Characterization of Rhizoctonia solani Kuhn in rice-based cropping system

Thirty three isolates collected from sheath blight (ShB) diseased rice plants from different localities of the Philippines and culms of rice plants were identified as Rhizoctonia solani (RS). Perfect type of fusion by self-anastomosis and imperfect type of fusion by different isolates from rice and other hosts involved in rice-based cropping system were observed. Type of colony fusion by the same isolate was indistinct or clearing zone formed by different isolates were described corresponding to true type of perfect and imperfect fusion, respectively. Types of reaction on potato dextrose agar (PDA) were useful to detect easily the actual type of anastomosis for large number of isolates in a shorter time. The results indicated the independent existence of RS isolates, occupying their own entities under wide variability, in rice-based cropping system in tropics and infection of a rice plant by more than one isolate. RS isolates from rice plants had extracellular phenoloxidase activity and were thiamine autotrophs. Cultural characters observed on different media by each isolate were stable. Single isolates and isolates from the same region could form the single cluster. RSI15 and RSI16 anastomostically closely related isolates found in one group when clustered based on continuous variables became separated into different groups based on binary variables. The buoyant sclerotia of RS, causal pathogen of sheath blight (ShB) of rice dehydrated at zero RH and 25 deg C retained about 44% of initial water 17 days after incubation. Fluorescent Pseudomonas (FP), Aspergillus niger, and Humicola fuscoatra var. fucoatra inhibited the colony growth of R. solani. Trichoderma spp. showed the mycoparasitic activity. Viability of sclerotia which had been placed about 4 to 5 cm under soil surface of upland and lowland soil was reduced when buried between 28 and 42 days in submerged condition. Less of viability was higher in lowland soil than in upland soil under submerged condition. Sclerotia maintained their viability in intermittent watering regimes to a high degree till 56 days after burial