Issue 14, August 13, 2012
Bur Oak Blight
The U of I Plant Cinic confirmed its first sample of Bur Oak Blight (BOB). The infected tree is located in Lake County, IL. Bur Oak Blight is a fungal disease caused by a newly described pathogen, Tubakia iowensis. Several species of Tubakia are known to infect Bur oak (Quercus macrocarpa), however, T. iowensis is the only species known to cause severe leaf blight. The variety Quercus macrocarpa has been reported to be especially susceptible to the disease.
Bur Oak Blight is a fungal leaf disease with the earliest symptoms appearing in June. The pathogen over-winters as pustules on diseased leaf petioles that remained attached from the previous growing season. In the spring, fungal spores are produced and released from the pustules coinciding with bur oak leaf development and expansion. Heavy rainfall has been reported to be an important factor with spore production and dissemination. The primary infection occurs before the leaves are fully developed. However, there is a latent period between infection and when the first symptoms appear in June.
Initially, infections may be limited to the lower braches. Symptoms intensify from year to year and progress from the lower branches to the entire crown. In June, the first symptoms appear as purple-brown spots on the underside leaf veins. In July, the lesions expand, and purplish necrotic veins become noticeable on the upper leaf surface as well. As the veins are killed, wedge shaped necrotic areas appear on the leaf blade. Coalescing lesions may cause the leaves to die. Severely infected trees may have significant leaf mortality or leaves with scorched appearance. Many, but not all, leaves killed by BOB remain attached to the tree into the winter, well after healthy bur oaks have dropped their leaves.
Purple-brown necrotic veins on the upper leaf surface associated with Bur Oak Blight.
Wedge-shape lesion symptomatic of Bur Oak Blight.
Branch with Bur Oak Blight infected leaves.
Diseased leaf retention is currently one of the best ways to identify this disease in the field. Look for leaves and petioles attached from the previous growing season. Infected petioles will have black pustules or scars from previously attached pustules. The other species of Tubakia, that infect bur oak, are not known to infect petioles or cause petiole blight.
Branch with diseased petioles still attached from previous growing season. Note the small black pustules.
Several successive years of severe infection and defoliation have been reported to kill trees. Some of the reported trees died as a result of secondary invaders such as the two-lined chestnut borer. Trees infected with BOB appear to have increased susceptibility to these secondary invaders.
- If you suspect a BOB infection, have the disease diagnosis confirmed by a laboratory. The leaf blight and scorching symptoms of BOB can be confused with oak wilt and oak anthracnose symptoms. Disease confirmation is important for providing disease control strategies recommendations. Disease samples can be submitted to the University of Illinois Plant Clinic. Information on general sample submission can be found on the plant clinic web site. A $15 fee will cover diagnostic services for the sample.
For Bur Oak Blight sampling, collect branches and twigs with symptomatic and healthy leaves. Be sure to include branches with petioles from previous growing season still attached (See fourth photo, for example)
- Raking diseased leaves may have little effect on controlling the disease. The primary infection occurs from the abundant spores produced from diseased petioles that remain attached to the tree.
- Boosting tree vigor may help the tree to limit and prevent secondary invaders. Pruning and removing branch dieback has been suggested to help reduce borer populations
- For high value trees, Iowa State University found trunk injections of propiconazole to be effective at controlling the disease. Applications require specialized equipment and will need to be made by a certified professional.
- Injections should be made in late May or early June just after the leaves have fully expanded
- The recommended application rate is 8-10 mls per 1" DBH. Higher applications rates reportedly resulted in phytotoxicity to leaves. The rate will also need to be adjusted if the tree has significant branch dieback in the canopy.
- One application should last several years. Iowa State currently recommends repeat application only after a severe outbreak re-occurs.
References and Further Reading
US Forest Service Pest Alert: Bur Oak Blight (Adobe PDF)
Published Research Article--Harrington T, McNew D, Hye Young Y. Bur oak blight, a new disease on Quercus macrocarpa caused by Tubakia iowensis sp. nov. Mycologia January 2012;104(1):79-92.