Issue 16, September 16, 2015
Bur Oak Blight
Bur Oak Blight (BOB) is a fungal leaf disease caused by the pathogen Tubakia iownesis. The U of I Plant Clinic has received several BOB samples over that past month. This is a relatively newly described pathogen, identified by Dr. Tom Harrington of Iowa State University. Several species of Tubakia are known to infect oak and cause minor leaf spots. Tubakia iowensis is the only species known to attack leaf petioles. Additionally, the pathogen will only infect bur oak, particularly Quercus macrocarpa var. oliviformis. Results from his research suggest that the pathogen is native to the region and has only recently become problematic due to a significant environmental change. Moist springs, with significantly more precipitation, have likely created an environment favorable for disease development. His research lab has also been keeping track of counties and states with trees confirmed to have BOB. As of May 2015, nine counties in Illinois have had confirmed BOB cases: Jo Daviess, Carroll, Stephenson, Ogle, DeKalb, Lake, Cook, and Hancock. A map with the current disease distribution can be found at the following link: http://www.public.iastate.edu/~tcharrin/BOB.html.
Symptoms
Bur oak blight is a late season leaf blight. The earliest symptoms of Bur Oak Blight first appear in June as purple-brown spots on the underside leaf veins. In July, the spots expand, and purplish necrotic veins become noticeable on the upper leaf surface (Image 1). The most notable symptoms appear in August and September as leaf veins are killed as the infection progresses and a characteristic wedge shaped necrotic area develops on the leaf blade (Image 2). Coalescing lesions and expanding vein necrosis may cause the leaf to die. Severely affected trees may have significant leaf mortality and/or leaves with a scorched appearance (Image 3). Extensive premature defoliation is common, but many diseased leaves remain attached to the tree into the winter, well after healthy bur oaks have dropped their leaves.
Image 1. Purple-brown necrotic veins on the upper leaf surface associated with Bur Oak Blight
Image 2. Wedge-shape lesion characteristic of Bur Oak Blight
Image 3. Bur oak with lower canopy affected by Bur Oak Blight
The retention of diseased leaves favors the pathogen and its spread. This pathogen overwinters as pustules on the still attached diseased leaf petioles. In late April and May, fungal spores are produced and released from the pustules about the same time as new leaves are developing and expanding. Heavy rainfall promotes spore production and disseminates this primary inoculum by splashing the spores to newly expanding leaves. 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.
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 (Image 4). The other species of Tubakia that infect bur oak are not known to produce these overwintering pustules.
Image 4. Branch with diseased petiole 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. Death has also been attributed to secondary invaders such as the two-lined chestnut borer. Trees infected with BOB appear to have increased susceptibility to these secondary invaders.
Management Strategies
- Fortunately, not all bur oaks will be affected by this disease. Many bur oak trees have a resistance to this pathogen. It's not uncommon to observe a healthy unaffected bur oak growing in close proximity to a severely infected tree.
- 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 other common disease such as: oak wilt, oak anthracnose, and environmental stresses. Disease confirmation is important for providing accurate disease control strategies and recommendations. Suspect BOB samples can be submitted to the University of Illinois Plant Clinic. Information on general sample submission can be found on the plant clinic website (http://web.extension.illinois.edu/plantclinic/). 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 Image 4 for example)
- Raking diseased leaves will 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 ml 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 recurs.
References and Further Reading
US Forest Service Pest Alert: Bur Oak Blight (http://na.fs.fed.us/pubs/palerts/bur_oak_blight/bob_print.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.
(Travis Cleveland)
Author:
Travis Cleveland