Recently, oak trees throughout North Carolina are falling victim to a disease, evident by browning leaves and premature leaf drop. When encountering a situation like this, forest health professionals handle it like a detective case, examining the available evidence, ruling out suspects, and utilizing laboratories for confirmation. In this case, while there are several diseases that might exhibit similar symptoms (e.g., Cercospora and tar spot), the prevalence on oak trees narrows the search. Another hint is the time of year that it is occurring: late summer and early fall.
One of the first oak leaf diseases suspected in this type of situation is anthracnose. In addition to oaks, anthracnose diseases affect a variety of other species such as dogwood and sycamore. However, the symptoms of anthracnose are typically seen in the early part of the growing season, ruling this one as not likely.
Another possible culprit is bacterial leaf scorch, which does occur in the late growing season. This is a serious bacterial disease which may result in tree death over time. However, this disease most often affects all the leaves on one portion of the crown whereas the symptoms seen in this case are scattered throughout the crown, with some unaffected leaves remaining on the same branches with affected leaves. Again, while possible, the evidence does not point this way.
A pine oak (left) and post oak (right) suspected to have Tubakia leaf spot disease.
This leads us to Tubakia leaf spot. The symptoms of this disease normally arise in late summer as scattered leaf spots, often with a yellowish area surrounding them (described as a ‘halo’). However, these spots may conjoin to form large brown areas of dead leaf tissue (which is what is observed in several of these cases). Sometimes these brown areas may be wedge-shaped and the edge delineated by one of the veins in the leaf. In addition, they typically affect leaves throughout the crown but not necessarily all the leaves on any single branch. In more severe cases, the tree may lose many of its leaves early leading to concern from homeowners.
A shield-shaped spore-producing structure can confirm the identity of Tubakia leaf spot. Image: Bruce Watt, Univ. of Maine, Bugwood.org.
This description seems to fit our case but the only way to be sure is by finding something to confirm the presence of the organism causing the disease. In the case of Tubakia leaf spot, the spore-producing structure is identifiable and can sometimes be seen with the aid of magnification. It is a shield-shaped structure found in the middle of the leaf spots and is easily confirmed in a laboratory. Bingo! In our case, we have a winner! This is good news — Tubakia leaf spot is primarily a cosmetic disease that causes premature leaf drop and is often easily controlled. Because this disease spreads by windblown and rain-splashed spores from the fallen leaf material, prompt clean up and disposal of these leaves can decrease disease occurrence next year. Burying, burning, or composting the leaf material has proven effective in reducing its spread.
In this instance, the culprit was relatively harmless, however all are not. Don’t hesitate to call your N.C. Forest Service County Ranger for assistance in diagnosing problems with your trees!
Sources
Bush, Elizabeth A. (2010) Bacterial Leaf Scorch of Landscape Trees. Virginia Cooperative Extension Publication 3001-1433.
Olson, Jennifer. Anthracnose and Other Common Leaf Diseases of Deciduous Shade Trees. Oklahoma State University Cooperative Extension Service Publication EPP-7634.
Proffer, T.J. (1990). Tubakia Leaf Spot. Florida Department of Agriculture and Consumer Services, Division of Plant Industry, Plant Pathology Circular No. 337.