A distinctive strain of tobacco necrosis virus (TNV) of unknown source was repeatedly isolated from water of the River Avon (Warwickshire) and two of its tributaries (R. Swift and R. Alne) using a technique developed for the concentration and isolation of water-borne bacteriophages. The same strain was isolated from the rivers Cam and Thames and from Lake Esthwaite (Cumbria) together with tomato bushy stunt virus.The TNV strain, designated Chenopodium necrosis (TNV-CN) was mechanically transmissible to C. amaranticolor and C. quinoa in both of which it caused local lesions and systemic infection. TNV-CN caused no infection when inoculated to tobacco (Nicotiana tabacum cv. White Burley) plants.The virus was not adsorbed to soil, could be isolated from leachate of soil in which systemically infected C. quinoa were grown and C. quinoa plants became infected when grown in soil watered with suspensions of the virus.The virus was not transmitted by Myzus persicae but was vectored by the zoospores of a lettuce isolate of Olpidium brassicae.TNV-CN was infective after 10 min at 85 "C, 3 wk at 20 OC and when diluted to lov8 but not Purified virus preparations contained c. 26 nm isometric virus particles.TNV-CN contained single-stranded RNA (mol. wt 1.5 x lo6) and one protein (mol. wt c. 26-4 x lo3) which co-electrophoresed in polyacrylamide gels with the protein of the D strain of TNV (TNV-D). Analytical centrifugation of TNV-CN indicated a single component virus with the same sedimentation coefficient = 115s) and buoyant density (1.385) in a CsCl gradient as those of TNV-D.TNV-CN and TNV-D were indistinguishable serologically.

SUMMARY Lettuce big‐vein disease is transmitted from diseased to healthy plants by zoospores of the lettuce root‐infecting fungus Olpidium brassicae. A laboratory technique based on microscopical examination of Olpidium Zoospores is described for assaying the toxicity of chemicals to zoospores. Chemcials found to kill zoospores in <1 h included copper (4 μ/ml), zinc (10μ/ml), diluted preparations of carbendazim (methyl‐2‐yl‐benzimidazole carbamate) as Bavistinand a formulation of Bavisitin containing no carbendazim. Bavistn controlled the disease when introduced at a concentration of 0.6 g/litre into a lettuce crop grown in a re‐circulated film of nutrient. Various surfactants inlcuding Agral, Cetrimide, Deciquam, Ethylan CPX, Hyamine 1622, Manoxol/OT and sodium lauryl sulphate were toxic to zoospores at concentrations of 1–10 μ/ml.

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SUMMARY Tomato bushy stunt virus (TBSV) of unknown source was isolated from water of the River Thames, near Oxford. The isolate designated TBSV‐T was mechanically transmissible to several tomato (Lycopersicon esculentum) cvs and to other species including Petunia hybrida, pepper (Capsicum annuum). eggplant (Solanum melongena), Nicotiana clevelandii, Chenopodium amaranticolor and C. quinoa in which it caused systemic symptoms. It caused no infection of globe artichoke (Cynara scolymus) or Pelargonium domesticum. The virus was not adsorbed to soil and could be isolated from leachate of soil in which systemically‐infected tomato or C. quinoa plants were grown. Tomato plants became infected when grown in soil watered with virus suspensions. TBSV‐T was infective after 10 min at 80°C but not at 90°C and when diluted to 10‐5 but not to 10‐6. Purified virus preparations contained C. 30 nm isometric particles. In gel‐diffusion serological tests, TBSV‐T reacted with homologous anti‐serum and with antiserum to petunia asteroid mosaic virus but not to pelargonium leaf curl virus. Seed‐borne infection (50–65%) of TBSV was demonstrated in plants grown from seed of symptomlessly‐infected tomato fruit. TBSV was isolated from symptomlessly‐infected tomato fruit imported from Morocco during October‐April 1981. One of the isolates (TBSV‐M) was indistinguishable from TBSV‐T in host range, symptomatology and serological reactions. TBSV was also found in tomato plants growing extraneously in primary settlement beds at sewage works; such plants having been derived from undigested seeds in sewage. Because of its ‘alimentary‐resistance’ in man, it is possible that one ecological route whereby TBSV enters rivers is by man's consumption of TBSV‐infected tomatoes and eventual sewage dispersal into rivers.

Carbendazim applied at the rate of z g per plant to the roots of tobacco (Nicotiana tabacum cv. White Burley) plants before infection with tobacco mosaic virus (TMV) caused very considerable reduction in the severity of disease symptoms in systemically infected leaves but did not affect their virus content. Leaves of untreated, infected plants had a greatly reduced chlorophyll content IOO days after infection whereas the chlorophyll content of leaves of infected plants treated with carbendazim was similar to that of normal uninfected leaves. Carbendazim had no effect on the infectivity of TMV in vitro or on the local lesion reaction of N . glutinosa plants when inoculated with TMV.Carbendazim was applied to lettuce cv. Cobham Green at a total rate of 0-1 g per plant before and after they were infected with beet western yellows virus and the plants were then grown on in the field. At harvest time (50 days after infection) almost all the treated virus-infected plants were of a normal green appearance, whereas the untreated controls were almost all very severely yellowed and unmarketable.