Spores (or conidia) cause the damage. The spores will germinate on the leaves and draw the sap from the leaf cells, which is why true mildew develops in the leaf cells. These leaf cells form the food source for the development of the fungi. The hyphae that develop during the germination process enable the spread of the spores. This way, the life cycle can continue for a long time. True mildew can only develop on living material.
