Management of Sheath Blight and Enhancement of Growth and Yield of Rice with Plant Growth-Promoting Rhizobacteria

Abstract

Sheath blight (ShB) of rice caused by Rhizoctonia solani, causes significant yield losses worldwide. Strong sources of genetic resistance are not available for ShB, and the disease is currently managed through use of chemical fungicides. Fungicidal management of ShB often gives inconsistent results and is not economical. Indiscriminate use of fungicides and chemical fertilizers to increase rice yields creates several concerns relating to environmental hazards, pathogen resistance, leaching losses, and destruction of beneficial microflora. Use of plant growth-promoting rhizobacteria (PGPR) as biocontrol agents is gaining popularity in managing rice diseases and in enhancing growth and grain yields. The objectives of this study were to 1) screen various PGPR strains for suppression of R. solani, and enhancement of rice seedlings vigor and select elite PGPR strains, 2) to evaluate the elite PGPR strains for suppression of ShB and for enhancement of growth and yield of rice under field conditions, and 3) to determine the mode of action of the elite strain for its disease suppressing and growth-promoting activities. Seventy PGPR strains with known activities on other crop-pathosystems were screened for in vitro antagonism against R. solani and for growth promotion of rice seedlings. The majority of the strains significantly suppressed the mycelial growth of pathogen, and improved rice seedling vigor and growth under in vitro conditions. Four strains completely inhibited sclerotial germination of R. solani under in vitro conditions. Of 70 strains, 31 strains significantly suppressed ShB lesions when tested in a detached leaf assay. Among these, one elite strain Bacillus subtilis MBI 600 was superior. Strain MBI 600 was produced in commercial proprietary liquid formulation and designated as Integral® and tested for its growth promoting characters, and found to produce only siderophores and negative for chitinase, cellulase, HCN, IAA and phosphate solubilization. Integral was compatible to various commonly used fungicides such as propiconazole, validamycin, benomyl, tricyclazole, mancozeb, hexaconazole, carbendazim and azoxystrobin. Nursery and field trials were conducted in randomized block design with eight replications to assess the efficacy of Integral at A. P. Rice Research Institute, Maruteru, India during 2009 against ShB of rice CV. Swarna. Integral was applied as a seed treatment (ST), seedling root dip (SD) and foliar spray (FS) at concentrations of 2.2 x 108 and 2.2 x 109 cfu ml-1. Seedling growth parameters and ShB severity were measured by calculating the highest relative lesion height (HRLH) at 90 days after transplanting. Seed bacterization with Integral resulted in enhanced root (9.3 to 14 cm) and shoot lengths (37 to 45 cm) over the control (8.4 and 36 cm, respectively) in the nursery. On a transplanted crop in the field, ShB severity was significantly lower when Integral was applied as ST + SD + FS at 2.2 x 109 cfu ml-1 (19.2 to 26.5), followed by at 2.2 x 108 cfu ml-1 (24.5 to 29.4) compared to the control (56.2 to 69.7). The ShB severity in carbendazim treated plants ranged from 16.8 to 19.8. Besides, the tiller production per plant was significantly higher in Integral treated plots at 2.2 x 109 cfu ml-1 (12.3 to 12.9) compared to the control (10.0 to 10.5). Highest grain yields were recorded in Integral treated plots at 2.2 x 109 cfu ml-1 (5922 to 6207 kg/ha) compared to the control (3925 to 4199 kg/ha). Scanning electron microscopy studies from an interaction between Integral and R. solani showed that Integral caused loss of structural integrity, maceration, shriveling, and reduction in hyphal width of R. solani. Deterioration of inner sclerotial filaments was observed when sclerotia were treated with Integral. Seed colonization studies showed that Integral was able to survive on rice seeds for up to six days following seed treatment. Integral seems to be a good root and rhizosphere colonizer. Overall, Integral significantly reduced the ShB severity, and increased seedling vigor and grain yields in rice under field conditions and seems to have a potential for commercial application for rice ShB disease management.