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Separation of strains of citrus tristeza virus (CTV), differentiated by their double-stranded RNA (dsRNA) profiles, was obtained by graft-inoculating citron plants from a Mexican lime that had been recently aphid-or graft-inoculated with a mild CTV isolate (T-385). Up to 24 sub-isolates with differing dsRNA profiles were obtained from the aphid-inoculated lime. Some of these sub-isolates induced stronger symptoms in several citrus species than the original T-385 isolate. One sub-isolate, T-385-33, was mild in Mexican lime, but induced stem pitting on sweet orange. Inoculation of this isolate on Mexican lime, sour orange and Eureka lemon induced mild or no symptoms when inoculum was taken from citron, but very severe symptoms when the inoculum was from sweet orange. Mexican lime and sweet orange plants co-inoculated with T-385-33 from sweet orange in combination with the other 23 sub-isolates showed mild symptoms. The results obtained suggest that there is natural cross-protection among sub-isolates in the original T-385 isolate. RESULTS dsRNA profilesA mild CTV isolate from the IVIA collection, T-385 (Moreno etal., 1991), was graft-inoculated to two citron plants, which were kept in the greenhouse. When these plants were periodically analysed for dsRNA content, variations in the electrophoretic profiles could be observed (Fig. 1).The original T-385 field isolate was aphidtransmitted to a Mexican lime plant and 2 months later, when the lime first reacted positively io CTV by ELISA, was graft-inoculated to 40 citron plants. When these citron plants were individually analysed 2 years after inoculation, up to 24 different dsRNA profiles could be identified (Fig, Plant Pathology 42,(48)(49)(50)(51)(52)(53)(54)(55)(56)(57)

Dredging of the Albufera Lake, a very important natural area of Eastern-Spain, has been proposed to remediate the silting, but a very large amount of sediments would be generated. To assess the feasibility of applying these to the sandy agricultural soils surrounding the lake, three rates (180, 360 and 720 tha -1 ) of four different sediments, corresponding to different degrees and sources of contamination, were tested by mixing with a soil obtained from the area of potential application. The effects on the soil properties and yield and nutrient contents of lettuce (Lactuca sativa L.) and tomato (Lycopersicon esculentum Mill.) were studied. As the most relevant changes, sediments improved the soil waterretention and cation-exchange capacity of the mixture, but increased its salinity and heavy metal contents.Yield of lettuce increased with parallel to the sediment applications whereas tomato growth and yield remained unaffected. Significant effects were also found on the nutrient contents of the plant tissues, depending on the sediment and application rate used, but no heavy metal accumulation in plants could be detected. According to the results, the application of appropriate rates of sediments to the agricultural soils surrounding the lake seems to be a sound practice to avoid problems arising from the disposal of large amounts of dredged material, and to improve the properties of the sandy soils of the area.