Infection with beet yellows virus depressed the dry‐matter yield of sugar‐beet plants by decreasing both leaf area and net assimilation rate (N.A.R.). It did not reduce the number of leaves. The lower N.A.R. of infected plants may occur because photosynthesis is slowed by chlorosis of the leaves or by changes associated with it. Plants infected at the end of June had 30‐50% of their leaf area yellowed from mid‐August. Later infection caused less yellowing. The yellowing almost sufficed to account for the decrease in N.A.R., if yellowed parts of leaves do not photosynthesize. However, the similar diurnal fluctuations of carbohydrate in the laminae of healthy and infected leaves suggests that photosynthesis may not be much slowed by infection; if so, the decrease in N.A.R. indicates a large increase in respiration rate, especially of the root. The effects of yellows on leaf area and N.A.R. appear to be independent; late‐sown plants suffered a greater reduction of leaf area, but a smaller reduction in N.A.R., than early‐sown plants; similarly treated plants showed the same reduction of leaf area in two successive years, but the decrease in N.A.R. was much greater in the second. Most of the loss of dry matter was in the root; the dry weight of the petiole (including stem tissue) was also decreased but the dry weight of leaf lamina was little affected. Yellows greatly increased the reducing sugars in the leaf lamina, and caused smaller increases in sucrose and starch. These effects appeared when the leaves showed only etch symptoms. The increase in starch was greatest at this time, but the effect on sugar content subsequently increased with the development of yellowing. Yellows reduced the water content, nitrogen content and the arealweight ratio of the leaf lamina. The change in nitrogen content, but not the others, could be accounted for by the rise in carbohydrate. Yellows had no effect on the water content of petiole and root; it increased the nitrogen content of these parts. It reduced the sucrose content of the root, but the loss of sugar yield was mainly attributable to decreased root weight. Infection with beet mosaic virus decreased the dry weight of sugar‐beet plants only when nitrogenous fertilizer was applied. As with yellows infection, the loss of dry matter resulted from reductions in both leaf area and N.A.R. Unlike yellows, mosaic depressed the dry weight of leaf lamina, as well as of petiole and root. Mosaic had no effect on carbohydrate content, water content or arealweight ratio of the leaf lamina, or on water content of petiole and root. It increased the nitrogen content of all parts of plants that received no nitrogenous fertilizer, but not of nitrogen‐treated plants. It slightly increased the sucrose content of the root. Infection with beet mosaic virus at the end of June caused a 20 yo loss of sugar yield from plants that received nitrogenous fertilizer. Yellows infection of similar plants at the same date reduced the sugar yield by 50%.

Multiple regression analysis of the data described in a previous paper identified alate Myzus persicae of the spring and summer migrations as the most important factor affecting spread of beet yellows virus in the sugar-beet root crop in England. Apterae of M. persicae spread little virus, and the contribution of alatae and apterae of Aphis fabae was negligible. A simple mathematical model of the spread of infection was developed. Assuming that the crop is visited by N aphids at a time when the proportion of plants infected is kO, the predicted proportion of infection for a time 3-4 weeks later (k,) is k l = k o +~o o (~-k o ) (I-e-Nz),

SUMMARYTwo isolates of groundnut rosette virus from East Africa (GRVE1 and GRVE2) and from West Africa (GRVW1 and GRVW2) were transmitted by Aphis craccivora obtained from West Africa. A third isolate from West Africa (GRVW3) was not transmitted by A. craccivora from three widely separated sources. GRVW1, GRVW2 and GRVW3 caused leaf‐symptoms in groundnut of a mosaic pattern in light and dark green. GRVE1 and GRVE2 caused chlorosis or chlorosis and leaf distortion as well as mosaic symptoms. Groundnut plants with GRVW1 could not be infected by means of aphids with GRVE1, and GRVE1 gave similar protection against GRVW1, which suggests that they are strains of the same virus.All isolates were transmissible manually from groundnut to groundnut (Arachis hypogea), Trifolium incarnatum and T. repens, and caused systemic infection. Inoculated Nicotiana clevelandii and N. rustica developed symptoms but virus could not be recovered from them. Chenopodium amaranticolor, C. hybridum and C. quinoa showed local lesions on inoculated leaves. Virus could be acquired by aphids from groundnut or Trifolium repens infected by means of aphids, but not from those infected by manual inoculation. Virus could not be recovered from T. incarnatum manually or by aphids, but was transmitted by cleft‐grafting from clover to groundnut.Saps extracted in borax buffer plus zinc sulphate at pH 9 from plants infected with GRVW1 and GRVE1 remained infective at 18° C. for 1 week, and at — 20° C. for up to 4 weeks. Virus could be recovered from frozen leaves. Buffered saps lost infectivity when heated above 50° C. for 10 min.; most were still infective when diluted 1/10 and some at 1/100.Electron micrographs of partially purified preparations contained spherical particles 25–28 mμ in diameter. There were usually only about five per microscope field and they resembled those of some other viruses.

The loss of total carbohydrate (sugars and starch) per cent of residual dry matter (dry matter less total carbohydrate) during a period of darkness from leaves of sugar-beet plants infected with yellows virus was as great as that from the leaves of healthy plants. T h e conclusion of previous workers, based on the results of the Sachs iodine test for starch and the occurrence of ' phloem gummosis ' in infected plants, that starch accumulates in infected leaves because translocation is prevented by blockage of the sieve-tubes, is therefore incorrect.Older leaves of infected plants had a higher content of reducing sugars and sucrose, and usually but not invariably of starch, both at the beginning and end of the dark period, than comparable leaves of healthy plants. By far the greater part of the increase was in reducing sugars. In leaves taken in late September from infected plants growing in the field, 20 yo or more of the total dry matter was present as reducing sugars. T h e reducing sugars in both healthy and yellows-infected leaves were shown by paper chromatography to be glucose and fructose in approximately equal amounts.Accumulation of carbohydrate in infected leaves is probably not a passive consequence of differences in carbohydrate production, distribution and utilization, but is attributable to changes in the physiology of the cells of the leaf.The carbohydrate content of sugar-beet leaves was little affected by infection with beet mosaic virus.Yellows-infected leaves had a lower water content per cent of fresh weight than healthy leaves. This was accounted for by the higher carbohydrate content of infected leaves, for the ratio of water:residual dry matter was not affected by infection or was slightly reduced. This implies that hydration was independent of carbohydrate content. * Throughout h s paper 'healthy' is used to signify 'not infected with virus'.

A survey of aphids and virus diseases of sugar-beet root crops in eastern England was made between 1940 and 1948. Prior to 1943 the observations were made on fertilizer experiments; from 1943 onwards they were made on commercial fields selected for position in relation to beet and mangold seed crops. The incidence of beet yellows increased with increasing numbers of Myzus persicae, but not of Aphis fabae. The relation with M . persicae was sufficiently close to suggest that it is the most important, possibly the only important, vector of beet yellows virus.Beet mosaic virus also increased with increasing numbers of M. persicae, but the relation was not close enough to exclude the possibility of other vectors.Numbers of A. fabae on sugar beet were slightly, but consistently, depressed by the use of salt as a fertilizer. Other fertilizers had variable effects. Neither aphids nor virus are likely to be greatly affected by fertilizers. Beet yellows is most prevalent in areas where seed crops are grown, but within these areas nearness to individual seed crops did not appear to increase its incidence.M . persicae were more numerous on sugar beet in seed-crop areas than elsewhere, and this alone might account for the prevalence of yellows. Beet mosaic virus is more closely associated with seed crops than is beet yellows. It is most prevalent near to seed crops within the seed-crop areas.

S U M M A R YCarrots that show symptoms of carrot motley dwarf contain two viruses, carrot mottle virus (CMV) and red-leaf virus (RLV). CMV cannot be manually inoculated to carrot, but can be to some other members of the Umbelliferae, as well as to some species of the Solanaceae, Leguminoseae and Chenopodiaceae. T h e host range of RLV is limited to the Umbelliferae, and it is not manually transmissible but was transmitted by grafting. Cavariella aegopodiae Scop. transmits RLV alone but will transmit CMV only from plants infected with both viruses. Thus aphids were unable to transmit from coriander plants manually inoculated with CMV, but after these plants were infected with RLV by aphids, virus-free aphids acquired and transmitted both viruses from them.Aphids remain infective with both viruses for 1-2 weeks, and retain infectivity through the moult. A minimum total of about 9 hr. is needed for acquisition and transmission ; vector-efficiency increases with increasing feeding times up to several days.T h e viruses causing motley dwarf become attenuated in the glasshouse after continued aphid-transmission ; avirulent isolates protect their hosts against infection by virulent ones, Infectivity of saps from CMV infected plants.was increased by extraction at high p H in the presence of a trace of Zn, and was associated in carrot with particles 30 m p in diameter. Water-phenol extracts are almost as infective as extracts in buffer, but are inactivated by 0.02 ,ug./l. pancreatic ribonuclease.Parsnip mottle virus (PMV) resembles CMV in many ways, but also differs in some important respects. Unlike CMV it infects celery and parsnip, and it is transmitted by C. pastinacae as well as C. aegopodiae, the only vector of CMV (in combination with RLV). PMV is transmitted by aphids from plants infected with it alone, whether these plants were infected by aphids or by manual inoculation.Carrot plants infected first with PMV and then with the motley dwarf virus complex developed symptoms of motley dwarf, but in coriander the reverse happened. CMV and PMV appear to interfere with each other's multiplication in the hosts, but CMV is dominant in carrot and PMV in coriander. They have some properties of distantly related strains.

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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.