Podosphaera leucotricha (Ellis & Everh.) E. S. Salmon
Lesions begin as small, circular, tan to orange blemishes approximately 3–5 mm in diameter. The discoloration is due to discoloring of the fuzz on the fruit (A). As lesions enlarge, the interior, reddened fuzz is shed. At this point, lesions appear green and smooth, surrounded by a tan to orange halo. Lesions may continue to expand and may coalesce with other lesions to form larger smooth patches of infection. Eventually, lesions stop expanding, harden, and turn orange-brown.
Widespread; the disease occurs wherever powdery mildew of apple occurs.
Rusty spot can be confused with powdery mildew infection caused by Sphaerotheca pannosa (Wallr.:Fr.) Lév.or Podosphaera clandestine (Wallr.:Fr.) Lév.
Fungicide programs targeted for control of brown rot or peach scab will often provide appreciable control of rusty spot. Programs geared specifically for powdery mildew control will also be effective. Rusty spot is caused by the same fungus that causes powdery mildew on apple. Therefore, controlling powdery mildew infections on neighboring apple trees will reduce the incidence of rusty spot.
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