S U M M A R YDahlia mosaic virus (DMV) infected twenty-five of the eighty-five plant species from four of eighteen families inoculated, but only dahlias were found naturally infected. DMV infected fourteen members of the Solanaceae, Amaranthaceae and Chenopodiaceae, and eleven of twenty-nine Compositae. Verbesina encelioides was the best plant for diagnosis, assay and source of virus. Systemically infected hosts contained ovoid intracellular inclusions 2.5-10 pm in diameter which were shown by electron microscopy to consist of a finely granular, vacuolated matrix containing numerous virus particles. V. encelioides sap was sometimes infective after dilution to 1/2000 but not 1/3000, after heating for 10 min to 75 "C but not 80 "C, and after 4 days at 18 "C or 32 days at 2 "C. Sap from infected dahlia, Zinnia elegans or Ageratum houstonianum rapidly became non-infective, but extracts made with 0.05 M sodium thioglycollate or 0.03 M sodium diethyldithiocarbamate remained infective for
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Detection, field occurrence and identity of the virus Detection.Dahlia leaves contain much phenolic material; this, and the activity of dahlia leaf phenoloxidase, are both increased by virus infection (Martin, 1954;Lawson & Taconis, 1965). Early failures to transmit DMV from infected to healthy dahlias by mechanical inoculation of sap (Brierley, 1933), and transmission rates of 10% or less obtained later (Brierley & Smith, 1950), probably resulted from the rapid inactivation of virus by o-quinones or other complex phenolic materials. Inactivation can be de-