Willow Oak disease - Ask Extension
Hello,
I have a couple willow oaks exhibiting poor leaf growth. There is a pustule like growth on almost all of the branches. Any idea what this is ...
Knowledgebase
Willow Oak disease #641470
Asked May 26, 2020, 7:54 PM EDT
Hello,
I have a couple willow oaks exhibiting poor leaf growth. There is a pustule like growth on almost all of the branches. Any idea what this is and how to treat?
Thanks,
Fred Sheckells
Harford County Maryland
Expert Response
Hello Fred,
The bumps on the oak's twigs are scale insects (in this case, a type of Lecanium scale). They feed on plant sap and in high numbers can stress a tree and lead to stunted growth.
If this is a mature tree or one large enough that you cannot reach most branches, an arborist should be consulted regarding treatment options. They will have spray equipment that can reach higher branches, or will use injectable (or trunk-absorbed) pesticides that homeowners cannot obtain.
If this is a young tree where you can reach the branches, spraying with horticultural oil (in summer) and dormant oil (in winter only) can help reduce the population. Crawlers, their most vulnerable "hatchling" stage, would be the targets of a summer spray. For Lecanium scales, this period starts around mid-June, and the crawlers will have moved out onto the leaves to feed. While the window for crawler treatment extends through much of the summer, earlier treatment will avoid harming beneficial scale predators/parasites that will arrive to help control their numbers. In winter, the slightly heavier concentration of oil (often called "dormant oil") can be used without risking plant damage to smother overwintering scale.
https://extension.umd.edu/hgic/topics/soft-scales-trees-and-shrubs
Miri
The bumps on the oak's twigs are scale insects (in this case, a type of Lecanium scale). They feed on plant sap and in high numbers can stress a tree and lead to stunted growth.
If this is a mature tree or one large enough that you cannot reach most branches, an arborist should be consulted regarding treatment options. They will have spray equipment that can reach higher branches, or will use injectable (or trunk-absorbed) pesticides that homeowners cannot obtain.
If this is a young tree where you can reach the branches, spraying with horticultural oil (in summer) and dormant oil (in winter only) can help reduce the population. Crawlers, their most vulnerable "hatchling" stage, would be the targets of a summer spray. For Lecanium scales, this period starts around mid-June, and the crawlers will have moved out onto the leaves to feed. While the window for crawler treatment extends through much of the summer, earlier treatment will avoid harming beneficial scale predators/parasites that will arrive to help control their numbers. In winter, the slightly heavier concentration of oil (often called "dormant oil") can be used without risking plant damage to smother overwintering scale.
https://extension.umd.edu/hgic/topics/soft-scales-trees-and-shrubs
Miri
