What’s killing my oak tree?

During the summer, we received a large number of oak samples in the diagnostic clinic. While the symptoms and associated problems ranged from minor leaf spots to more serious oak diseases, accurate diagnosis of three of these problems can save nearby trees.

Oak Wilt
Oak wilt is a fungal infection (Ceratocystis fagacearum) affecting most oak species. In Missouri, oaks in the red oak group (northern red oak, pin oak, black oak, shingle oak etc.) are most susceptible. Often, the first indication occurs when leaves turn brown or yellow from the tips and outer edges inward. Leaves will drop starting in the upper canopy as the disease advances. When the bark is scraped away from affected branches, dark streaks will sometimes be present in the sapwood. Oak wilt is spread by beetles that are attracted to infected trees and wounds on healthy trees in the spring. Spread of the disease can also occur rapidly to healthy trees through the roots when infected trees are found nearby. There is very little that can be done to save an infected tree. Infected trees in the red oak group may die within a month or less after symptom expression, whereas oaks in the white oak group may survive several years. Rapid identification of the problem, removal of infected trees and protective measures for nearby trees are important control measures.

Bacterial Leaf Scorch
Bacterial leaf scorch is a problem in Missouri and other parts of the Midwest. Bacterial leaf scorch (Xylella fastidiosa) begins with leaves that turn brown at the margins, usually late in the season. In contrast with oak wilt the oldest leaves on a shoot are often the first to develop symptoms, and leaves will be held on the tree for some time after scorching occurs. Affected oaks will often survive for several years, producing leaves with a normal appearance each spring. However, laboratory tests are needed for positive diagnosis. Submit fresh, symptomatic leaf tissue in late summer or early fall before fall leaf drop to a plant diagnostic lab. Xylem feeding insects such as leafhoppers, treehoppers, and spittlebugs can spread the infection to healthy trees, however controlling these insects has not been shown to reduce infections. Fertilization, irrigation and tree injections may prolong the life of diseased trees.

Oak Decline
Oak decline is a complex condition caused by many factors, including predisposing conditions such as old age, poor fertility, droughts, floods, or root disturbances. Then a secondary agent such as an insect or disease has the opportunity to attack. Here in Missouri, secondary agents commonly include red oak borer, carpenter worm, two-lined chestnut borer, Armillaria root disease and hypoxylon canker. Symptoms vary depending on secondary agent as a contributor. Leaves will turn brown and may or may not fall. Early fall coloration of leaves, late leafing in spring, production of foliage in clumps, and branch dieback are common symptoms. Trees with decline may die within a few years or even survive with only dead branches. Routine maintenance can promote vigor and allow a tree to survive for many years.

Submitting Samples to a Diagnostic Clinic

• Select symptomatic limbs from trees.
• Dead limbs and those in an advanced state of decline cannot be used for isolation.
• Samples should be 0.5 to 2 inches in diameter and 6 inches long for oak wilt testing.
• Send in leaves from sampled limbs, especially for bacterial leaf scorch testing.
• Keep samples cool during sampling and shipping, but never freeze tissue.
• Ship samples in sealed plastic bags. Do not add water or wet materials to the sample.
• A dry paper towel wrapped around leaves can help protect the sample in shipping.
• Submission forms are available at your county extension office or on the web at http://soilplantlab.missouri.edu/plant/index.htm
• Samples can be sent to: Plant Diagnostic Clinic; 23 Mumford Hall; University of Missouri; Columbia MO 65201
• General diagnosis is $15.00, additional $10 fee for fungal isolation such as oak wilt culture, additional private lab fees for bacterial leaf scorch testing, however all testing frequently not necessary on each sample.
• Ship quickly or bring in person to the laboratory. The samples should be shipped on a Monday, Tuesday or Wednesday to avoid a weekend in shipping.
• Results from the isolations will be available in 2 to 3 weeks. Reply time depends on how fast the fungus develops in culture.
• Questions: Simeon Wright, 573-882-3019

Additional information on other oak problems we often see in the diagnostic clinic can be found at http://soilplantlab.missouri.edu/plant/diseases/oak.htm