In July 1999, leaf mosaic and distortions were observed on watercress in the region of Camp de Masque, in the eastern part of Mauritius. Electron microscopy of crude sap preparations revealed the presence of 720 nm, flexuous filamentous particles. Virus detection by reverse-transcription polymerase chain reaction (RT-PCR) and virus sequencing by A. Mackenzie (Research School of Biological Sciences, Australian National University) confirmed the identity of the causal pathogen as Turnip mosaic virus (TuMV) in November 1999. A survey was initiated in January 2000, covering the 44 major watercress ponds across the island. Two hundred seventy-five samples (231 symptomatic; 44 symptomless) were collected from 22 localities. TuMV was detected by double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) using commercial antisera (Agdia Inc.). Plant extracts were prepared by cutting approximately 20 g of leaf samples into small pieces, from which 1.0 to 1.5 g were used in the evaluation. Eighty-one percent of symptomatic samples (187 out of 231) were TuMV positive; all of the symptomless samples were TuMV negative. Symptoms on infected watercress included leaf mosaic, mottling, distortions, general yellowing, and plant stunting. TuMV has since been detected on all three commonly grown watercress varieties in Mauritius: Brède Doux, Brède Blanc, and Constance. Under local conditions, TuMV affects the quality and thus the commercial value of the crop. Additional hosts of TuMV among local brassicas are also being determined, and to date the virus has been detected in turnip (Brassica campestris spp. rapa), pak choi (Brassica campestris spp. chinensis), and petsai (Brassica campestris spp. pekinensis).
