The identification and isolation of Impatiens necrotic spot virus (INSV) from prickly pear cactus (Opuntia microdasys)

In February 2001, 15-month old plants of Opuntia microdasys (Lehm.) var. albata were received from a specialist UK cactus grower. Over 80% of the plants (which had been raised as cuttings) of the same stock had necrotic symptoms, ranging from small (2-4 mm) necrotic spots up to large (1-2 cm) necrotic lesions. As the nursery involved had a persistent infestation of Western flower thrips (Frankliniella occidentalis), the samples were tested by ELISA for the two tospoviruses which occur in the UK and which are transmitted by this vector: Tomato spotted wilt virus (TSWV) and Impatiens necrotic spot virus (INSV). INSV, but not TSWV, was detected using both polyclonal (Loewe) and monoclonal (Adgen) antibodies; infection by INSV was confirmed by RT-PCR when, using INSV-specific primers (Mumford et al., 1996), a PCR product of the correct predicted size (620 bp) was generated. The virus was also transmitted mechanically to a range of indicator plants which developed characteristic symptoms of INSV. Infected Chenopodium quinoa leaves with necrotic lesions were shown to be infected by INSV when subsequently tested by both ELISA and RT-PCR. The biological, serological and molecular properties of this Opuntia isolate are very similar to those of previously characterised INSV isolates from the UK and the Netherlands (Weekes et al., 1998). This is the first report of a tospovirus infecting a member of the genus Opuntia.

A second virus detected in the Opuntia samples by electron microscopy had flexuous, filamentous particles 520 nm long. Such virus particles were also detected in mechanically-inoculated leaves of C. amaranticolor. This virus has yet to be identified, but its experimental host range and the size and morphology of its particles indicate that it is probably Cactus virus X (CVX), a potexvirus previously reported from Opuntia spp. (Brunt et al., 1996). The symptoms, if any, caused by this potexvirus are unknown. However, CVX occurs symptomlessly in cactus species (Bercks, 1971) and thus alone is unlikely to have any significant deleterious effect on infected plants.