Phytophthora cinnamomi Populations Affecting Avocado in California Show Low Differentiation, Phenotypic Variability, and Introductions from Mexico.

Abstract

Phytophthora root rot (PRR), caused by Phytophthora cinnamomi, is one of the most devastating avocado diseases worldwide. We previously reported a group of genetically distinct, more virulent, and less fungicide-sensitive isolates corresponding to the A2 clade II clonal population in southern avocado-growing regions in California that may have originated from Mexico. To test this hypothesis, we assessed the genetic and phenotypic diversity of the California and Mexico avocado P. cinnamomi populations. We found that P. cinnamomi populations at these two locations belong to the two previously described A2 clades. Higher genetic differentiation was detected among Mexican isolates when compared with the populations from California, however, most Mexican isolates clustered with the California A2 clade II populations providing evidence for the Mexican origin of these isolates in southern avocado-growing regions. With the exception of one diploid Mexican isolate, all isolates were estimated to be triploid based on inferred allele ratios. Despite clonality, wide phenotypic variability was detected within and among populations regarding growth rate, optimal growth temperature, fungicide sensitivity, and virulence. Interestingly, contrasting virulence patterns were found depending on the host and infected tissue utilized. This study reveals the adaptative capacity of clonal P. cinnamomi populations to local environments and control methods. Pathogen migration between Mexico and California was also inferred which highlights a pathway that should be surveilled if further emergence is to be avoided. Monitoring and characterizing the P. cinnamomi populations is critical to develop effective and durable PRR control methods to ensure the sustainability of the avocado industry worldwide.