Sample of the week (November 19, 2018): Maple Tar Spot

Tar spot symptom on silver maple leaf

Tar spot symptom on silver maple leaf

Tar spot is caused by several fungal species in the genus Rhytisma. Tar spot is generally a cosmetic disease and does not cause any health problem on trees. Early infection on leaves appear as yellowish spots approximately 1/8 inch in diameter and may be inconspicuous in early summer. As the disease progresses in mid to late summer, these spots enlarge, turn dark in color and become raised forming tar-like structure on the leaves. The disease may cause premature defoliation, however this does not have any significant effect on the tree health. Tar spot pathogens overwinter on plant materials and are spread by wind on to new foliage as leaves begin to emerge in the spring.

Tar spot can be controlled by raking and removing plant debris in the fall. This will help to destroy overwintering inoculum that can infect new leaves in the following spring. Fungicides labelled for tar spot control is rarely recommended since the disease does not cause any severe damage to the tree.

For appropriate diagnosis, the MU Plant Diagnostic Clinic can help you confirm if your plant has this disease. Please visit our website http://plantclinic.missouri.edu/ and follow the instructions for collecting, packaging and shipping samples to the clinic.