Rusty spot is a disease of peach fruit only. It is caused by the powdery mildew fungus that attacks apple, Podosphaera leucotricha. Peaches growing near apple trees are most susceptible. The disease gets its name because the infected area looks like rusty water residue. The small, circular, orange-rusty lesions develop on the fruit that enlarge and may cover the entire fruit surface. No symptoms occur on leaves and stems.
Infections begin in late spring, during warm days, cool nights, and some form of moisture. The spores are spread by wind, dripping dew, rain, and irrigation.
Rusty spot infections (from apple mildew) affect only fruit of susceptible peach and nectarine cultivars between shuck split and pit hardening. Nonglandular, pubescent peach varieties are the most susceptible to infections.
- Thin, rusty-orange cobwebby patches on fruit surface fuzz
- Cracked fruit where infections occur
- Reduced yield
Monitor fruit for development of white patches or rusty-orange patches of mycelium on peach and nectarine fruit surfaces from post-bloom through pit hardening.
A control spray program likely will be needed if several colonies per tree are found. Fungicides work as a preventive, and will help to stop the spread to additional fruit. Adequate management of rusty spot was achieved with three to four fungicide applications including the full bloom treatment in the most favorable conditions for disease. Include the nearby apple trees in the spray program treatment.