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Integrated Pest Management · Agriculture and Natural Resources

University of California

Bacterial spot (Xanthomonas arboricola pv. pruni)

Bacterial spot on almond leaves and nuts.

Bacterial spot on almond leaves and nuts. Photo by Roger Duncan. Used by permission.

(Published 7/13)

Closeup of bacterial spot on almond leaves.

Closeup of bacterial spot on almond leaves. Photo by Brent Holtz. Used by permission.

Bacterial spot lesion on almond stem.

Bacterial spot lesion on young almond twig. Photo by Brent Holtz. Used by permission.

Bacterial spot is a relatively new almond disease in California. It has currently been found predominantly on the cultivar Fritz in Colusa, Merced, Stanislaus, and San Joaquin counties. The disease has also been observed on sweet cherry and other stone fruit crops in San Joaquin and Stanislaus counties. Research is ongoing to determine management options.

2013 Bacterial Spot on Almond Field Day (PDF)

Symptoms

Symptoms first appear in mid-April to early May. Amber-colored gum exudes from nuts with reddish lesions on the hull surface, similar to anthracnose and leaffooted plant bug feeding.

  • Anthracnose = amber gum, sunken lesions, and pink or orange spores in the lesions
  • Leaffooted bug feeding = clear gum and feeding puncture through hull and into shell

In bacterial spot, leaves become spotted, especially where water collects (e.g., along leaf margins), turn yellow, and drop prematurely. Green twigs (less than a year old) can have visible lesions or cankers.

Biology

See also: Bacterial Spot of Almond and other Prunus species. (PDF)

Bacterial Spot needs wetness to spread; bacteria are spread from cankers or mummies by dripping dew and splashing or wind-blown rain to newly emerged leaves. It overwinters on mummies and possibly in twig cankers.

Preliminary Management Guidelines

Practice prevention measures as for other bacterial diseases. Research in other crops suggest the following preliminary management guidelines may reduce bacterial spot. Research is needed to determine the benefits of in-season bactericide treatments.

  • During the season, blow fallen nuts into the center between rows and grind them up into small pieces that easily degrade.
  • Practice sanitation when moving between orchards by brushing off shoes; sweep out all nuts from trailers or hoppers.
  • If possible, harvest before fall rains.
  • After harvest, defoliate trees to reduce inoculum and improve the visibility of mummies and coverage of dormant sprays.
  • Remove and destroy mummy nuts; pole or shake and then disc or mow.
  • Copper plus oil applications before winter rain may help prevent disease.
  • A delayed-dormant copper plus oil application may help prevent disease.

More information

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