Carrying capacity of component crops on Rhizoctonia solani Kuhn inoculum in relation to multiple cropping

Inoculum of Rhizoctonia solani was evaluated in both field and laboratory experiments in the Department of Plant Pathology, IRRI [International Rice Research Inst., Los Banos, Laguna, Philippines]. Upland field had generally more sclerotia (R. solani) count than lowland field conditions. Higher disease intensity in upland compared to lowland fields was observed. There were no significant difference in disease intensity of rice sheath blight, banded leaf blight of corn and Rhizoctonia blight of mungbean between minimum and maximum cultural practices. The ineffectiveness of either minimum or maximum cultural practices on crop grain yields was also observed. Minimum cultural practices enhanced greater sclerotial density and more serious disease infection compared to maximum cultural practices. The mungbean-rice-mungbean cropping pattern with minimum cultural practices had higher capacity to carry the viable sclerotia as compared to their counterparts. Therefore, rice sheath blight intensity in mungbean-rice-mungbean was higher than that in corn-rice-mungbean patterns. There was negative correlation between rice sheath blight density and rice grain yield. Cultural characteristics of Rhizoctonia solani isolates varied in the mycelial growth, the size, the number and the pattern of sclerotia production. Isolates of corn and corn-rice plots reduced sclerotia in circular pattern, and most of them were aggregated and effused sclerotia. Isolates of mungbean and mungbean-rice plots formed solitary sclerotia and distributed over the surface of agar medium. The variation in virulence among ten isolates of R. solani were also observed. Corn (upland) plot isolate was the most virulent when inoculated with agar disc. Isolates from mungbean and mungbean-rice (upland) plots were the most virulent when inoculated with sclerotia. Size and weight of R. solani sclerotia observed before planting rice were closely correlated to rice sheath blight incidence while the specific weight number and the viability were closely correlated to mungbean Rhizoctonia blight