Bacterial Spot on 'Fritz'

Written by Brent Holtz, David Doll, Roger Duncan, and Themis MichailidesWe 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.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.