Shot Hole Disease

The main symptoms of shot hole on nectarine occur on twigs and buds, but fruit lesions may develop when spring weather is wet. Twig symptoms first appear as small, purplish black spots. These turn brown as they enlarge, often having a light center with a purplish brown margin. Tiny, dark brown bumps develop at the center of each lesion. These bumps are spore-forming structures called sporodochia and are easily seen with a hand lens. When buds are affected, the scales turn dark brown or black and the buds may be covered with a shiny layer of exuded gum. Buds killed by bacterial blast have a similar appearance but tend to be much blacker, and nearby foliage of the affected shoot is wilted. Shot hole can be distinguished on nectarine by the presence of tan twig lesions with dark margins, usually accompanied by profuse gumming.

Fruit and leaf symptoms look much like those of twig lesions. They are small spots, purplish at first, and turning light brown in the center as they enlarge. Sporodochia form in leaf lesions but not in fruit lesions. Leaf lesions may be surrounded by a light green or yellowish zone; in many cases the brown tissue in the center will fall out, leaving the "shot hole" that gives the disease its name.

Wilsonomyces carpophilus survives on infected twigs and buds. Spores are produced throughout winter and are spread by splashing rain and wind. The disease is favored by prolonged wetness in fall to mid-winter (twig blight). Summer rain or sprinkler irrigation encourages fruit infection. There is more infection low in the tree where fruit stay wet longer.

Shot hole is managed primarily with fungicide treatments to protect buds and twigs from infection. In orchards where twig infections are prevalent, the efficacy of the dormant treatment can be improved by pruning out and destroying infected wood. If the orchard is sprinkler irrigated, be sure to angle sprinkler heads low enough to keep from wetting the canopy.

Treatment with Bordeaux mixture and some formulations of copper are acceptable for use in an organically certified orchard.

Spray at leaf fall or from November 15 to December 1 before winter rains to protect against twig infections. This spray often will also control leaf curl. If needed, make a spring application during bloom to prevent fruit and leaf infection. If weather is wet, look for fruiting structures in leaf lesions. If any are present, treat with a product, such as pyraclostrobin/boscalid, chlorothalonil, or azoxystrobin, that is used for both brown rot and shot hole control.