Alejandro I. Hernandez
Graduate Student Researcher
Department of Plant Pathology, University of California, Davis, Kearney Agricultural Research and Extension Center
Parlier, CA, USA
Renaud Travadon
Department of Plant Pathology, University of California, Davis
Davis, California, United States
Rosa E. Jaime
Department of Plant Pathology, University of California, Davis, Kearney Agricultural Research and Extension Center
Parlier, CA, USA
Tawanda E. Maguvu
Department of Plant Pathology, University of California, Davis, Kearney Agricultural Research and Extension Center
Parlier, California, United States
Cameron A.T. Zuber
University of California Cooperative Extension Merced and Madera Counties
Merced, California, United States
Brent A. Holtz, Farm Advisor, UC Cooperative Extension
University of California Cooperative Extension San Joaquin County
Stockton, California, United States
Florent P. Trouillas
Professor
Department of Plant Pathology, University of California, Davis, Kearney Agricultural Research and Extension Center
Parlier, California, United States
California almonds are the state’s top agricultural export, representing 80% of the global almond supply and a farm gate value of $5 billion. Red leaf blotch (RLB), caused by Polystigma amygdalinum, is an emerging disease in California and poses a serious threat to almond production. Statewide surveys of almond orchards were conducted to document the geographical distribution and current spread of RLB in California. To investigate the temporal dynamic of ascospore dispersal of P. amygdalinum, spore traps consisting of microscope glass slides coated with petroleum jelly were placed in two almond orchards. Traps were collected weekly from January 15th until May 28th, 2025. In addition, to evaluate chemical and biological control of RLB we tested 22 fungicides encompassing FRAC groups 3, 9, 7, 11, 12, 52, and BM01/BM02. Field trials were conducted in three orchards with a history of RLB and fungicides were applied using either a backpack sprayer to entire tree canopies or hand-held spray bottles to selected branches. In each experiment, treatments were applied at petal fall, 2 weeks, and 7 weeks post petal fall. Orchard surveys indicated that RLB is present in 8 counties across California’s central valley. Spore trap experiments showed the detection of ascospores as early as mid-January, and with a sharp increase in March, April, and May. Mixed fungicides (FRAC groups 3+7; 3+11; 7+11) and FRAC 3-triazoles were most effective at managing RLB. This research provides fundamental knowledge to further understand the epidemiology and control of this emerging disease in California.