INTERACTION BETWEEN BEET CURLY TOP IRAN VIRUS AND THE SEVERE ISOLATE OF BEET CURLY TOP VIRUS IN THREE SELECTIVE SUGAR BEET CULTIVARS

The severe strain of Beet curly top virus (BCTV-Svr, genus Curtovirus) and Beet curly top Iran virus (BCTIV, genus Becurtovirus) are the causal agents of beet curly top disease in Iran. The interaction between the two viruses in three sugar beet cultivars (cvs) Brigita, Dorothea and 7233 was examined by evaluating disease severity (DS), plant growth and the titre of viral DNA in infected plants. Sugar beet cultivars differed significantly (P < 0.001) in DS. Infected 7233 plants showed less severe symptoms compared to those observed in plants of cvs Brigita and Dorothea. Cultivar 7233 was less affected by BCTV-Svr infection compared to BCTIV infection or mixed infection of both viruses. However, there was no significant difference in DS index (DSI) between Brigita and Dorothea infected by either one or both viruses. Brigita and 7233 cultivars showed the highest and lowest reduction in vegetative growth indices, respectively. Consequently, Brigita cultivar was considered a highly susceptible, Dorothea a susceptible and 7233 a tolerant cultivar. The highest plant growth reduction in all three cultivars was caused by mixed infection. Analysis of the real-time PCR data showed that in single infection of each cultivar with either BCTV-Svr or BCTIV, the titre of BCTV-Svr DNA was higher than that of BCTIV DNA. However, in mixed infection of all three tested cultivars, BCTIV DNA titre increased relative to BCTIV-only infection, while BCTV-Svr DNA titre dropped relative to a BCTV-Svr-only infected plant. The results of this study indicate a form of interference phenomenon for this virus-host system with more severe symptoms in mixed than in single infections and with the highest BCTIV and the lowest BCTV-Svr DNA titres, respectively, in mixed infection.