Biological Characterization and Management of Rhizoctonia oryzae-sativae Causal Agent of Aggregate Sheath Spot Disease of Rice

Abstract

Plant pathogenic fungus Rhizoctonia oryzae-sativae (teleomorph Ceratbasidium oryzae-sativae) caused aggregate sheath spot disease of rice. The disease is considered as minor on rice. However, in rice–wheat/potato cropping system the disease has emerged in higher incidence in North-West region of Bangladesh. Research studies of R. oryzae-sativae in particular are still limited hence very little information concerning the fungus is available in Bangladesh. Therefore, the present study was undertaken with aims to generate preliminary information on the population biology and structure of R. oryzae-sativae. Perhaps this is the first comprehensive report in which thirty isolates of R. oryzae-sativae collected from rice fields of ten different districts of Bangladesh. Multidisciplinary characterization approach or technique viz. culture morphology, virulence test, somatic compatibility groups (SCGs) and DNA fingerprinting were adopted. Isolates were confirmed using R. oryzae-sativae specific diagnostic markers in which a single band of 1200 bp was amplified. Data analysis of the mycelial growth rate separated the isolates into two distinct groups relatively slow and faster. Pathogenicity test on cultivar BR11 revealed that all the tested isolates were pathogenic and found significant correlation between mycelial growth rate and relative lesion height (RLH). There were 27 SCGs from 30 isolates indicated 90% diversity among the isolates. Variable number tandem repeat (VNTR) and Repetitive-element Polymerase Chain Reaction (Rep-PCR) were conducted using MR, RY, GF and BOXA1R primers respectively. DNA bands of four primers ranged from 0.25 to 2.21 kb. A combined dendrogram was constructed by NTSYS-pc software (2.20 e) which separated the isolates into three groups at 69.6% similarity level. Among them, all isolates placed in two major clusters except isolate RA-1, which placed in cluster group III. Fast growing isolates have been placed in Group II, while slow growing isolates in cluster group I. The similarity coefficient values of the dendrogram profile ranged from 0.36 to 0.98 with an average of 0.67. The effects of eight fungicides with six concentrations were tested in vitro on mycelial growth of R. oryzae-sativae. R. oryzae-sativae found sensitive to all the fungicides tested. Carbendazim showed strong activity against the pathogen with the lowest LD (lethal dose) 90 values (0.8 ppm) and LD50 (0.1 ppm) in vitro. All the fungicides significantly reduced disease development in field trials but only Carbendazim, Tebuconazole and Trifloxystroin-tebuconazole increased rice yield. However, aggregate sheath spot reduced yields by up to 14.74% under Bangladeshi conditions. The maximum growth of the pathogen on PDA measured at 6.0-9.0 pH level. The highest mycelial growth was recorded in dextrose (0.47 mm/hr) followed by sucrose (0.44 mm/hr). The best mycelial growth rate of R. oryzae-sativae found in peptone (0.43 mm/hr) followed by yeast (0.37 mm/hr). Among the tested phytoextracts, garlic and henna found most effective against R. oryzae-sativae. None of the tested germplasm or cultivars found immune to aggregate sheath spot disease. T. Aus germplasm Akuee, Kalosoti and T. Aman varieties Kurchi jira, Kurchi binni, Shabichi dhan, BRRI dhan39, BRRI dhan40, BRRI dhan46 found moderately susceptible while no susceptible variety found in Boro season. Multidisciplinary characterization of R. oryzae-sativae revealed that isolates of Bangladesh origin is not genetically identical.