Alder trees can be host to the alder gall mite which produces these wart-like structures on the leaves. The galls are spherical and little more than 2 mm in diameter. Each wart has a narrow opening on the underside of the leaf through which the mite will leave when fully grown in the autumn. The mite spends the winter in an alder cone or in the bark of the tree and then, when the leaves appear in spring, promptly lay their eggs on a leaf, the warts appear and provide a home for the next generation of mites.
The galls are green at first but become red with age. They can affect an entire leaf stopping its development but not every leaf on the tree will be infected thus leaving the tree undamaged and so able to host the mite over the winter and support new mites the following year. It is all in balance.
It is not uncommon and if you look at alders in leaf there is pretty good chance you will find it eventually.
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