Phytophthora cinnamomi populations collected from avocado in the United States exhibited high adaptive capacity to climate and control methods

Authors: HOYT, BENJAMIN - University Of California, Riverside; SALAS, SAVANNAH - University Of California, Riverside; CRANE, JOHNATHAN - University Of Florida; NAVIA-URRUTIA, MONICA - University Of Florida; GAZIS, ROMINA - University Of Florida; CANO, LILIANA - University Of Florida; ADHIKARI, ACHYUT - University Of Hawaii; TIAN, MIAOYING - University Of Hawaii; JIFON, JOHN - Texas A&M University; Serrato Diaz, Luz; Goenaga, Ricardo; ADASKAVEG, JAMES - University Of California, Riverside; MANOSALVA, PATRICIA - University Of California, Riverside

Submitted to: Frontiers in Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2026
Publication Date: 6/22/2026
Citation: Hoyt, B.K., Salas, S., Crane, J., Navia-Urrutia, M., Gazis, R., Cano, L.M., Adhikari, A., Tian, M., Jifon, J., Serrato Diaz, L.M., Goenaga, R.J., Adaskaveg, J.E., Manosalva, P. 2026. Phytophthora cinnamomi populations collected from avocado in the United States exhibited high adaptive capacity to climate and control methods. Frontiers in Plant Science. 17:1838248. https://doi.org/10.3389/fpls.2026.1838248.
DOI: https://doi.org/10.3389/fpls.2026.1838248

Interpretive Summary: Phytophthora root rot (PRR) caused by Phytophthora cinnamomi is a serious disease that affects avocado trees worldwide. As global demand for avocados has increased, production has expanded.However, this increase in production has also been linked to a rise in cases of PRR. The United States is one of the largest consumer of avocados worldwide, but growers are facing new challenges because some populations of P. cinnamomi are becoming harder to control. In previous research, we studied populations of this pathogen in California and found strains that were highly aggressive and resistant to potassium phosphite, a fungicide commonly used to manage the disease. In this study, we expanded our research to include samples from Florida, Texas, Hawaii, and Puerto Rico. We evaluated how quickly the pathogen grows at different temperatures, how sensitive it is to six different fungicides, and how aggressive it is when infecting plants. Our results showed significant differences among populations from different states. Isolates from Hawaii, Florida, and California grew best at higher temperatures (28–30°C), suggesting they have adapted to warm climates. We also found more resistance to potassium phosphite inCalifornia and Florida, likely due to repeated use of this fungicide. In addition, we established baseline sensitivity levels for several other fungicides that are not widely used in avocado production. Some isolates were more virulent(more capable of causing severe disease on avocado seedlings and pear avocado fruits), especially those from California and Puerto Rico. Overall, our findings showed that even genetically similar (clonal) populations of P. cinnamomi can adapt to local environmental conditions and disease control practices. This adaptability increases the risk of the appearance new fungicide-resistant and more aggressive strains. Continued monitoring and research are essential to develop effective, long-term strategies to protect the U.S. avocado industry and ensure its sustainability.

Technical Abstract: Phytophthora root rot (PRR) caused by Phytophthora cinnamomi is a major production challenge impacting the global avocado industry. With recent higher demand for avocados, a rise in production has occurred resulting in an increase of PRR incidence and severity worldwide. The United States is the top avocado consumer, though domestic growers are facing clonal A2 P. cinnamomi populations challenging their current PRR control methods. We previously characterized the avocado P. cinnamomi populations in California and reported potassium phosphite-resistant and highly virulent isolates collected from southern growing regions. In this study, we expanded our phenotypic characterization to isolates from other avocado producing states and territories by comparing their radial growth rate at different temperatures, in vitro sensitivities to six different Oomycota fungicides, and virulence. Significant variability was detected among P. cinnamomi populations for each phenotypic trait within and among states. Isolates from Hawaii, Florida, and California exhibited higher optimal growth temperatures (28°C and 30°C) when compared to isolates from other states which exhibited a decrease in radial growth at 28°C, suggesting adaptation to warmer climates. More potassium phosphite-resistant isolates were found in California and Florida, reflecting the continued overuse of this fungicide in these major production states. Significant variability was also detected regarding in vitro sensitivities to other Oomycota fungicides, possibly revealing natural variation of isolates, since some of these fungicides are not heavily used or registered on avocado. We reported the first baseline in vitro sensitivities to mefenoxam, oxathiapiprolin, fluopicolide, ethaboxam, and mandipropamid of P. cinnamomi populations from Florida, Texas, Hawaii, and Puerto Rico. Finally, a wide range of virulence among isolates was detected using avocado seedlings and D’Anjou pear fruits with isolates from California and Puerto Rico being the most virulent. Our study highlights the adaptative capacity of clonal P. cinnamomi populations to local environments and control methods suggesting the potential for emergence of new fungicide-resistant and more virulent populations threatening the national avocado industry. The continued surveillance and phenotypic characterization of P. cinnamomi populations in the United States are critical for the development of effective and durable PRR control methods to ensure industry sustainability and profitability.