Tobacco yellow dwarf and bean summer death diseases apparently are confined to Australia, and the causal pathogens are transmitted by the leafhopper Orosius argentatus. The diseases were caused by viruses. Purified preparations obtained from Datura stramonium plants, infected with either tobacco yellow dwarf virus (TYDV) or bean summer death virus (BSDV), contained viruslike particles occurring in pairs about 20 nm .times. 35 nm in size. On sedimentation in sucrose density gradients, the geminate particles of TYDV and BSDV each formed discrete zones which were associated with infectivity. TYDV preparations have an UV absorbance typical of a nucleoprotein, contain a single centrifugal component with a sedimentation coefficient of 76S, and yield a single structural protein species of MW 27,500. The yield of TYDV was about 100-250 .mu.g/kg of tissue, and of BSDV about 20-50 .mu.g/kg of tissue. In addition to producing similar host reactions and having similar vector relations, TYDV and BSDV are serologically related, and are therefore considered likely to be strains of 1 virus. TYDV and BSDV have many properties in common with beet curly top virus (CTV), and a distant serological relationship was established between TYDV and CTV.