A virus that causes chlorotic streaks on ryegrass leaves was transmitted by the eriophyid mite Abacarus hystrix (Nalepa). Virus‐free mites acquired the virus in 2 hr. feeding on infected ryegrass and the proportion that became infective increased with increased feeding time up to 12 hr.; vectors lost infectivity within 24 hr. of leaving the infected leaves. All instars of A. hystrix transmitted the virus.The virus was transmitted by manual inoculation of sap to other species of Gramineae, including oats, rice, cocksfoot and meadow fescue, but none of these hosts seemed to contain as much virus as ryegrass; their saps did not precipitate specifically with antiserum prepared against the virus in ryegrass, whereas sap from infected ryegrass precipitated up to a dilution of 1/32. Infective sap of S22 Italian ryegrass contained flexuous rod‐shaped particles; the dilution end‐point of the virus was about 1 in 1000; the virus was inactivated when held for 10 min. at 60°C. and most of its infectivity was lost after 24 hr. at room temperature.

Comparisons were made of the host ranges, interactions in infected plants, and effects on yields of cereals of two isolates of barley yellow‐dwarf virus, one avirulent, RV, obtained from Rothamsted farm (Watson & Mulligan, 1956) and the other virulent, KV, obtained from Kent. They resembled each other and the American yellow‐dwarf viruses in their ability to infect wild grasses but differences were found when the infection tests were made using inbred lines of the same grass species. KV infected one variety each of rice, rye and maize, and caused symptoms in each.When plants were first infected with RV and later, when symptoms had fully developed, with KV, they suffered the same loss of yield from KV as did plants infected for the same length of time with KV alone. Therefore plants infected with RV were not protected against infection with KV. Similarly, aphids (Rhopalosiphum padi L.), fed first on sources of RV and then on KV, transmitted mainly KV so there was no evidence of protection in the insect vectors.Effects on yield of cereals were related to the time of appearance and intensity of symptoms. The effect of RV was not only less than that of KV but was also more variable.

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Some barley yellow-dwarf (BYD) viruses isolated from cereal crops in Great Britainwere transmitted by Rhopalosiphum padi, L. and others were not. Sitobion fragariae (Walker), S. avenue (Fabricius), and Metopolophiurn dirhodum (Walker) all transmitted viruses of both types, but they usually transmitted those of which Rhopalosiphum was a vector less readily than did R. padi. The transmissibility of a virus by a given aphid species was not affected by transmission with another, less efficient, vector species. Neomyzus circumj?exw (Buckt.) and Rhopaloriphum maidis (Fitch) transmitted the few viruses with which they were tested.A few R . padi acquired virus from infected leaves during 3 0 min. feeding and inoculated healthy seedlings during 15 min. feeding, but the minimum total time taken to acquire and transmit was 10 hr. and 32 hr. were needed for about half the aphids that were able to acquire and transmit virus to do so. This may indicate the existence of a short latent period of the virus int the vector, although the evidence is not conclusive. The times spent on infected plants influenced the results more than those spent on healthy ones; many transmissions occurred with short feeding times on healthy plants so long as the time spent on infected leaves was long, but the reverse was not true. Nymphs of R. padi that moulted after they left infected plants on which they fed long enough to become infective, infected slightly fewer plants than adults fed for the same times.Watson & Mulligan (1960) compared the host ranges of two barley yellow-dwarf viruses, one virulent (KV), the other avirulent (RV) and described how they interact in host plants and vectors and how they affect yields of cereals. The present paper compares the behaviour of the same two viruses when transmitted in varying conditions by Rhopalosiphum padi L. and describes the transmission of other British isolates of BYDV by different species of aphids. MATERIALS AND METHODSThe viruses were isolated from plants or leaves collected in the field, by feeding virus-free aphids of one or more species first on the infected leaves and then on healthy test seedlings of Blenda oat. The methods of handling the aphids and maintaining them in virus-free culture have previously been described (Watson & Mulligan, 1960). When a virus was being tested with two different aphids halves of the same infected leaves were used as sources of virus (Rochow, 19583).The viruses are distinguished in this paper by the district from which they came.Two isolates from Rothamsted farm are called RV 54 and RV 59, the years in which they were isolated. Other isolates were from Kent (KV); Harlech, North Wales (HV); Bristol (BV); Hexton, nr. Luton, Beds (XV); Woburn Experimental Farm, Beds (WV). 45-2