The isolation of Arabis mosaic virus from Mexican orange blossom (Choisya ternata) in the UK

In September 2001, a sample of pot-grown Choisya ternata (family Rutaceae) obtained from a nursery in southern England, was received at the Central Science Laboratory. The sample came from a stock where 10% of all plants were exhibiting abnormal stunting. The sample received was also suffering from apical die-back of the young shoots. Initial detailed examination, including transmission electron microscopy (TEM), failed to identify the presence of any non-viral pathogens or to detect any virus particles. However, following mechanical inoculation onto a range of indicator plants, systemic mosaic symptoms were observed on cucumber (Cucumis sativus). Subsequent inoculation from this cucumber, onto a wider range of indicators, gave symptoms on Nicotiana clevelandii (systemic mottle and necrotic flecking) and N. benthamiana (local ringspots). Again no particles were seen by TEM in these hosts. Given the types of virus commonly found in woody ornamentals (Cooper, 1979), the lack of visible particles and the indicator reactions, a nepovirus infection was suspected. Symptomatic leaves from the indicators were tested by ELISA, using a range of five different nepovirus antisera. Only Arabis mosaic virus (ArMV) was detected (OD405 was over three times the healthy mean), using a polyclonal antiserum kit (Loewe Biochemica, Germany). This antiserum has been extensively used over many years and has always proved to be ArMV specific. Leaves from certain other indicators, including Chenopodium quinoa and N. tabacum, also tested positive for ArMV by ELISA, despite the fact that no symptoms were observed on these hosts. While asymptomatic ArMV infection does appear to be common in many indicator species (Schmelzer, 1963), it is interesting that all previously reported ArMV strains gave symptoms on C. quinoa, including the distinct hop-strain (Schmelzer, 1963; Murant, 1970). Following the ELISA test results from indicators, the original Choisya sample was tested for ArMV by ELISA. While no virus was detected in the leaves, ArMV was confirmed in the roots. The reason why the virus was no longer detectable in leaves is unclear, but might be related to seasonal effects; the ELISA test was carried out in November, several months after the initial sap inoculation. This is the first published report of ArMV (or of any other virus) being isolated from Choisya spp.