What the Heck is That?

Many Linden trees in the Touraine are currently sporting weird red growths on their leaves, like tiny fingers pointing upwards. 

 

Underside of a leaf.

They are called Nail Galls (Fr. Galles cornues) and they are caused by microscopic mites in the Eriophyes genus. There are not just a few galls on affected trees, but several dozen on the upper surface of each affected leaf. The galls can be up to 15 mm long and range in color from pale green and yellow to red. On the underneath of the leaf you can spot a tiny hole, and inside the gall's hairy centre there will be an invisible worm like mite measuring 0.2 mm long.

 

Upper side of a leaf.

The galls start appearing at the end of spring. The mites are present all over Europe, but particularly in France and Great Britain. Each species of Eriophyes is particular to one, or occasionally two, species of Linden. The galls pictured are on a neighbour's Common Linden Tilia x europeus (Fr. Tilleul commun), so they are probably caused by Eriophyes tiliae. But it's impossible to say for sure without expert knowledge of the group and microscopic examination. Common Linden is a hybrid, so it can be affected by several species of mite, which also affect the parent Linden species.

If your tree is affected, don't worry. There are biological controls (other predatory mites) but they are not very practical to apply, and normally it is not worth employing them. The Eriophyes mites and their activities do not hinder the growth of the tree or harm it in any way. They just make it look a bit weird for a while.