Tubakia leaf spot in Oaks
Overview
Tubakia leaf is a fungal disease affecting oak trees (Quercus spp). Black, red, and pin oak trees are most severely affected, but all oaks and some other trees, including maple, apple, and elm, can be affected by the fungus. Most recently several new species have been characterized in oak belong to the white oak group (See table).
Symptoms and signs
Symptoms of Tubakia leaf spot may begin to appear in mid-summer, but may not be noticeable or widespread until late summer or early fall. Symptoms consist of circular leaf spots ranging from ¼ - ½ inch in diameter and red-brown to black in color. Spots may grow into one another and form larger, irregularly shaped blotches. Spots may have dark rings or a yellow ‘halo’ around them. Spots can also appear on leaf veins. This may cause death of the vein, leading to collapse of the leaf tissue. This is the cause of defoliation associated with Tubakia leaf spot. Defoliation may become severe on stressed trees, but this occurs late in the season when trees would begin to drop leaves anyway. The disease can cause small cankers on small branches or twigs. Signs of the disease may not be easily visible to the naked eye. Fruiting bodies of the fungus are very small, dark discs that can be found inside lesions on either side of the leaf.
Disease cycle
The fungus overwinters in fallen leaves and sticks. If this debris is not removed, fungal spores will be spread in the spring by wind and rain splash. Trees are infected early in the season but will not show symptoms until mid- to late-summer or early fall.
Type of sample needed
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents can be located at the NPDN website. If you have a sample from outside of Iowa, please DO NOT submit it to the Plant & Insect Diagnostic Clinic without contacting us.
Management
Maintaining plant health: planting location, planting depth, watering practices, nutrients
Tubakia leaf spot is a cosmetic disease and does not affect the overall health of the tree since it occurs so late in the season. Trees experiencing stress such as drought or nutrient deficiency are more susceptible to the disease. Iron deficiency makes trees particularly susceptible. Newly transplanted trees are more prone to infection than older, more established trees. Mulch along with proper watering should be used to aid in the establishment of new trees. Tips for caring for newly planted trees can be found in this article. Trees should be pruned to promote air flow through the canopy. Guidelines for proper pruning can be downloaded free from https://store.extension.iastate.edu/product/6191. The fungus needs warm, wet conditions to thrive, so an open canopy with airflow is cooler and moisture dries more quickly. Trees that are severely affected and experiencing high levels of defoliation can be fertilized slightly more than normal to encourage new growth.
Scouting
Symptoms may begin appearing in mid-summer but may not be noticeable until late summer or early fall. Symptoms may appear first and be more severe on lower and inner parts of the tree since they are often more warm and moist than upper and outer parts of the tree.
Sanitation
Remove (rake) and destroy infected plant material in the fall or winter, before the fungus begins spreading spores in the spring. Leaves and sticks can be buried, burned where allowed, or composted. For more information on composting, follow this link: https://store.extension.iastate.edu/product/Composting-Yard-Waste
Chemical management
Fungicide applications are not recommended since this disease occurs late in the season and does not threaten the overall health of the tree.
| Tubakia species (columns) | ||||||
|---|---|---|---|---|---|---|
Oak (Quercus) species (rows) | T. iowensis* | T.hallii | T. macnabbii | T. tiffanyae | T. dryina | T. americana |
| Bur oak, Q. macrocarpa | x | x | x | x | x | |
| Post oak, Q. stellata | x | x | ||||
| N red oak, Q. rubra | x | x | x | x | ||
| White oak,Q. alba | x | |||||
| English oak,Q. robur+ | x | |||||
| Swamp white oak, Q. bicolor | x | x | ||||
| Shingle oak, Q. imbricaria | x | x | x | |||
| Black oak, Q. velutina | x | |||||
This table contains information found at: Farr, D.F., & Rossman, A.Y. Fungal Databases, U.S. National Fungus Collections, ARS, USDA. Retrieved July 3, 2019, from nt.ars-grin.gov/fungaldatabases/. This table is not fully comprehensive; species of oak not included in the table may be susceptible to Tubakia spp.
More information
Harrington, T.C. & McNew, D.L. Antonie van Leeuwenhoek (2018) 111: 1003. https://doi.org/10.1007/s10482-017-1001-9
More on Tubakia leaf spot on the articles from Texas, Wisconsin and Illinois
By Becca Baker (ISU Agronomy Graduate student) and Lina Rodriguez Salamanca
Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on July 8, 2019. The information contained within may not be the most current and accurate depending on when it is accessed.