Rusty Spot
Podosphaera leucotricha  
Rusty spot on peach fruit.


I. Introduction: Rusty spot of peach is characterized by the presence of rust-colored spots that can cover the entire surface of the fruit. 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.

II. Symptoms: Symptoms of rusty spot are recognized only on the fruit. On young, green fruit, the earliest symptoms appear as small, orange-tan spots approximately 1/8 to 1/16 inch (3-5 mm) in diameter (photo 2-59). This rust color is superficial and is due to 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 to shed, leaving a fuzzless, smooth, bald center 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.

III. Disease Cycle: Since the cause of rusty spot is not clear, no disease cycle is known. There is evidence that implicates the apple powdery mildew fungus as the rusty spot pathogen, since infection on peach is more severe in blocks located adjacent to mildew-susceptible apple cultivars. 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.

IV. Monitoring: Monitor as described for peach and nectarine powdery mildew.

V. Management: The incidence of rusty spot is reduced with wettable sulfur applied beginning at shuck split and continuing at 8- to 10-day intervals.  Controlling mildew in adjacent apple orchards reduces rusty spot in peach orchards. 

Chemical control - commercial growers

Chemical control - home orchardists (pdf file - Acrobat Reader required)

Text prepared by J.W. Travis, J.L. Rytter, and K.S. Yoder

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