Phytophthora pluvialis hosting Phytophthora pluvialis RNA virus 1 shows morphological changes and hypervirulence to Pinus radiata needles

Phytophthora species infect a wide range of plants causing major economic and ecosystem impacts. Phytophthora pluvialis, causal pathogen of red needle cast (RNC) disease of Pinus radiata, hosts Phytophthora pluvialis RNA virus 1 (PplRV1). All tested P. pluvialis isolates contained PplRV1. To assess the impact of this virus on its host, virus-free cultures of P. pluvialis isolates NZFS 3052 and NZFS 4018 were generated using ribavirin and hyphal tipping to produce the PplRV1a cured culture 3052-C and the PplRV1b cured culture 4018-C. Phenotypically, PplRV1-infected isolates produced more aerial hyphae, sporangia, and hyphal swelling than PplRV1-cured isolates. The vertical transmission of PplRV1a or PplRV1b via zoospores from their parent P. pluvialis isolate was 100%. A qPCR-high-resolution melting assay, developed to differentiate the two PplRV1 variants and the P. pluvialis genotypes (NZFS 3052 and NZFS 4018), provided no evidence of PplRV1a transmission between the P. pluvialis NZFS 3052 isogeneic isolates. In contrast, there was PplRV1b transmission between the isogenic isolates of P. pluvialis NZFS 4018. A detached pine needle assay revealed that the RNC symptoms on pine needles inoculated with PplRV1b-infected P. pluvialis were more severe than those inoculated with the PplRV1b-cured isogenic P. pluvialis isolate. These combined results suggest that PplRV1a has no obvious effect on pathogenicity whereas PplRV1b may induce hypervirulence in P. pluvialis. This understanding of the impact of PplRV1 on P. pluvialis pathogenicity extends our knowledge about interactions between Phytophthora and its viruses and may lead to alternative control options for this pathogen.