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 secondeconomically 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 ofresistance to the pathogen in rice field conditions in northern iran. thirty seven vs twentythree rice cultivars respectively from iranian local (il) and iranianimproved (ii) cultivarsalong with ten foreign resistant (fr) cultivars were field grownat the rice research instituteof 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, theplants were kept wet until the evaluation time through 2-5 times of daily water spraying, whileapplying 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 infectedtiller (it), number of dead and green leaves(dl and gl), number of tillers (nt), tillerangle (ta), stem width (sw), and plant height (ph). results indicated that, the group of frcultivars 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, usingupgma 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 (foreignsusceptible), moosatarom, hassansaraei, jamshidjo, khazar, shiroodi, d4 (il and ii), andfujiminori was recorded as susceptible. analysis of the relationship between plant traits anddisease response indicated that rlh and sss were not significantly correlated with theassessed morphological characteristics as regarded fr cultivars, whereas a positive significantcorrelation was observed between sss and ta (r = 0.20, p. value= 0.033), and a negative onebetween rlh and ph (r = -0.29, p. value= 0.002) as detected on local cultivars.