Subterranean clover (Trifolium subterraneum) is grown as a pasture legume in several temperate regions of the world where the soils are acidic and infertile, and the rainfall is winter dominant and less than 600 mm annually. It is particularly important in southern Australia where more than 16 million ha have been sown with this species as the pasture legume component.Nine viruses have been recorded infecting subterranean clover in the field. These are alfalfa mosaic, bean yellow mosaic (pea mosaic), beet western yellows, clover yellow vein, cucumber mosaic, pea enation mosaic, soybean dwarf (subterranean clover red leaf), subterranean clover mottle and white clover mosaic. In addition there is an important problem referred to as subterranean clover stunt that was assumed to be caused by a virus but whose aetiology is still unknown. The importance of these diseases is reviewed and details on their epidemiologies are outlined together with details on progress towards their control and some comments on matters worthy of attention in the future. Reference is also made to several exotic viruses known to infect subterranean clover experimentally that could possible cause problems if introduced into Australia.

Samples of 2077 grasses and cereal plants representative of the Poaceae found in Tasmania were collected from a wide range of habitats throughout the State. Each sample was examined for infestation with aphids and then checked for infection with viruses causing barley yellow dwarf by both aphid transmission and serological tests. Aphid species found among the samples were Hyalopterus pruni, Rhopalosiphum maidis, R. padi and Sitobion fragariae. R. padi transmitted a vector non-specific type of barley yellow dwarf (PAV) and a vector specific type (RPV), either alone or together, while S. fragariae transmitted PAV alone and occasionally RPV when present in plants together with PAV. The other aphid species did not transmit. A total of 189 samples contained virus. Incidence was greatest in samples from the Bambusoideae subfamily (31%) and least in the Arundinoideae (4%). There was no difference in the level of infection between the native and introduced species that were represented. The Arundinoideae and Panicoideae were predominantly infected with RPV types, while the Pooideae, with the exception of Dactylis glomerata and Poa pratensis, were predominantly infected with PAV types. Many more infected plants contained both PAV and RPV (11.9%) than would have been expected had the two types of virus infected independently (2.5%). No infected plants were found among samples from 25 of the 56 species tested, and some of these may prove useful in breeding for resistance to barley yellow dwarf viruses.

A range of crop plants, pasture legumes and weeds, mostly with yellows symptoms similar to those caused by luteoviruses, were collected from the field around Tasmania and checked for infection with beet western yellows virus (BWYV) and subterranean clover red leaf virus (SCRLV) using aphids and indicator plants. BWYV was recovered from 216 of 897 plants tested, representing 30 different species and including 16 not previously recorded as natural hosts. SCRLV was recovered from 163 of 637 plants, representing twelve species including four not previously recorded as natural hosts. BWYV was isolated most often from composites and crucifers, while SCRLV was recovered most frequently from legumes. Eight plants were found infected with both viruses together. In host range studies, Tasmanian isolates of BWYV caused symptoms in lettuce, subterranean clover and sugar beet like those seen on these plants in the field from which the virus was isolated, and were thus similar to isolates of BWYV from North America. The Tasmanian isolates of BWYV were also closely related serologically to Californian isolates of BWYV; these were serologically quite distinct from isolates of SCRLV, and both these groups were serologically distinct from legume yellows virus and from potato leaf roll virus.

A leaf-roll disease of broad bean, similar to that induced by bean leaf roll virus (BLRV) in Europe, is common in Tasmania. Subterranean clover red leaf virus (SCRLV) was transmitted to subterranean clover test seedlings by using its most efficient vector, Aulacorthum solani (Kalt.), from 84% of 204 randomly selected affected broad bean plants. The disease was reproduced in broad bean with SCRLV in controlled aphid transmission tests. Effects of infection on yield were severe, as further pod set was markedly reduced after symptoms of infection developed. There was some variation among cultivars in their response to infection. The virus occurred as commonly in green pea crops as in broad bean. In pea it caused a top yellowing, but most commercially grown pea cultivars had some tolerance. Subterranean clover stunt virus (SCSV) caused symptoms in broad bean and pea which were also similar to those induced by SCRLV. However, SCSV was rarely found infecting plants in Tasmanian pea and bean crops. BLRV, SCRLV and SCSV share many properties in common and with some other viruses.

Seventy one seed-lines representing 23 species of papilionoid legumes and 17 species of nonlegumes were collected and distributed to four countries; Australia, New Zealand, the Netherlands and the U.S.A. In each of these countries plants were grown from the seeds and their suscept~ility to a total of eight persistent aphid isolates transmitted viruses was assessed. The viruses were a strain of beet western yellows (BWYV) from Glycine maxin Illinois, legume yellows (LYV) in California and a virus in Michigan (MiAV) from Medicago sativa, from Pisum sativum causing leaf roll and top yellows in New Zealand (PeLRV-NZ) and the Netherlands (BLRV), isolates of subterranean clover red leaf from New Zealand (SCRLV-NZ) and Tasmania (SCRLV-T); and Subterranean clover stunt (SCSV) from Tasmania.The relationships between the eight viruses as indicated by their host reactions were assessed using computer classification techniques. SCRLV-NZ and SCRLV-T were the most similar. They had moderately wide host ranges that included some non-legumes. A second group comprized BWYV and PeLRV-NZ. These were typical of most beet western yellows virus strains in that they infected Brassica napus, Capsella bursa-pastoris and Stellaria media. MiAV and BLRV also formed a pair. They generally induced severe symptoms on the hosts which they infected and had host ranges confined to legumes except that BLRV also infected Claytonia perfoliata and Erodium spp. The relationships of LYV and SCSV were not consistent. They paired together in some classifications, but SCSV sometimes grouped with the SCRLV isolates. Both had host ranges confined to legumes, caused severe symptoms in most hosts and were often difficult to recover from affected plants. LYV had some affinities with BLRV and MiAV.The tests indicated a set of test plants which were most useful for propagating and identifying persistent aphid-transmitted viruses from legumes. Two, P. sativum cv. Onyx and Trifolium subterraneum cv. Bacchus Marsh were susceptible to all isolates. Ten others distinguished between t]he isolates and were Arachis hypogea, Beta vulgaris, C. bursa-pastoris, G. max cv.