The scarring caused by the fungus responsible for Rusty Spot is unable to stretch as the fruit enlarges. This results in a tear or break in the skin of the fruit. Gum is produced at the site of the break.

Symptoms of Rusty Spot on Peach

Note Discolored Fuzz	Note web-like Russetting on fruit after fuzz is wiped away

`Rusty Spot' is a term used to describe the dark reddish or rusty-brown spots which develop on the fruit of peaches and nectarines. When the discolored fuzz is wiped off, a web-like russetting of the skin of the fruit is seen.

The cause of rusty spot is uncertain, although many plant pathologists believe it to be the apple powdery mildew fungus. In the late 1970's, Dr. Norm Luepschen, Fruit Pathologist at the Orchard Mesa Research Center reported this problem was more severe on peaches near apple trees heavily infected with the powdery mildew organism Podosphaera leucotricha. Peaches within one-quarter mile of mildew infected apples appear to be susceptible to this disease problem.

Infection of peach and nectarine fruit 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. This is especially true when high humidity and showers occur during spring and early summer. 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. Gum will ooze from the cracks developing in the skin of the fruit,

To prevent `rusty spot' of peach and nectarines, avoid planting susceptible peach varieties within one-quarter mile of highly mildew susceptible apple varieties. Planting highly susceptible apple varieties with highly susceptible peach and nectarine varieties usually results in increased `rusty spot' problems.

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.

References used:

Larsen, H.J., A.G. Gaus, R.J. Zimmerman, & M. Rogoyski (eds). 1998. Colorado Tree Fruits Pest and Crop Management Guide. XCM-41. Colorado State University Extension.
Owawa, J.M., E.I. Zehr, G.W. Bird, D.F. Ritchie, K. Uriu, & J.K. Uyemoto (eds). 1995. Compendium of Stone Fruit Diseases. American Phytopathological Society.
Travis,J., J. Rytter, & K. Hickey. No Date Given. Fruit Pathology Penn State Department of Plant Pathology. Web Page at http://www.cas.psu.edu/docs/CASDEPT/PLANT/fpath/stonefrt/rustspot.html