تنوع در بيماريزايي جدايه هاي RHIZOCTONIA SOLANI چغندرقند

Relative virulence of 131 Rhizoctonia solani isolates was evaluated in in vitro and in greenhouse conditions. Of these, 44 isolates belonged to AG-2, 58 to AG-4 and the rest of them were classified as unknown AG-group. For in vitro tests, germinating seeds of a susceptible line (261) were placed on the periphery of actively growing colony of R. solani on Water Agar (WA). Disease severity of seedlings was measured 3, 5, and 7 days after inoculation on the basis of 0-4 scale. On the basis of Area Under Disease Progress Curve (AUDPC), R. solani AG-4 isolates were grouped into 26, AG-2 isolates, into 17 and unknown isolates into 11 using Duncansʹ multiple range test. High correlation was found between 2nd recording date (five days after inoculation) and AUDPC (r=0.92, P= 0.05). Virulence degrees of the selected isolates of clustering analysis from AG-4 and AG-2 and unclassified isolates were compared. Results showed that the isolate Rhl24 (AG-4) collected from Kermanshah was the most virulent, while Rh 157 (AG-4) isolate collected from Khorasan was the least one. Studies on the virulence of selected isolates of AG-4 and AG-2 (belonging to high and low virulence group in in vitro test) in greenhouse condition, in which seedlings and adult plants were inoculated continued different degree of virulence and also showed that the range of variability in the virulence of AG-4 isolates were more than of AG-2 isolates. The Rhi 57 (AG-4) was the least pathogenic isolate on seedling and adult plants. The AG-2 isolates were more virulent than AG-4, when inoculated on adult plants. In the third part of this research, an experiment was conducted using mixed isolates of AG-* Corresponding author 4 and AG-2 to inoculate adult sugar beets. A mixture of 4 high virulent and 4 low virulent isolates of AG-4 used as treatments I and 2. Similarly, the same mixtures of AG-2 isolates employed as treatments 3 and 4. Results indicated that, there was no major difference in the degree of virulence between highly virulent isolates of AG-4 and AG-2 groups. In an in vitro test, the interaction of 4 isolates l)elonging to AG-4 and AG-2 of varying degree of virulence with 6 sugar beet genotypes of different levels of resistance to Rhizoctonia was evaluated. The results showed that sugar beet genotypes with various degree of resistance are more easily differentiated by inoculating AG-2 isolates than AG-4.