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Maize streak virus (MSV) was purified by homogenising infected leaf tissue in 0.01 M pH 3.9 phosphate buffer and clarifying the extract with n-butanol (7 ml/Ioo ml extract). Purified preparations contained particles 20 nm in diameter, some occurring singly, but most occurring in pairs, forming structures of 30 x 20 nm. The sedimentation coefficients of single (R)/I : *I*: S/S: S/Au.

A virus obtained from sweet potatoes in Kenya, Uganda and Tanzania was transmitted by inoculation of sap and by whiteflies (Bemisia tabaci).It infected forty-five of I 19 plant species in fourteen of thirty-six plant families. It was propagated in Nicotiana glutinosa and N . tabacum, in which diagnostic symptoms of vein clearing, leaf curling and distortion developed. Chenopodium quinoa was a good local lesion host.Different seedling lines of sweet potato differed greatly in their susceptibility to infection and in symptoms produced; some developed leaf mottling and were stunted, some were symptomless, and some appeared immune.The virus was transmitted by dodder (Cuscuta campestris) but not by aphids, or through seed of Ipomoea nil or N . clevelandii. Sweet potato sap contained strong inhibitors of infection, and a low concentration of virus.Virus-free cuttings of sweet potato were obtained by thermotherapy (4-5 wk at 35 "C), or by meristem-tip culture. The virus remained infective in sap of N . tabacum after dilution to 10-3, or after 10 min at 55 "C (but not 60 "C), 3 but not 7 days at 18 "C, or 42 but not 49 days at 2 "C. Infectivity was abolished by sonication or U.V.irradiation, by 2 % formaldehyde or 2 % tri-sodium orthophosphate, and was greatly decreased by 20% CHCl, or 20% ether. Purified virus preparations were obtained from N . tabacum by clarifying phosphate buffer extracts with n-butanol, virus precipitation with polyethylene glycol, and differential centrifugation. The virus sedimented as one band in density gradients, and produced a single sedimenting boundary in analytical centrifugation (sozo, 7 155s). It contained one polypeptide species of mol wt 37 700, and preliminary digestion experiments suggested a single-stranded RNA.Antisera prepared against the virus reacted specifically in precipitin tube tests with titres of 1/16 384, but no serological relationships could be found between the virus and fourteen viruses of the potato virus Y group.Electron micrographs showed straight, filamentous particles c. 950 nm

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.