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Powdery mildew on a rose cutting (Rosa sp.). Powdery mildew is a fungal disease that affects the leaves and stems of a plant. The infected plant first develops white powder-like spots, which increase in size and become thicker as many spores form. The disease then spreads throughout the length of the plant. (KEYSTONE/SCIENCE PHOTO LIBRARY/DAVID ROACH/SCIENCE PHOTO LIBRARY)