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SUMMARY Cassava latent virus (CLV) is almost entirely confined in East Africa to upland cassava‐growing areas west of the Rift Valley, where it is often associated with cassava mosaic disease (it was isolated from 27 of 38 cassava plants with mosaic, but not from 24 without mosaic). However, it is not the causal agent, because it was not recovered from any of 31 mosaic‐diseased plants in coastal districts. All attempts to return CLV to cassava failed. The host range of CLV appears to be limited to Euphorbiaceae (Manihot) and Solanaceae (Nicotiana, Datura, Nicandra, Solanum). N. clevelandii proved the most useful assay and propagation host. The dilution end‐point of CLV was about 10‐3, thermal inactivation point about 55°C, and longevity in vitro about 3 days. CLV was purified by clarification of leaf extracts with butanol/chloroform mixtures. Purified preparations (A 260/A 280 ratio c. 16) contained numerous 30 20 nm paired particles with a sedimentation coefficient (s20w) of 76 S. Treatment with RNase and DNase showed that the viral nucleic acid is DNA; CLV closely resembles maize streak virus but is not related to it serologically. The cryptogram for CLV is D/1: 0.8/*: S/S: S/*, geminivirus group.

SUMMARY Viruses occurring in Cassia bicapsularis in Northern Tanzania, in Voandzeia subterranea in north western and eastern Tanzania, and in Phaseolus lunatus in the Kenya highlands, were all serologically related to peanut mottle virus. Their host ranges, and the symptoms they induced in test plants, were very similar, and they differed only in degree of virulence in some host species. The Voandzeia isolate did not infect groundnut, and only the Phaseolus isolate infected two species in the Cucurbitaceae. All the isolates infected Chenopodium amaranticolor, a species which formerly was reported as being immune to peanut mottle and thus considered of diagnostic value. In Africa, variation in peanut mottle virus isolates seems to be associated with host species and ecology, and there is at present no evidence for naturally occurring variants within a host species as occurs in groundnut in America. Three of the four isolates were purified by homogenising together infected leaf tissue, chloroform and 0.5 M sodium citrate buffer containing 1% 2‐mercapto‐ethanol at pH 8, in the proportion 1: 1:2 respectively, and precipitating the virus from the clarified homogenate with 5% w/v polyethylene glycol. When centrifuged in sucrose density gradients such preparations gave a single, bright specific light scat‐tering zone with no haze.

Polyacrylamide gel electrophoresis indicated that maize streak (MSV) and cassava latent (CLV) viruses each contain one species of protein and two of RNA. The estimated protein mol. wt is 28000 for MSV and 34000 for CLV. The mol. wts obtained for the two RNA species using formamide-containing gels were the same for the two viruses: 1-7 X 10(6) and 1-3 X 10(6). It is suggested that the viruses have a two-part genome and that the tendency of their nucleoprotein particles to form pairs favours the delivery of complete genomes to sites of infection.

Clitoria yellow vein virus (CYW) was found in Clitoria ternatea andAbrus precatorius in coastal districts of Kenya, but was not detected in food legume crops. When transmitted by inoculation of sap, C Y W infected many species in the Papilionaceae, commonly causing yellowing of secondary and smaller leaf veins. All the economically important food legumes grown in the area of occurrence were very susceptible, so that CYW is potentially very important. The virus also infected okra (HzXwus aculentus) and species in the Solanaceae, but none of many species of Cucurbitaceae. C Y W is serologically closely related to cocoa yellow mosaic and kennedya yellow mottle viruses, and more distantly to okra mosaic and desmodium yellow mottle viruses. Other properties of C Y W typical of the tymoviruses include particle morphology (particle diameter c. 28 nm; two components) with sedimentation coefficients of 50s (top) and 19 S (bottom); molecular weight of protein sub-units c. 20000; thermal inactivation point c. 72 "C; and longevity in vitro c. 3 wk. I 0 0 Insusceptible: Capsicum annuum, C. frutescens, Datura fmox, D. stranooniwn, Lycopersicon escuhtum, N i c d a physabdes, N i c o t k a glutinosa, N . tabacum, Petunia hybrida, Physalis florihna, Solanum melongena. hirsutum. Properties in vitro clevelandii retained infectivity at IO-' , but not IO-*. 70 "C for 10 min, and abolished after 10 min at 72 "C. Dilution end-point. When diluted with distilled water, crude infective sap of N. Thermal inactivation point. Infectivity was much decreased when sap was heated at Longevity in vitro. Infectivity was lost after 3 wk at 18-20 "C. Physico-chemical properths Electron microscopy. Electron micrographs of both leaf-dip and purified preparations (Plate, fig. 7) of C Y W showed isometric particles c. 28 nm diameter, some penetrated by negative stain ('empty' particles), and some not ('full' particles). This is characteristic of the tymoviruses, in which these particles are referred to as top and bottom components respectively. Density gradient centrifugation. Two specific light-scattering zones were produced in density gradients, 6 mm (top component) and 13 mm (bottom component) below the meniscus. The ultraviolet absorption spectrum of the bottom component was typical of a nucleoprotein, with a maximum at 260 nm and a minimum at 239 nm. The 260 : 280 absorption ratio for the bottom component was 1-69 ; the Emax./Emin. ratio was 1-46. Molecular weight of protean subunits. Polyacrylamide gel electrophoresis indicated the molecular weight of the CYW protein subunits to be c. ~O O O O .Analytical ultracentrifugation. The schlieren pattern of an unfractionated preparation of C Y W (Plate, fig. 8) showed two components with sedimentation coefficients of 109s (bottom component) and 50s (top component). Serological relationships.In gel diffusion tests, purified CYVV reacted with homologous antiserum to a dilution of 1/512. Morphology and sedimentation coefficients of the two components of CYW suggested it may be a tymovirus (Harrison et al. ...