One of the putative products has significant sequence similarity with viral movement proteins. None of the putative proteins encoded by GRV RNA seems to be a structural protein. In genome organization and in the amino acid sequences of its potential products, the RNA of GRV is similar to that of carrot mottle mimic umbravirus, and to the umbravirus-like RNA-2 of pea enation mosaic virus.

Groundnut plants with symptoms of rosette disease contain groundnut rosette virus (GRV), but GRV is transmitted by Aphis craccivora only from plants that also contain groundnut rosette assistor virus (GRAV). Two main forms of rosette disease are recognised, 'chlorotic rosette' and 'green rosette'. GRV cultures invariably possess a satellite RNA and this is the major cause of rosette symptoms: satellite-free isolates derived from GRV cultures from Nigerian plants with chlorotic or green rosette, or from Malawian plants with chlorotic rosette, induced no symptoms, or only transient mild mottle or interveinal yellowing, in groundnut. When the satellite RNA species from GRV cultures from Nigerian green or Malawian chlorotic rosette were reintroduced into the three satellite-free isolates in homologous and heterologous combinations, the ability to induce rosette symptoms was restored and the type of rosette induced was that of the cultures from which the satellite RNA was derived. Thus different forms of the satellite are responsible for the different forms of rosette disease. Other forms of the satellite induce only mild chlorosis or mottle symptoms in groundnut. Individual plants may contain more than one form of the satellite, and variations in their relative predominance are suggested to account for the variable symptoms (ranging from overall yellowing to mosaic) seen in some plants graft-inoculated with chlorotic rosette.

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Transmission through seed of crop and weed plants seems to be characteristic of nematode-borne viruses. It occurred with tomato black ring virus (TBRV) in nineteen species (thirteen botanical families), with arabis mosaic virus (AMV) in thirteen species (eleven families), with raspberry ringspot virus (RRV) in six species (five families), and also, in more limited tests, with tomato ringspot, cherry leaf roll and tobacco rattle viruses. A remarkable feature was that infected seedlings, except those containing tobacco rattle virus, often appeared healthy. The occurrence and extent of seed-transmission depended on both the virus and the host plant. In many progenies more than IO%, and in some I O O~~, of seedlings were infected. The viruses were transmitted through at least two or three generations of seed of those host species tested. After 6 years' storage, TBRV-and RRV-containing seed of Capsella bursa-pastoris and Stellaria media germinated to give infected seedlings.In controlled crossing experiments with strawberry and raspberry, virus was transmitted to seed from both male and female parents but, at least in raspberry, the presence of competing virus-free pollen much decreased the ability of pollen from infected plants to set seed. There was no evidence that healthy mother plants became infected when their flowers were pollinated with infected pollen.

S U M M A R YRaspberry bushy dwarf virus (RBDV) was transmitted to raspberry seed both through the pollen and through the ovule and it infected plants pollinated with infected pollen. It did not infect plants prevented from