Bacterial Spot on ‘Fritz’

Written by Brent Holtz, David Doll, Roger Duncan, and Themis Michailides

Bacterial Spot of Almond 1

Figure 1: Nuts infected with Bacterial Spot of Almond. Photo by M. Moran

We have visited and received samples from orchards in the counties of Merced, Stanislaus, and San Joaquin Counties that have been experiencing symptoms of amber colored gum exuding from almonds. The damage has been predominantly on the variety ‘Fritz,’ but reports are coming in of similar damage on ‘Monterrey’ and ‘Padre.’

Over the past few years, we have observed these symptoms at about the same time in May.  The damage looked similarly to damage by Leaffooted Bug (LFB-Leptoglossus clypealis) or anthracnose. Concern was raised when ‘Fritz’ containing orchards sprayed proactively three times for LFB or anthracnose experienced the same symptoms.   Symptomatic nuts were sampled and submitted to Dr. Themis Michailides, a Plant Pathologist at the Kearney Research and Extension Center.  Dr. Michailides managed to isolate Xanthomonas arboricola pv. pruni from these infected tissues.  X. arboricola pv. pruni is a plant pathogenic bacteria capable of causing the disease ‘bacterial spot’ of Prunus species, such as almond.  We will have to verify that this pathogen is in fact causing these symptoms, but the evidence looks pretty convincing.

Bacterial spot of almond 2

Figure 2: Nuts infected with Bacterial Spot of Almond. Note the size of the lesion and the lack of a small feeding hole into the kernel. Photo by M. Moran.

Symptoms of infected nuts include the production of an amber colored gum from a spots on the hull (Figure 1). Cutting into the hull, there is no presence of LFB feeding, but there is a lesion about the size of a pencil eraser (Figure 2). These lesions do not continue to enlarge, become sunken, orangish in color, or exude an orange slime like anthracnose. Nuts do not fall from the tree and are in close proximity to mummy nuts from past years (most likely also infected).

Management for bacterial spot will be much different than controlling LFB or anthracnose. It may involve trying to reduce inoculum levels by destroying mummies and spraying dormant copper treatments to reduce overwintering inoculum.  Unfortunately, bacterial diseases are very difficult to control.  We have no evidence to date that LFB vectors this pathogen, but it is a concern and will be researched. It is important to note that the ‘Fritz’ variety has been shown to be susceptible to bacterial spot in Australia.

If you have any questions, please contact your local farm advisor. We will be having a field day to highlight what we do and don’t know about this new disease in California shortly.

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6 thoughts on “Bacterial Spot on ‘Fritz’

  1. If you suspect that you may have bacterial spot in your almonds, please contact your local farm advisor. The key symptom with this disease is amber colored gum balls on the hulls which is in contrast to clear gum produced in response to leaf footed bug injury. We would like to get a handle on how widespread this problem is. Pay particular attention to the fritz variety. Please watch for announcements for the July 12 field day to be held in the Ripon area.

  2. The same bacterium was also isolated in our laboratory in 2006 from samples submitted by FA John Edstrom. In Australia in 2003/2004, the disease was severe in Fritz and NePlus but they also found it in Nonpareil, Mission, and Carmel cultivars in lower severity. The Australians claim that management of the disease is done with copper sprays at pink bud, bloom, and before (within 7 days) each spring-early summer rain event. Also in Australia, people involved with almond research noticed that same nuts and leaves fall prematurely.

    1. Steve,
      The gumming will be clear, with minimal discoloration. It may have a “pigs-tail” curl. If the gumming is amber in color, then there it could be bacterial spot. Check out the video on leaf-footed plant bug for more details.

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