Detection of latent or symptomatic tobamoviruses in Hoya spp.

Hoya tobamovirus-2 (HoTV-2) was reported from infected samples from 2011 in several Hoya species (Gaafar et al., 2020; Richert-Pöggeler et al., 2018) in Germany in mixed infection with other viruses. Infected leaves were used to mechanically inoculate different experimental host plants. Plants were kept at 18-22°C for symptom evaluation. The virus showed no symptoms in susceptible plants. The only exception was the infection of Nicotiana benthamiana, which developed a mild mosaic and epinasty of systemic leaves 3-4 weeks post-inoculation (p.i.). At 14 days p.i., leaves were examined by electron microscopy. Rod-shaped virions were detected, confirming the systemic infection. A phylogenetic analysis of the virus showed its relatedness to Youcai mosaic virus (YoMV). Therefore, antisera for YoMV was used for immuno-electron microscopy. The antisera reacted with HoTV-2. The experiment showed that the virus was able to replicate and move systemically in the host plants without inducing any apparent symptoms. A similar Tobamovirus was detected in several Hoya species in Florida in 2021 with necrotic foliar lesions. Infected leaves were used to mechanically inoculate experimental host plants. Necrotic local lesions were observed on N-gene tobacco. Sequence analysis of partial replicase, movement protein, and coat protein gene sequences showed ~96% nucleotide identity with German HoTV-2 and Hungarian Hoya necrotic spot virus isolates. Further research is needed to fully characterize these recently detected Hoya-infecting tobamoviruses.