I live in Toronto near the lake. During the past month, there have been a lot of leaves dropping from my red oak due to oak gall. Is there anything that can be done to help my tree along in subsequent years? The tree is a huge heritage tree that is over 200 years old. I’m worried the leaf drop is harming the health of the tree. Is it time to call in an arborist? I’m attaching a photo of some leaves with oak gall. Thanks!
Thank you for contacting Toronto Master Gardeners about your heritage Oak tree. The most common oak trees in Toronto are White Oak (Quercus alba), Northern Red Oak (Quercus rubra) and Black Oak (Quercus velutina). The leaf in your photograph appears to be from the Northern Red Oak, which also produces acorns with saucer-shaped cups. There are many different types of galls that can affect an oak tree: twig and stem galls, leaf galls, oak apple galls, roly poly galls, wool sower galls, vein pocket gall and leaf pocket gall and jumping oak gall.
Your photograph shows the galls attached to the twig and leaf petiole. There are some galls which occur specifically on Northern Red Oak.
1. Leaf blister galls are caused by mites, midges and wasps which attack the leaf tissue. The galls cause small yellow or red blisters. If the galls are confined to leaves, then it is more of a cosmetic, rather than health problem. Given the size of the tree, an arborist could check upper limbs for larger, more serious galls.
2. Large oak apple gall is a large, globular swelling on red oaks which appear on the vein or petiole of the leaf. The globe can measure up to 25 – 35 mm. The oak apple gall wasp, Biorhiza pallida, is a tiny wasp that causes growths, or ‘galls’, on oak twigs. Fortunately this infestation rarely affects the health of the tree. Photo below:
3. Gouty oak gall is caused by the cynipid wasp and can be an aggressive problem. This galls measure 50mm in diameter, grow together and can encircle entire branches. If there are galls on leaves and branches, then the health of the tree may be endangered. Consulting a professional arborist would be the next step. Possibly affected branches can be cut out or permitted insecticidal sprays applied to the affected areas. Removing and destroying all leaves from the base of the tree throughout the season will remove some of the host from your area. Photo below:
For comparison, there is the Oak Petiole Gall that affects species of white oak with photograph below:
Since your oak tree is a large heritage specimen, it would give you peace of mind to consult a certified arborist to diagnose the extent of the oak gall. Landscape Ontario (landscapeontario.com/) is a good source to find reputable landscape trades.
Your oak is a precious, living piece of Toronto history and hopefully it will grace your garden for many more years.
June 28, 2023