The effect of alfalfa mosaic virus (A1MV) on lucerne forage yield was investigated in experimental plots in which all plants were indexed for freedom from seed‐borne A1MV. Plants which were artificially inoculated with A1MV were tested for successful infection prior to transfer to the plots. Inoculated plots yielded 14‐8‐22‐8 (fresh weight) and 15.0 18.1 (dry weight) less than uninoculated plots. Isolate A1MV‐I reduced the yield of five lucerne cultivars and yield was decreased by several A1MV isolates in cv. Vertus. Yield decreases tended to be greatest at the first harvest and progressively smaller at the second and third. Despite aphicide treatments and a cereal barrier crop, ELISA indexing showed natural infection developed in control plots. Most known infected plants failed to produce symptoms and yield decreases were associated with predominantly symptomless
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