Leaf warts?

The leaves on the lower branches of my beautiful Tilleul have begun to sprout multiple growths,- the only way I can describe them is pyramid shaped warts! They grow out on the upper side of the leaf. The tree appears to be healthy otherwise, and the leaves remain green, although these 'things' are yellowish. Is this a problem or is it normal?
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coykiesaol


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wrote:

Linden Eriophyid gall mites.
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