Welcome Aquitaine,
Hope i'm not doing late! Please don't cut your plants so early, looks like they still need 2-3 weeks more!
The white marks in the leaves are powder mildew. Try to clean the area before start growing next year, check the other plants and trees nearby, maybe they are also being affected by the fungus. Apply sulfur powder over the affected plants and trees, and also over your ganja plants in the very first days of flowering (first 1-2 weeks or so, when the plants have only a few pistils). This will protect for 1-1.5 months of flowering, usually enough for short flowering indicas. But it's more difficult with longer flowering sativas , sometimes the sulfur doesn't last enough to protect the whole flowering and later, the sulfur cannot be applied in mid flowering.
Unless you want to go with chemical systemic insecticides, cleanness, this and other organic remedies (like equisetum arvense extract) are the only way to prevent (but not to fully avoid) powder mildew nowadays, it's becoming a serious problem in many places.