Bacterial Spot - A New Disease of Almond in the San Joaquin Valley

Written by: Roger Duncan, Brent Holtz, David Doll and Themis MichailidesEarlier this spring, we received reports from growers and pest control advisors that they had observed a few San Joaquin, Stanislaus and Merced County almond orchards with large amounts of amber-colored gum balls exuding from the hulls.  The damage has been predominantly on ‘Fritz,’ but there are reports of similar damage on ‘Monterey’ and ‘Padre.’  In some orchards, Fritz is severely affected while the Nonpareil and other pollinators are very clean. Samples from affected orchards were sent to University of California plant pathologists, Themis Michailides and Jim Adaskaveg.  Both scientists confirmed that the lesions were caused by the bacterium, Xanthomonas arboricola pv. pruni, the bacterium that causes bacterial spot of almond and stone fruit. This is the first report of this disease on almond in the San Joaquin Valley.  Although these orchards have apparently had the problem for multiple years in a row, the symptoms were misdiagnosed as leaf footed bug feeding injury or anthracnose.  Dr. Michailides isolated the same organism from an almond orchard in Colusa County in 2006.The symptoms of bacterial spot include multiple lesions on the hulls with large balls of amber colored “gum” or “sap”.  Over time, these spots can grow into slightly depressed lesions on the hull.  Eventually the infected nuts may shrivel and fall from the tree.  Occasionally angular leaf spots can be seen, but this is not (so far) a major part of the disease and can be hard to find.  Bacterial spot can be confused with leaf footed bug feeding injury or anthracnose.  Unlike leaf-footed bug feeding injury which exudes clear balls or strings of gum, injury from bacterial spot causes amber colored gum.  In addition, cutting into a hull damaged by leaf footed bug often reveals evidence of a puncture wound through the hull and into the shell.  This characteristic is absent with bacterial spot.  Because this is a bacterial disease, there are no fungal spores present.  This is in contrast with anthracnose-infected almonds which generally have pink or orange colored spores present within the lesions.  In addition, anthracnose often leads to shoot death.  This has not been observed so far with bacterial spot.Bacterial spot is a common problem in Australia and growers there have been forced to abandon the two most severely affected varieties, Fritz and Ne Plus, due to extensive crop loss. Because we have no history of this disease in California, the only information we have is from Australia.  Unfortunately, bacterial diseases are very difficult to control and intensive spray programs with copper and Mancozeb have not controlled bacterial spot in Australia.  University of California farm advisors and specialists will establish several trials next year in an effort to develop management guidelines for this important disease in California.The University of California Cooperative Extension will host a ‘Bacterial Spot of Almond’ Field Day July 12 at 12700 East Graves Road in Manteca.  Attendees will learn to identify the symptoms and we will discuss what we know about management options.