Some years only a small portion of the fruit is injured. Some years, major fruit damage can occur if weather favors the production and dissemination of mildew spores. Spores of the fungus land on the fruit and begin to grow. As the fungus spreads it creates a network of marks on the surface tissue of the fruit. The fungus, however, is unable to continue growth and eventually dies.
Rusty spot is seen only on the fruit.
The skin of the fruit becomes hard and leathery beneath the discolored fuzz and tends to crack as the fruit enlarges. No information was available on the susceptibility of Nectarines to 'Rusty Spot' at the time this was published. The control of powdery mildew on apples with fungicides and treating highly susceptible peach and nectarine cultivars during the early fruit development stage is recommended. Colorado Tree Fruits Pest and Crop Management Guide. Colorado State University Cooperative Extension.