The cause of rusty spot is uncertain, although many plant pathologists believe it to be the apple powdery mildew fungus, Podosphaera leucotricha. Many observations have shown that peach orchards with rusty spot are usually next to apple orchards that are infected with powdery mildew.
Rusty spot is recognized only on the fruit. The earliest symptoms on young green fruit appear as small, orange-tan spots. This symptom is due to a change in the color of the fuzz or hairs on the fruit. These first spots may become noticeable three to four weeks after shuck fall. The discolored area enlarges slowly, and the older discolored hairs begin disappearing, leaving a fuzzless, smooth, center spot surrounded by a non-uniform band of orange to tan hairs. Finally, the spots become quite spread out leaving brownish or reddish centers of hard, smooth skin that appear somewhat like a bruise from a limb rub.
Since the cause of rusty spot is not clear, the disease cycle is unknown. Although most infection seems to occur from petal fall to one month after shuck fall, some new spots on the fruit may continue to appear up to the time of harvest. There does not seem to be any relationship between weather conditions and the number of diseased fruit.
Choose varieties that are not susceptible to rusty spot. Spotting on the fruit of some varieties appears to be due to the powdery mildew fungus. Rio-Oso-Gem, Jerseyqueen, Jefferson, Washington, Redskin, and Loring often are affected. Sulfur, Benlate, Topsin-M, or Funginex used at full labeled rates may reduce rusty spot incidence.