Study on Partial Resistance to Sheath Blight Disease (Rhizoctonia solani AG1- IA) in Iranian and Selected Exotic Cultivars of Rice

Rice sheath blight disease caused by Rhizoctonia solani AG1- IA is the second economically important disease of the crop in most rice- growing regions of the world، including Northern of Iran. The disease is mainly controlled through application of fungicides. The present study was performed to identify the candidate cultivar (cultivars) as source of resistance to the pathogen in rice field conditions in Northern Iran. Thirty seven vs twenty three rice cultivars respectively from Iranian Local (IL) and Iranian Improved (II) cultivars along with ten Foreign Resistant (FR) cultivars were field grown at the Rice Research Institute of Iran. They were inoculated with the mycelial mass of a virulent isolate (G309) of pathogen، 45 days after being transplanted. To supply favorable humidity for disease development، the plants were kept wet until the evaluation time through 2-5 times of daily water spraying، while applying a overhead water sprayer system available on the field. Assessment of cultivars، reaction was done by recording the Sheath Blight Severity as appeared on the sheath (SSS)، number of Lesions on the Sheath (LS)، Relative Lesion Height (RLH)، percentage of Infected Tiller (IT)، number of Dead and Green Leaves (DL and GL)، Number of tillers (Nt)، Tiller Angle (TA)، Stem Width (SW)، and Plant Height (PH). Results indicated that، the group of FR cultivars significantly differed from groups (IL) and (II) cultivars by exhibiting less SSS، LS، DL vs more GL. Cluster analysis of the cultivars as based on RLH، SSS and DL، using UPGMA method revealed 4 groups. One of the grpups comprised of: Jasmine 85، Teqing، Pecos and Tetep was observed as resistant، whereas، another one containing Swarna (Foreign susceptible)، Moosatarom، Hassansaraei، Jamshidjo، Khazar، Shiroodi، D4 (IL and II)، and Fujiminori was recorded as susceptible. Analysis of the relationship between plant traits and disease response indicated that RLH and SSS were not significantly correlated with the assessed morphological characteristics as regarded FR cultivars، whereas a positive significant correlation was observed between SSS and TA (r = 0. 20، P. value= 0. 033)، and a negative one between RLH and PH (r = -0. 29، P. value= 0. 002) as detected on local cultivars.