Correlation between virus accumulation and symptom severity in commercial lily plants revealed by inoculation with a single infectious variant of plantago asiatica mosaic virus

Plantago asiatica mosaic virus (PlAMV) is an important pathogen that causes necrotic disease in commercial lilies (Lilium spp.) and spreads through the global lily trade. While genome sequences of several PlAMV isolates from necrotic lily plants have been reported, biological studies of its infection dynamics in lilies have been limited by the lack of a single variant capable of infecting lily plants and inducing symptoms. In this study, we sought a reproducible PlAMV inoculation system in lilies and investigated relationships between virus inoculum concentration and infectivity, as well as virus accumulation in plants and symptom severity. Using virion-associated nucleic acid isolation, we obtained a PlAMV isolate called ‘KK’ that reliably infected lilies and induced necrotic symptoms. Mechanical inoculation of Lilium cv. ‘Casa Blanca’ was performed in a screenhouse during two growing seasons, spring (April to August) and autumn (August to November), in 2023-2024 in Japan. Reverse transcription quantitative PCR (RT-qPCR) analysis confirmed almost 100% infection in KK-inoculated plants. By quantifying virus accumulation in the infected leaves and assessing necrotic symptoms, we found differences between leaf positions and a positive correlation (r = 0.67) between disease severity and virus accumulation. However, a considerable number of plant samples exhibited low disease severity despite substantial level of virus accumulation, suggesting that virus accumulation is not sufficient for the expression of necrotic symptoms. This study establishes a quantitative pathosystem for PlAMV in lilies and provides a basis for future research into the mechanism underlying symptom development and disease management strategies in commercial lily production.