First Detection of Potato Spindle Tuber Viroid in Natural Isolates of Potato Blight Agent Phytophthora infestans

Phytophthora infestans is the oomycete that causes potato blight, an important disease. The potato spindle tuber viroid (PSTVd) is a dangerous pathogen of many plants, including potato. We have previously shown that PSTVd can be transmitted from infected potato plants into the Ph. infestans mycelium, replicated within the mycelium, and then transmitted to other potato plants upon their infection with Ph. infestans in laboratory conditions. The objective of this work was to check the hypothesis that PSTVd transmission, preservation, and replication in Ph. infestans are possible to occur in natural conditions during long-term coevolution of the host and pathogen in the Solanum spp.-Ph. infestans system. A screening test for PSTVd was performed in 111 natural Ph. infestans isolates obtained from potato plants, which represented various cultivars, had signs of potato blight, and were collected from industrial potato fields of the Moscow, Vologda, and Bryansk regions and breeding and variety test plots of the St. Petersburg and Moscow regions in 2020 and 2022. Using RT-PCR with PSTVd-specific primers, 42 Ph. infestans isolates collected in 2020 were tested after five passages and 69 Ph. infestans isolates collected in 2022, after a single passage on rye agar. Diagnostic amplicons were detected in 8 and 50 isolates, respectively. Some of the amplicons were visually assessed as minor amplification products, apparently resulting from nonspecific priming on a host Ph. infestans gene, which codes for a hypothetical protein-coding mRNA in Ph. infestans and other oomycetes. Eight amplicons were sequenced to verify the PSTVd presence in Ph. infestans isolates. Three amplicons corresponded to the complete PSTVd genome and five, to its part (~260 bp). The nucleotide sequences of cloned amplification products were identified to species in the BLAST system and deposited in GenBank. The amplicons obtained with the PSTVd-specific primers were identified as PSTVd sequences in all Ph. infestans isolates examined. The majority of the nucleotide sequences were phylogenetically related to BLAST sequences of PSTVd strains originating from Russia; several strains showed similarity to strains from other countries (France, China, and West African countries). The results demonstrate that PSTVd was for the first time detected in natural (field) Ph. infestans isolates and offer new opportunities for studying the intricate multilevel host-parasite interactions.