Peculiarities of Interactions in the Solanum spp/ Potato Tuber Spindle Viroid (PSTVd) Pathosystem and Symptoms of Mixed Viroid/Viral Infection

Aims: To evaluate responses to four strains of potato spindle tuber viroid (PSTVd) and to mixed viroid/viral infection of modern potato cultivars.

Study Design: Four PSTVd strains were used to evaluate the tolerance of potato varieties: VP35 (LC523658), VP87 (LC523667), FP10-13 (LC523676), and NicTr-3 (LC654171). These strains were chosen based on their different geographic origin and their different aggressiveness against a highly susceptible tomato cv. Rutgers.

Place and Duration of Study: Laboratory of Plant Resistance to Diseases of All-Russian Institute of Plant Protection, St. Petersburg, between January 2022 and May 2024.

Methodology: The study included 39 modern potato cultivars of different origin. Transcripts of four PSTVd strains were used to inoculate tomato plants of the cv. Rutgers. Before PSTVd inoculation with tomato sap, all potato plants were tested for viral infection by ELISA. The presence of PSTVd in infected plants was verified by RT-PCR as well as by RT-qPCR.

Results: The strain-specificity in the response of 39 cultivars to viroid infection was revealed. Five types of symptoms on tubers were identified at 90 dpi. Five cultivars were identified in which, after the first inoculation of plants with all PSTVd strains, normal in shape tubers were formed. Average tuber weight for PSTVd- infected plants was more than twice and a half time less compared to mock-inoculated plants (P < 0.05). All plants of the next generation derived from infected but normally shaped tubers showed strong symptoms of disease. After three cycles of continuous propagation in tomato cv. Rutgers PSTVd strains became less aggressive to potato cultivars. Mixed viroid/viral infection lead to an increase manifestation of tuber deformation only for certain cultivars.

Conclusion: The strain specificity of the majority of cultivars in terms of the degree of symptom development after the first inoculation was established. Five cultivars were symptomless after the first inoculation with four strains but were found to have severe viroid symptoms in plants obtained from PSTVd-infected tubers. No synergism in the manifestation of mixed viroid/viral infection symptoms on tubers was found.