Characterization of sugar beet Rhizoctonia isolates in Iran

During 2001-2005, 79 isolates of Rhizoctonia from seed rot, seedling damping –off, root rot and dry rot canker of sugar beet were collected. Based on morphological characteristics, number of nuclei per cell and hyphal diameter, isolates were classified in two groups, binucleate (4 isolates) and multinucleate (75 isolates). Among 75 multinucleate isolates, 27 isolates were identified as AG4, 28 isolates as AG2, two isolates as AG3 and one isolate as AG5.The remaining of the isolates(17 isolates) did not anastomose with any of tester cultures. Most of isolates of AG2 had been isolated from root rot symptoms, but AG4 isolates from damping –off and root rot symptoms. From dry rot canker symptoms with concentric ring on the root surface of sugar beet, 5 Rhizoctonia isolates were collected, that did not anastomose with any tester cultures. Optimum temperature for mycelial growth ranged from 25 to 30, but for dry rot canker isolates was above 30 °C. Pathogenicity test was carried out in in vitro condition. Representative isolates from each AG and each geographic location were selected for pathogenicity test. Results showed that the isolates varied in their virulence.AG4 isolates were highly virulent on seedling and more virulent than AG2 isolates.AG3 and AG5 isolates caused superficial discoloration on seedling. From binucleate isolates, two of them were as pathogenic as dry rot canker isolates. Virulence variation of selected isolates of different anastomosis groups were also investigated on mature sugar beet plants under greenhouse conditions. Results showed that both isolates of anastomosis groups 2 and 4 had the ability to cause root rot, however, the disease severity of anastomosis group 2 (AG-2) on mature plants was higher than those of AG-4 group.