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Notes: Isolate DX of potato virus X (PVX) caused the typical reactions of a group three strain, systemic top necrosis in presence of potato hypersensitivity gene Nx and mosaic with gene Nb. When tubers harvested from DX-infected plants of the Nx: Nb cultivars Maris Piper and Pentland Dell were, grown on, most progeny plants were healthy, but some had systemic top necrosis caused by DX while others had mosaic symptoms in upper leaves and severe necrosis in lower ones. Infective sap from the plants with mosaic and necrosis always caused similar symptoms when inoculated to Pentland Dell. Only group four PVX strains can cause systemic infection without top necrosis in Nx:Nb cultivars, so the affected plants all contained a group four strain. That the severe necrosis of their lower leaves resulted from a shock reaction rather than from isolate DX also being present was indicated because the plants partially recovered, inoculations from near to the dilution end-point failed to demonstrate any separation of strains and behaviour on back-testing to Pentland Dell was unchanged after passage twice through tubers of this cultivar. Also, although DX caused severe necrotic symptoms in tubers of both Nx: Nb cultivars, these symptoms did not develop in tubers of plants with mosaic and necrosis. When cultured serially in Nicotiana glutinosa, the group four strain eventually reverted to group three.

Notes: Serological tests were made using a simple kit consisting of dropper bottles filled with latex sera or buffer, a plastic mixing plate, plastic pipettes, wooden cocktail sticks, a black glass reaction plate and polythene bags, packed in a plastic sandwich box. Potato viruses X, S and Y were readily detected in infected potato leaves in tests done both in the laboratory and in the field. Results were obtained in 4–10 min.

Notes: Since the mid 1950s, there have been many developments in serological techniques for testing for plant viruses. Certain of these can provide great savings in time, labour and cost in routine testing situations and are well suited for use by advisory services, certification schemes and healthy stock programmes, statutory and quarantine authorities, and plant breeders. The recent developments are described under the headings flocculation in liquid media, gel diffusion, labelled antibodies and electron microscope serology, Flocculation tests depend on observation of aggregates formed in a liquid medium. The microprecipitin and chloropiast agglutination tests are simple forms which are still widely employed routinely. However, sensitivity is greatly improved by using flocculation tests in which antibodies are adsorbed to the surface of larger inert carrier particles, such as polystyrene latex spheres, tanned red blood cells or bentonite. Latex particles are the most widely used of these because they are easy to sensitise with antibodies, little antiserum is required and the sensitised latex can be stored for years without loss of activity. The latex test is also very simple to do, is well suited for routine checks on either small or large numbers of samples which can be grouped and can be used with many virus–crop combinations.Gel diffusion tests depend on observation of precipitin lines formed in an agar gel medium. They are of two main types, single (= simple) diffusion and double diffusion. Both can be used effectively with viruses which have isometric or ‘near isometric’ particles but not with those which have elongated particles because these do not diffuse readily through agar. Recent developments which involve breaking elongated particles into sub-units or fragments which can diffuse through agar now permit gel diffusion to be applied with almost all viruses. Various treatments are useful in breaking particles, the most effective of which employ detergents or other disruptive chemicals such as pyridine, pyrrolidine and ethanolamine, However, although gel diffusion tests which incorporate one or other of these treatments have been employed routinely for testing for viruses in several different crops, such tests are relatively insensitive, require much antiserum and conditions must be carefully controlled to avoid formation of nonspecific precipitin lines.Antibodies can be labelled to make virus—antibody aggregates readily observable, or to obtain increased sensitivity in testing, or both. The only form of labelled antibody test widely employed in routine screening for plant viruses is enzyme-linked immunosorbent assay (ELISA), in which antibody is labelled with an enzyme and positive results are observed as an enzyme-mediated colour reaction. Since its introduction to plant virology in 1976, ELISA has been very widely applied in situations where large numbers of either individual or grouped

Notes: Controlled environment cabinets were used to determine light.and temperature conditions needed for full expression of potato leaf roll virus (PERV) symptoms in potato plants grown from eyeplugs taken from tubers of cv. Désirée. PLRV symptom development was poor under conditions in which growth was etiolated. In general, typical rolling of lower leaves developed faster and was more distinct at 18 than at 22°C, and when light intensity was increased, but the two continuous light regimes used gave better results than any of those involving 10-or 18-h photoperiods. At 18°C, distinct symptoms suitable for scoring during tuber indexing developed in all infected plants after four to five weeks or five to six weeks with continuous light at 7000 or 4000 lux respectively, but only after six to eight weeks with 18 h/day at 7000 lux, which was the best of the 10- and 18-h regimes. Yellowing and shedding of lower leaves developed faster in healthy plants kept under either of the continuous light regimes than in healthy plants kept for 18 h/day at 7000 lux or in infected plants kept in any of these three regimes, and was very obvious after only six weeks. In terms of symptom appearance in infected plants and defoliation m. healthy ones, a regime of 23 h/day at 7000 lux gave similar results to those obtained with continuous light at 7000 lux. In trials done in winter in a heated glasshouse illuminatcd by mercury vapour lights, continuous illumination had the same effects: distinct PLRV symptoms and obvious loss of lower leaves devefoped after only five weeks in all the infected and healthy plants respectively.