of an infestation include small emergence holes in the bark, discolored shoots, sawdust like frass, and sap oozing from the tree. The reason why it can be so difficult to spot an infestation is because signs of bark and wood boring beetles often occur after the beetles have been attacking the tree for a period of time.
Wood Boring/Bark Beetle Maintenance
Prune and dispose of bark beetle-infested limbs
Maintain Healthy Trees
Remove the tree as a last resort
Once a tree has a beetle infestation is can be difficult to manage. Insecticides often fail because the beetles are protected beneath the bark of the tree. When removing the tree or pruning infested branches it’s important to destroy them properly. If the wood is not chipped, burned or removed from the property immediately beetles can quickly emerge and attack other trees in the area. Do not pile infested wood next to another tree or shrub.
Canker cause a dark or discolored area on the trunk or branches of an oak tree. The infected area may appear flattened or sunken with swollen and cracked edges. Besides for being unsightly, cankers can cause die back of branches and even kill an oak tree if not properly maintained. Symptoms include dying or dead branches, wilted leaves (still attached) among healthy foliage, and discolored bark, sunken and cracked bark.
Managing cankers can be tricky. One method to treat this condition is to use Cambistat a Paclobutrazol based growth inhibitor. Paclobutrazol is a proven chemical to treat and stop the spread of cankers. This growth inhibitor works by halting the above ground growth of the tree. Essentially, you will be robbing the canker of the food and nutrients it needs to develop. This procedure lasts for approximately 3 years. During this time the root systems of the tree will expand rapidly. After 3 years the above ground growth of the tree will return. The growth will be so rapid that hopefully the tree will grow over and seal the canker. As a last resort remove the tree completely if the canker has done significant damage to the trunk. Cankers are caused by a fungus which can be easily spread to other trees in the area by insects or wind.
Galls are abnormal growths on oak trees. Galls often occur on leaves twigs, branches or roots. Galls can be caused by a variety of insects. Generally, Galls do not harm a tree, however, they can be very unsightly. Symptoms of galls include irregular plant growths.
Do nothing; live with the problem.
Destroying fallen leaves
Destroying gall-infested twigs and branches
Watering and fertilizing
Despite the unattractive appearance of galls, their presence is usually not harmful to the host plant. Unless the galls appearance are causing unattractive landscaping they often do not need to be treated. To prevent galls from forming on trees its important to regularly prune branches and destroy fallen leaves and branches where insects can spawn and eventually move into the tree. Insecticides can also be used to kill the insects causing the galls.
Gypsy moths are not a native species in the United States. They were brought over in the late 1800′s for silk spinning. They have become one of the most destructive insects in the United States causing nearly $1 billion dollars in damage to trees. Gypsy moths attack the vegetation on oak trees and if they become established gypsy moths can defoliate all the trees in the area of outbreak. Symptoms of a gypsy moth infestation include egg masses on tree trunks and branches, caterpillar sightings in late spring, holes in leaves, and defoliation.
Gypsy Moth Maintenance
Burlap covering to capture caterpillars and pupae
Removing egg masses
Before taking action it is important to note how severe the infestation is. When observing the property count how many egg masses there are. If the number reaches ten or more swift action may be required. When treating an outbreak it is most common to remove egg masses and apply pesticides. Sevin SL carbaryl is a great insecticide to combat a gypsy moth infestation. Another common practice is to place a piece of burlap coated with insecticide around the trunk of an oak tree. Caterpillars climbing the tree will get caught in the burlap. Homeowners may choose to use pheromone traps to attract and capture gypsy moths.
Oak wilt is a very deadly fungal infection that almost always kills oaks. Red oaks are especially susceptibility to the disease and once infected can be killed in just a few short weeks. White oaks can have a stronger resistance to the disease, but it is still deadly. Oak wilt symptoms start at the leaf margins and progress inward. Young leaves wilt turning pale green and brown remaining attached for a period of time. It usually begins on one branch and quickly engulfs the entire tree. Oaks infected with oak wilt decline very rapidly.
Oak Wilt Maintenance
Unfortunately, options are very limited when an oak wilt infection occurs. Red Oaks have nearly a 100% mortality rate and often the best way to fight an outbreak is to remove the infected tree before the disease can spread to nearby trees either through the root systems or from insects. A preventive measure to protect trees from oak wilt is to use fungicides. If an outbreak occurs it can be beneficial to protect other trees using a fungicidal treatment. It is recommended to remove the infected tree before the disease can spread to other nearby trees on the property.
Tubakia Leaf Spot
Tubakia leaf spot is not a very deadly disease and is often just an unattractive annoyance. Symptoms of Tubakia leaf spot include Small, brown, dot-like fruiting bodies forming on the necrotic tissue of leaves. Recently, Bur oak blight has become an increasing problem. Bur oak blight is a type of Tubakia leaf spot that can cause mortality if left untreated.
Tubakia Leaf Spot Maintenance:
Remove infected leaves and dead twigs
Replace the plant
To treat Tubakia leaf spot simply prune infected (dead or dying) leaves and branches from the tree. In rare cases a fungicidal treatment or removal of the oak will be necessary. Usually, young oaks are the only trees that would require this extreme method of treatment when dealing with a leaf spot infection. Often the disease is just an unsightly nuisance to property owners. To treat bur oak blight studies have shown that injecting propiconazole fungicide in late May or early June (before symptoms appear in the summer) have reduced symptoms and can be used to manage high value trees.
Root rot is one of the most deadly diseases that can infect oak trees. Root Rot can be caused by a bacterial or fungal infection such as Rhizoctonia, Fusarium, Amillaria, or Ganoderma. Unfortunately, due to the close similarity between symptoms of bacterial and fungal root rot it’s tough to distinguish with the naked eye. One method of determining whether or not there is a fungal or bacterial infection is to take a sample of the roots and smell them. If the roots smell sour this indicates it is a bacterial infection. Symptoms of root rot include older leaves yellow and fall, margins of leaves die, roots become limp, honey colored mushroom around base of tree in the fall, die back of branches and limbs, and white fungal mat over roots. Butt rot which effects the root flare of a tree can also turn into a canker if left untreated (see canker maintenance for more information on how to treat this disease).
Root Rot Maintenance
Remove trees to prevent spreading
Growth Regulators (see canker maintenance)
Similar to oak wilt there is not much of an opportunity to treat root rot on an infected tree. When a bacterial infection occurs use a technique called air spading and remove the soil around the root flare of the tree. By removing the soil you will allow excess moisture to dry out and this might help slow or kill of the bacterial infection. There is little that can be done to stop the spread of fungal root rot. Usually, the best coarse of action is to remove the infected tree before the disease can spread to other nearby trees on the property.
Note: On mature oak trees (especially White Oaks) its important not to disturb the roots of a mature tree. Oak trees do not respond well to construction or landscaping/excavating that can destroy or damage the roots of the tree that has already become established.
Maintain Oak Trees To Keep Them Healthy
Oaks are generally very strong and resilient to fungal infections and pest infestations. The best method of treatment for all the diseases covered in this document is to use preventative measures. In most instances, trees that become ill are already in a weakened condition. Annual mulching and fertilizing can ensure the trees have the proper nutrients they need. During times of drought make sure oaks receive enough water to keep them strong.
Young Oak Trees
Pruning dead or dying branches can also help stop or slow the rate of infection of many diseases. On young oak trees structural pruning prevents the need for large pruning cuts in the future. It is recommended to prune trees during dormant winter months. For young oaks use a higher concentration nitrogen fertilizer to promote healthy plant growth. Young trees also need more watering, especially after planting (volume depends on species).
Mature Oak Trees
Young oaks and old oaks need different care then old oaks. Old oak trees need fine pruning if they were structurally pruned when young. Mature oaks trees do not go through large growth so fertilizers should be low nitrogen fertilizer.
Taking time to do proper oak tree maintenance a few times during a season will help keep trees healthy and free of disease and pests. Since oaks are so durable we tend to forget they do need some care to help keep them healthy. Monitoring trees over the course of the season helps make sure they are not suffering from and disease or pest infestations. Also, if a disease or pest infestation breaks out the earlier you can catch it the better chance you can stop it from destroying the tree or spread to other trees on the property.
Oak Tree Pruning Practices
Live Oak Tree Pruning