For the third summer in a row, the leaves of the city’s horse chestnut trees are covered in brown spots and turning brown. In another few weeks they’ll shrivel up, die, and fall prematurely. According to one source, this is caused by the larvae of a leafminer moth. Others say it’s the heatwave or even salt (although there’s not enough snow in Paris to need lots of salt). Supposedly the trees won’t die, but it sure looks butt ugly.
Update July 2021: the confirmed leafminer attack on horse chestnut trees (marronniers) continues throughout the city of Paris as well as the forests surrounding the Ile-de-France. Entire parks, like the Domaine de Versailles and the Parc de St-Cloud (pictured below), show the ravages by mid-July. There is no way to cure them all, so the trees are slowly replaced when possible in smaller parks and gardens in the city, but for the larger forests where this isn’t feasible, we will likely continue to see the leaves of the horse chestnuts do their annual early browning-shriveling-dying cycle indefinitely.