Thanks for the question.

There is no worry; there is nothing you should do; your basswood is quite healthy. What you are observing on the leaves are called galls. They have small mite larvae within them. Here’s the deal. Last year (2023) eriophyid mites (hard to identify the exact species) were present on your basswood tree where they overwintered under pieces of bark or some other nook or cranny of the tree. This spring (2024) all these mites emerged from their hiding places and mated with one another. Eggs were laid on the emerging leaves of your tree and larvae quickly hatched. These larvae burrowed into the leaf tissue. The leaves responded by enclosing the larvae with a form of leaf cancerous growth. A type of defensive response. This led to the appearance of the bumps and nodules which you are seeing. The tree is not harmed in this process. Soon from each gall will emerge an adult mite. They are incredibly small so don’t expect to see them. These adults will spend the rest of 2024 and through the 2024-25 winter living on your tree. In spring 2025 the cycle begins all over again. See:

https://hortsense.cahnrs.wsu.edu/fact-sheet/lindentilia-leaf-galls/

https://extension.umn.edu/yard-and-garden-insects/insect-and-mite-galls

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