Issue 13, August 3, 2015

Tubakia Leaf Spot

Tubakia leaf spot has been observed on many oak samples this year. 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 often confused with those of anthracnose. As a rule of thumb, oak anthracnose symptoms usually appear earlier in the growing season (May-June), while tubakia leaf spot occurs as a later season disease with symptoms first appearing in July and August.

Tubakia leaf spot lesions will vary with host susceptibility and environmental conditions. The lesions start as small water soaked areas. They become evident as they enlarge and transition to a reddish brown color (Photo 1). Severe infections can 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 called a pycnothyrium which is composed of mycelia and spores (Photo 2).

Northern red oak leaf with numerous Tubakia Leaf Spot lesions.

Tubakia pycnothyrium (Disk-like fruiting structures)

Moist, rainy weather and moderate temperatures promote infections and allow the spread of this fungus. The disease is much less common during years with predominately dry weather.

Tubakia leaf spot usually 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. Rake and remove fallen leaves to reduce disease occurrence during the following growing season, by reducing innoculum in the surrounding area. Promoting tree vigor and alleviating any potential stresses to the tree is also recommended. (Travis Cleveland)

Travis Cleveland

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