Oak Leaf Blister


For the first time, I have found Oak Leaf Blister on this spring’s leaves of my ~25 year old Red Oak. The tree is healthy, located in central Etobicoke, in the back garden. There is a large Norway Maple adjacent that has been plagued by tar spot over the last few years.

Is there any treatment or action I can take now? Research on the web suggests only removal of fall leaves and spraying a fungicide in the spring before leaf out. Are you hearing of other Red Oaks affected this year in Toronto? It has not been a wet spring!

Thank you


Thank you for contacting the Toronto master Gardener concerning your Red Oak Tree.

Yours is the first question concerning oak leaf blister ( oak leaf curl). Currently, the majority of the questions is this year’s boom in gypsy moth.

Oak leaf blister is a fungal disease caused by the fungus Taphrina caerulescens. This fungus causes bulges and blisters on the leaf’s surface. You are correct, in that this fungus is more prevalent when there is a cool wet spring. Fungal spores are spread by wind and splashing raindrops onto bud scales and twigs. The spores then remain dormant until the following spring where rain washes the spores onto the young developing leaves. For a complete description of the life cycle of this fungus refer to Missouri Botanical garden’s article on Oak Leaf Blister.

Leaves with numerous spots may fall prematurely to the ground. If this occurs early in the season the tree may leaf out later in the season. As with most diseases, maintaining tree vigor is the key to help reduce the impact of this disease. This means  keeping the trees well watered during severe drought and adequately fertilized. Also clearing up the fallen leaves to prevent further spread of the disease and pruning out dead or dying branches will help reduce the impact of this disease. Remember to sterilize your tools after pruning any diseased branches and do not out your diseased leaves in your compost bin.

According to Natural Resources of Canada fact sheet on Oak Leaf Blister  “Spraying a biological fungicide before the tree leafs out in the spring generally eradicates the disease. For information on the products registered for controlling specific insects or diseases, please contact the Pest Management Information Service of the Pest Management Regulatory Agency (PMRA), at the following toll-free number: 1-800-267-6315.

You may wish to contact a certifies arborist to spray the tree. To find a certified professional arborist in your area to help you visit the Ontario branch of the International Society of Arboriculture here.

May 26, 2021