UC Riverside

Phytophthora root rot (PRR), caused by Phytophthora cinnamomi, is a major threat to global avocado production, exacerbated by rising demand and expanded cultivation that has increased disease incidence and severity. The United States avocado industry faces clonal A2 pathogen populations that challenge current control methods with potassium phosphite-resistant and highly virulent isolates. We characterized U.SA. pathogen populations associated with avocado PRR and revealed the phenotypic plasticity and genetic structure of P. cinnamomi isolates from domestic avocado growing regions. Phenotypic characterization of isolates from California, Florida, Hawaii, Texas, and Puerto Rico showed wide variability in radial growth at different temperatures, in vitro sensitivities to six oomycete fungicides (potassium phosphite, mefenoxam, oxathiapiprolin, fluopicolide, ethaboxam, mandipropamid), and virulence on avocado seedlings and D’Anjou pears. Warmer-climate isolates (Hawaii, Florida, California) optimized growth at 28–30°C, while potassium phosphite resistance was detected in California and Florida isolates due to overuse, suggesting climate and chemical adaptation. With the exception of California, baseline in vitro sensitivities to these fungicides were determined for pathogen populations in these avocado growing regions. Virulence assays identified highly virulent isolates in California and Puerto Rico. Whole-genome resequencing linked U.S.A. and Mexican populations to the two A2 panglobal clonal lineages. Analysis of these clonal P. cinnamomi populations revealed all isolates were triploid and exhibited lower genetic differentiation between domestic avocado regions but higher differentiation between the U.S.A and Mexican populations. All isolates shared multilocus genotypes expressing large phenotypic variability within and among growing regions. Greenhouse trials assessed fungicide efficacy to control PRR on five UCR experimental rootstocks and two commercial rootstocks grafted to ‘Hass’. All fungicides lowered PRR incidence and soil propagules versus inoculated untreated controls, with oxathiapiprolin superior to the other chemistries and potassium phosphite more effective on resistant rootstocks. All UCR rootstocks combined with fungicides yielded better disease suppression, highlighting these combinations for integrative avocado PRR management. This work provides phenotypic and genetic characterization of U.S.A. avocado P. cinnamomi populations and diverse strategies to manage PRR. Ongoing pathogen surveillance, resistant rootstock development, and new fungicide registration are vital for durable PRR management to create a profitable and sustainable domestic avocado industry.