TVDVaRNA, Sat-TBTV and/or TBTV were always associated withTVDV. Tobacco plants infected with TVDV, TVDV + TVDVaRNA orTVDV + TVDVaRNA + Sat-TBTV did not show obvious symptoms.Only plants infected with the four causal agents showed typicalTBTD symptoms of foliar chlorosis and necrosis, internode short-ening and plant stunting. Tobacco plants infected with TVDV aloneonly showed mild vein distortion, less than the plants infected withboth TVDV and TVDVaRNA. Impact of TVDV or TVDV+ TVDVaRNAon the infected plants was minor. The plants infected with TBTVand Sat-TBTV showed symptoms very similar to the typical TBTDsymptoms, but without internode shortening and plant stunting. Itindicated that TBTV, or TBTV and Sat-TBTV might play importantrole in the TBTD symptom expression. The results also suggestedthat presence of the four causal agents might be necessary for thetypical TBTD symptom. More study is necessary to understand theroles of each of the four agents in TBTD.