There have been spectacular advances in virus handling techniques in recent years, with increasing emphasis on characterization by biochemical and biophysical criteria.
Such studies are now possible, not merely with the selected laboratory "guinea pig viruses" like TMV and TYMV, but with many crop pathogens.
Nevertheless, the test plant remains indispensable as the way of culturing plant viruses, and as the only means of demonstrating their infectivity.
Relatively few new indicator plants have been introduced in recent years.
Saponaria vaccaria 'Pink Beauty' (Hakkaart and van Olphen, 1971) reacts well with carnation mottle virus, being more sensitive to this virus than Chenopodium amaranticolor, though no more so than C. quinoa.
Silene armeria (Hakkaart, 1968) is a convenient diagnostic host for the 50 nm member of the carnation etched ring virus complex, and can be infected by sap inoculation or aphids.
If carnation mottle virus is also present, however, symptoms of etched ring may be obscured.
Although Ocimum basilicum reacts with local lesions to many viruses, it has diagnostic value with viruses such as cowpea aphid borne mosaic (Lovisolo, 1966). Hyoscyamus niger has been used as a local lesion host for a mosaic virus from Hippeastrum (de Leeuw, 1972). A Nicotiana hybrid (N. clevelandii x N. glutinosa) was susceptible to eleven viruses, most of which commonly infect Solanaceae (Christie, 1969); the plant has the more vigorous growth habit associated with N. glutinosa, but is not palatable to aphids and, under our conditions, proved susceptible to fewer viruses than N. clevelandii and gave much poorer yields of virus than were obtained from N. clevelandii.
Some unusual plants have been tried as indicator species, such as Crambe abyssinica (Horvath, 1969), but appear to offer no particular advantages over existing standard test plants with several viruses.
Seedlings of the tree Paulownia tomentos a were susceptible to all nine viruses tested (alfalfa mosaic, tomato black ring, tobacco ringspot, tomato spotted wilt, potato X, nasturtium ringspot, strawberry latent ringspot, arabis mosaic and cucumber mosaic), giving differential reactions of potential diagnostic, or separation value for viruses (Schmelzer, 1969).
Chenopodium quinoa is proving of increasing value and, of the species of Chenopodium tested, it is probably the most sensitive; moreover, several viruses readily infect C. quinoa but not other species and the inhibitors in C. quinoa are very slight compared with, for example, C. amaranticolor, so that viruses can be readily transmitted from C. quinoa to such inhibitor-sensitive