Objective in order to elucidate the influencing factors in the synergistic effect of co-infection of cucumber mosaic virus (CMV) and lily symptomless virus (LSV) in lily.

Methods Nicotiana benthamiana and Lilium davidii Duch were used as hosts, with CMV and LSV virions purified from the virus disease samples of L. davidii Duch as test viruses in this study. The friction inoculation and DAS-ELISA methods were used. The optimum concentration, temperature, relative humidity and the order of inoculation of CMV and LSV in N. benthamiana were studied, and the optimum conditions for the occurrence of synergism were selected. The relative expression of two viral coat protein (CP) genes in the inoculated new leaves of L. davidii Duch were detected and the synergistic effect was verified by RT-qPCR.

Results Subsequent co-infection experiments revealed synergistic conditions requiring 3.57% CMV and 12.50% LSV CMV ( 14.29 % ) and LSV ( 16.67 % ) under 25°C, 60% relative humidity, and sequential inoculation (CMV 48 h prior to LSV). The compound infection occurred earlier than the single infection, mainly manifested as severe mosaic, chlorosis, bright veins and other symptoms in uninoculated leaves, and curled upper leaves in the late growth stage. The concentration of CMV and LSV were 10.37 ng·mL⁻¹ and 3.66 ng·mL⁻¹, respectively, which were 1.45 and 1.11 folds that of the single infection, and the concentration of CMV was significantly higher than that of LSV, CMV was the passive synergistic virus, LSV was synergistic virus. After inoculation of L. davidii Duch scales, they were cultured under the selected optimal conditions for 60 d. RT-qPCR detection showed that the CMV-CP relative expression in the new leaves of L. davidii Duch infected with the two viruses was significantly higher than that of LSV-CP and the relative expression of CMV-CP and LSV-CP in the two viruses single. It shows that the synergistic effect conditions of the two viruses infecting L. davidii Duch were consistent with those of infecting N. benthamiana.

Conclusion N. benthamiana and L. davidii Duch unicolor were inoculated with 3.57 % CMV for 48 h and then inoculated with 12.50 % LSV under the conditions of temperature 25 °C and RH 60 %, it was most conducive to the occurrence of synergistic effect. The content of the two viruses was higher than that of single inoculation, and the content of CMV was significantly increased, which was a synergistic virus, and LSV was a synergistic virus. The results of this study are of great significance for further elucidating the synergistic effect rule of CMV and LSV co-infection host plants such as N. benthamiana and L. davidii Duch, and guiding the effective control of viral diseases.