Issue 15, September 7, 2016
Tubakia Leaf Spot
Tubakia leaf spot was briefly mentioned in the last issue. We continue to receive samples infected with this disease. The disease is caused by the fungal pathogen, Tubakia dryina. All oak species are susceptible to this disease, but those within the red oak group are more commonly affected. This leaf spot is often associated with stressed trees, especially pin oaks with symptoms of iron chlorosis. Other potential hosts include: maple, hickory, chestnut, redbud, ash, black tupelo, sourwood, sassafras and elm.
The symptoms of this disease appear similar to and are often confused with those of anthracnose. As a rule of thumb, oak anthracnose symptoms usually appear in late spring to early summer (May-June), while tubakia leaf spot occurs in late summer with symptoms appearing in July and August. Tubakia leaf spot lesions vary with host susceptibility and environmental conditions. The lesions start as small water soak areas. They become evident as they enlarge and transition to a reddish brown color (Photo 1). Severe infections may cause premature leaf drop, a symptom which can be alarming to those scouting for oak wilt. The Tubakia pathogen is fairly easy to confirm in a diagnostic laboratory with the aid of a microscope. It produces a distinctive disc-shaped fruiting body which is composed of mycelia and spores called a pycnothyrium (Photo 2). Symptoms tend to be most severe on the lower branches where moisture accumulates and remains for longer periods of time (Photo 3).
Photo 1. Northern red oak leaf with numerous Tubakia Leaf Spot lesions
Photo 2. Tubakia pycnothyrium (Disk-like fruiting structures)
Photo 3. Lower branches of an oak infected with Tubakia leaf spot (Urbana, IL. Sept. 2016)
Tubakia leaf spot is more prevelant in years with abundant rainy weather and moderate temperatures. These conditions promote infections and allow the spread of this fungus. The disease is much less common during years with predominately dry weather.
The disease symptoms may appear alarming, However, the disease develops late enough in the season that there are no long-term adverse effects on tree health. As a result, treatment with fungicides is not usually recommended. Raking and removing fallen leaves may reduce innoculum in the surrounding area, thus limiting disease occurance the following growing season. Promoting tree vigor and alleviating any potential stresses to the tree is also reccomened. (Travis Cleveland)