Diverse <i>Pseudomonas</i> Species Engage in Beneficial and Suppressive Interactions with the Kiwifruit Pathogen <i>Pseudomonas syringae</i> pv. <i>actinidiae</i> Across <i>Actinidia</i> Germplasm

In 2010, a Pseudomonas syringae pv. actinidiae biovar 3 (Psa3) incursion into New Zealand devastated susceptible kiwifruit ( Actinidia chinensis) orchards. In contrast, many Actinidia species maintained in germplasm collections were Psa3-resistant and showed limited symptoms. However, recent genome biosurveillance revealed the emergence of widespread leaf spot symptoms in Actinidia germplasm. Psa3 isolates were recovered from only 12% of symptomatic tissue collected, despite the frequent isolation of phenotypical Pseudomonas isolates on selective agar. Despite the poor recovery of Psa3 isolates, Psa3 was found in all symptomatic tissue through qPCR and metabarcoding analysis. Metabarcoding revealed stark differences in bacterial community composition from samples taken from symptomatic or lesion-free tissue from the same leaf. Host genotype also influenced community composition, but to a lesser extent. Whole-genome sequencing of 15 diverse Pseudomonas isolates revealed that many belonged to the P. syringae species complex. Curiously, the kiwifruit-associated P. syringae pv. actinidifoliorum (Pfm) was never recovered. Pathogenicity and competitive assays revealed that although individually these Pseudomonas isolates were not more pathogenic than Pfm or Psa3 on resistant Actinidia hosts, some could interact with Psa3 to improve their growth, suggesting that these isolates may form pathogenic consortia in these disease-associated phyllosphere communities on typically Psa3-resistant hosts. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .