Agriculture: Nectarine Pest Management Guidelines

Shot Hole Disease

  • Wilsonomyces carpophilus
  • Symptoms and Signs

    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.

    Comments on the Disease

    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.

    Management

    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.

    Organically Acceptable Methods

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

    Chemical Control

    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.

    Common name Amount per acre REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used.
     
    A. PYRACLOSTROBIN+BOSCALID
      (Pristine) 10.5–14.5 oz 12 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11) and Carboxamide (7)
      COMMENTS: To reduce the potential for resistance, do not make more than 5 applications of this or other Group 11 or 7 fungicides per season. Do not make more than 2 sequential applications of this product.
     
    B. BORDEAUX MIXTURE#
      10:10:100 Label rates See label See label
      MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
      COMMENTS: Not all copper compounds are approved for use in organic production so be sure to check individual products.
     
    C. CHLOROTHALONIL
      (Echo 720) 3.125–4.125 pt 12 See comments
      (Bravo Ultrex) 2.8–3.8 lb 12 See comments
      (Bravo Weather Stik) 3.125–4.125 pt 12 See comments
      MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
      COMMENTS: Do not use with or closely following oil sprays. Do not apply after shuck split.
     
    D. FIXED COPPER#
      (various products) Label rates See label See label
      MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
      COMMENTS: Not all copper compounds are approved for use in organic production so be sure to check individual products.
     
    E. ZIRAM
      (Ziram 76DF) 6 lbs 48 30
      MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
     
    F. AZOXYSTROBIN
      (Abound) 12–15.5 fl oz 4 0
      MODE-OF-ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.
    # Acceptable for use on organically grown produce.
    1 Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. In California, make no more than one application of fungicides with mode of action Group numbers 1,4,9,11, or 17 before rotating to a fungicide with a different mode of action Group number; for fungicides with other Group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode of action Group number.
    NA Not applicable.
    Text Updated: 06/10
    Treatment Table Updated: 09/15