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Shot hole lesions on leaf and young fruit of peach.

Peach

Shot Hole Disease

Pathogen: Wilsonomyces carpophilus

(Reviewed 4/10, updated 4/10)

In this Guideline:


SYMPTOMS AND SIGNS

The main symptoms of shot hole on peach 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 peach 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
In areas where orchards have a history of this disease, spray at leaf fall or from November 15 to December 1 before winter rains to protect against twig infections. This spray also helps 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 to Use R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. PYRACLOSTROBIN/BOSCALID
  (Pristine) 10.5–14.5 oz/acre 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/acre 12 NA
  (Bravo Ultrex DF) 2.8-3.8 lb/acre 12 0
  (Bravo Weather Stik) 3.125–4.125 pt/acre 12 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M5)
  COMMENTS: Do not use with or closely following oil sprays.
 
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 76DF 2–2.5 lb/100 gal water
(dilute spray)
48 30
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M3)
 
F. AZOXYSTROBIN
  (Abound) 12.3–15.4 fl oz/acre 4 0
  MODE OF ACTION GROUP NAME (NUMBER1): Quinone outside inhibitor (11)
 
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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 (for more information, see http://www.frac.info/). 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Peach
UC ANR Publication 3454
Diseases
J. E. Adaskaveg, Plant Pathology, UC Riverside
R. A. Duncan, UC Cooperative Extension Stanislaus County
J. K. Hasey, UC Cooperative Extension Sutter/Yuba counties
K. R. Day, UC Cooperative Extension Tulare County
Acknowledgment for contributions to Diseases:
W. D. Gubler, Plant Pathology, UC Davis
B. L. Teviotdale, Kearney Agricultural Center, Parlier

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