Field Note – Powdery Mildew of Almond
Written by Cameron Zuber, UCCE Merced County
Some interesting almonds came into theoffice and, no, this is not a setup for a joke.
In the middle of July, some almond fruit nuts from Le Grand, CA were brought into the Merced County UC Cooperative Extension office. There were various signs of possible pest damage and disease infection. These included black growth inside the hull indicating possible Rhizopus stolonifera fungus which may lead to hull rot; grayish-black spots indicating scab (Cladosporium carpophilum); and gumming which may indicate bacterial spot (Xanthomonas arboricola pv. Pruni), anthracnose (Colletotrichum gloeosporioides), or bug feeding.
However, the greatest amount of damage were odd areas of russetingand cracking on the hull.
After speaking with person who brought in the samples, there wasnot any indication of leaf or shoot die back near the nuts which starts to ruleout hull rot and lab tests did not show any fungi that would cause hull rot. Labtest also showed some instances of almond scab growths, but none of these pestor disease symptoms were at a level to be extremely concerned about. Also, nonehave been shown to cause the odd cracking on the hull, so what was the cause,should you be worried about it, and how do you fix it?
The cause was found through some digging and a lot of help from farmadvisors and plant pathologists. It was determined that the cause was mostlikely a false powdery mildew (or powdery mildew-like) fungus of the Acremoniumspecies. Normally these organisms live on dead or decaying organic matter(i.e., saprophytes), but do occasionally infect plant material and, foralmonds, result in the russeting and cracking seen in the above fruit nuts. Itis called “false” or “like” powdery mildew because it shows similar symptoms ofpowdery mildew on other crops like those seen in apples (Podosphaeraleucotricha) and peaches (Sphaerotheca pannosa) but is not directlyrelated to those organisms.
As to worry, the UC IPM website notes that, “the disease is rarely an economic problem on almonds in California,” and the damage to the fruit nuts that were brought into the office seemed to be located only in the hull.
At this point, fixing the issue would probably be moot as theredid not seem to be an immediate concern for the current year’s crop, and, aswith most things with pest and disease management, the solution is normally prevention.However, finding products that are effective for this specific fungus of Acremoniumspecies is difficult due to the challenge with identify it through labassessments. If you have this issue, please consult with your local farmadvisor or PCA. Current UC IPM recommendations for prevention are fungicideapplications during jacket-split and mid-spring (e.g., April and May inCalifornia).
We would like to acknowledge and thank Jim Adaskaveg (professorand plant pathologist with UC Riverside), Themis Michailides (plant pathologistand lecturer with Kearney Agricultural Research and Extension Center), andBrent Holtz (Farm Advisor with UC Cooperative Extension in San Joaquin County)for their help with identifying this disease. We would also like to give aspecial thank you to the grower for allowing us to share their story.
The Almond Doctor Field Note isa series of posts that will cover observations made from the field by staffresearch assistants of the University of California Cooperative Extensionoffice of Merced County. While Field Note posts may reference completed oron-going research, its intent is share notable observations from the field, notto summarize research findings or describe current recommended farmingpractices.