うどんこ病
Powdery mildew is a common fungal infection of apples and can occur in almost any apple growing climate. The spores from fungi preserved over the winter are released from the unfolding leaves of the buds. The spores, carried by the wind, infect leaves, blossoms, and fruit. The fungus spreads until it covers the whole leaf and then grows down twigs, covering them with a gray felt. This results in aborted blossoms, reduced finish quality, and reduced yield. The USDA has estimated that 40% of apple orchards in the East and 50% of apple orchards in the West are infected with powdery mildew. Without control of the fungus yields would drop by 65%.