One stem of my french lilac has bloomed white, while the rest are traditional french lilac purple with white edge. How can this be?

Question

The tree is about 10 years old and has always bloomed with the traditional french lilac colours.

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Answer 1

Your Lilac is a hybrid and Lilacs tend to grow suckers from their base. One of these suckers likely reverted to the the root stock as a white flower.
You can remove this keep the shrub from producing more white flowers.