Tubakia leaf spot on oak
Tubakia leaf spot is a late season leaf disease on bur oaks. In most years across southern Minnesota a late season leaf disease can be seen on individual bur oaks, usually after August 1st. The fungus Actinopelte dryina has had a recent name change to Tubakia dryina. This is the cause of the disease. This season the amount and severity of affected oaks varied greatly. The range was from light to very evident disease development. The 2003 southeast Minnesota drought complicated bur oak foliage, as leaf scorch was apparent in early September in many very dry areas. In some years the appearance of the leaf spot can be very dramatic as the entire tree turns brown except a few leaves in the very upper crown. The defoliation can reach 90% in a few short weeks. Affected oaks refoliate the following year.
Symptoms: During late July and August bur oaks scattered throughout southern and southeastern Minnesota exhibit the following symptoms.
- Leaves with multiple leaf spots that eventually turn brown.
- Brown or curled leaves most commonly in the lower crown. The infection progresses from the lower crown into the upper crown.
- Whole tree crowns can become brown with curled leaves.
There may be other factors involved including other insects and diseases as well as environmental factors. Trees affected by loss of foliage generally do not refoliate in the late season. This on average is good, as stored food resources are not used and available for the next spring. Additional stress from a multitude of factors, like drought or insect defoliation, may contribute to eventual dieback or decline.