If the weather is often humid, as it is in spring, this mould form it. If there are not too many infested areas, remove the leaves. Do not throw them into the garden because the spores can survive in the soil and multiply. Then spray the plants with an extract of Horsetail- Equisetum or another Equisetum species. At least in my country there are liquids of Equisetum to strengthen the plants. The mould does not like the silicon it contains. Alternatively, you can use a decoction of Tanacetum vulgare (I hope it grows in your area) or garlic. Tanacetum vulgare is also good against insects as it is poisonous for them. For Garlic, crush four cloves of garlic and pour over a litre of boiling water. Leave to cool and spray over. Repeat if necessary.
In the past, copper solution was used. In many European countries this is forbidden because it contaminates the soil.