1RARS, Shilongani, PO Box-33, Nagaon-782001, India; 2Assam Agricultural University, Jorhat-785013, India

Background and objectives
Sheath bight of rice caused by Rhizoctonia solani is one of the major constraints to rice production, with yield losses of between 10 and 36%, depending on the growth stage when the disease occurs [1]. Biological control offers an option as a component of IPM and Trichodermaspp. are being tested against various diseases caused by R. ;solani. However, a a viable method for commercial application in the field is yet to be found because of a lack of precise knowledge about the ecological conditions that favour multiplication and parasitism of the hyperparasites for specific host-pathogen-hyperparasite interactions. Since environmental factors limit the efficacy of biocontrol of soil-borne pathogenic fungi, studies were carried out to determine the effect of different soil moisture regimes and soil pH on the survival of T. ;harzianum and T. ;viride and their ability to antagonize R. ;solani leading to suppression of sheath bight.

Materials and methods
The effect of different moisture regimes on survival of T. ;harzianum and T. ;viride and development of sheath blight was conducted. Using pots in a factorial design, the treatments were combinations of: (1) rhizosphere effect, (2) soil moisture levels, and (3)inoculations. Two contiguous plots in lowland and highland were selected to study the antagonistic activity of T. ;harzianum and T. ;viride on R. ;solani. These were in lowland, irrigated conditions and highland rainfed conditions with two levels of soil pH in a split plot design with systematic arrangement of the whole plot. The whole plot treatments were land situation, randomization was not possible and arranged across the field. The subplot treatments were combinations of: (1) soil pH levels and (2) inoculations.

Results and conclusions
Sheath blight on tillers and sheaths was significantly less in submerged conditions than in 100% saturation formed by 50% saturation in the pot experiment. Again, infection was lower in the lowland irrigated field conditions compared with highland rainfed. Occurrence of sheath blight was reduced under low pH compared with that of neutral pH. Further infection of R. ;solani was significantly reduced in association with Trichoderma spp. Disease suppression was greater with T. ;harzianurn than in T. ;viride. Survival of T. ;harzianum and T. ;viride was greater in the presence of the rice plant and also in the highland rainfed conditions. Survival of Trichoderma spp. was greater under low pH than in neutral pH, and the population of T. ;harzianum was higher than that of T. ;viride in all the moisture levels and in all soil pH levels. Survival of R. ;solani was greatest in the presence of rice plants in 50% saturation formed by 100% saturation. It was least in submerged situations and survival was greater in neutral pH than in low pH. It was reduced in the presence of T. ;harzianum. Plant growth characters and yield were increased proportionately with the decrease of disease incidence. Thus, the studies showed that sheath blight incidence was less in lowland irrigated conditions/submerged situations which was associated with lower populations of R. ;solani and higher competitive saprophytic ability of Trichoderma spp.[2]. Reduction of sheath blight in low pH was related to increased population and antagonizing ability of Trichoderma spp., more than that of R. ;solani [3]. T. ;harzianum and T. ;viride suppressed sheath blight as well as survival of R. ;solani and the reduction was greater with T. ;harzianum than T. ;viride because of its greater population growth.

1. Roy AK, 1993. Indian Phytopathology 46, 197-205.
2. Rosales AM, 1984. Asta FFTC Book Series 26, 147-57.
3. Chet I, Baker R, 1980. Phytopathology 70, 994-98.