Field Note: Early Leaf Drop in Almonds from Peach Silver Mite

Written By Cameron Zuber, Research Assistant at UCCE Merced, Graduate student at UC Merced.

In the middle of October, a second leaf almond orchard in Parlier, CA presented with a lot of leaves on the ground in the ‘Sonora’ pollinator rows. It was a bit early for the trees to enter dormancy and deficient irrigation was not performed as the orchard was too young to have been harvested this year. Upon closer inspection the leaves on the trees seemed to indicate a pest or disease issue.

Early leaf drop in almonds is a concern as it may impact the flower buds that are developing on the tree which could then impact the vigor of the bloom and overall crop yields next year. The browning on the leaf could be an indication of salt burn or leaf scorch (https://thealmonddoctor.com/2014/07/25/salt-burn-vs-leaf-scorch/) and there were some leaves that had rust present (https://thealmonddoctor.com/2010/04/05/almond-leaf-rust-treat-now-to-prevent-late-season-defoliation/). However, the splotchiness of the browning did not seem typical of salt burn or leaf scorch and the rust was not prevalent enough to cause this degree of leaf drop.

The symptoms seemed more serve on the ‘Sonora’ trees, but the ‘Nonpareil’ trees were showing similar symptoms though less sever. Another odd symptom were the leaves seemed to have a silvery color which became more apparent when looking through a hand lens and when compared to a neighboring block’s leaves that were a healthy color.

The combination of splotchy browning and a silvery color started to indicate a potential culprit for the early leaf drop. Browning is a symptom of rust and a silvery color indicates thrips or leafhopper damage but having both indicated something else. Looking through a strong hand lens showed the presence of peach silver mite (Aculus cornutus). Peach silver mite can be distinguished by their teardrop body shape which is different from other mites typically found in almonds, like webspinning spider mites. However, a strong hand lens at 15x or higher is needed to even see peach silver mite which makes it difficult to identify the pest early in the season. Peach silver mite feeding causes a silver color on the leaf and during higher temperatures feeding spots may cause browning.

UC IPM (https://www2.ipm.ucanr.edu/agriculture/almond/Peach-Silver-Mite/) says the peach silver mite is normally considered a beneficial as it serves as a food source for mite predators that would feed on more harmful types of mite. However, peach silver mites may cause problems in younger orchards if populations reach a high enough level to cause early leaf drop. For this orchard, the amount of leaf drop was a concern and the ‘Nonpareil’ trees were showing similar symptoms to the ‘Sonora’ trees. Therefore, a treatment is being considered. UC IPM recommends a narrow range oil, wettable sulfur, or abamectin to treat peach silver mite. A zinc sulfate spray was already planned for this orchard to help with zinc deficiency issues. While not a recommended product for peach silver mite, the sulfur in zinc sulfate may help, so continued monitoring will be conducted after the spray to see if the peach silver mite population will drop to an acceptable level.

Special thanks to Brent Holtz (Farm Advisor with UC Cooperative Extension in San Joaquin County) for assistance with determining the issue with this orchard.

The Almond Doctor Field Note is a series of posts that will cover observations made from the field by staff research assistants of the University of California Cooperative Extension office of Merced County. While Field Note posts may reference completed or on-going research, its intent is share notable observations from the field, not to summarize research findings or describe current recommended farming practices. 

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