Suppression of soil-borne pathogens by composted municipal solid waste

The objectives of this study were to determine the level of suppressiveness of composted municipal solid waste (CMSW) against soil-borne plant pathogens, and to investigate the mechanism involved in this process. The suppressiveness of CMSW to Rhizoctonia solani and Fusarium oxysporum f.sp. vasinfectum was studied in greenhouse experiments using sandy soil amended with different levels of CMSW. R. solani and F oxysporum f.sp. vasinfectum disease severity in cotton plants was reduced in the presence of mature compost by 80 and 95 respectively. After 1.5 months of composting, CMSW still did not suppress R. solani, whereas 2 week-old compost already exhibited suppression off. oxysporum f.sp. vasinfectum. Reducing the microbial population of CMSW by autoclaving, gamma-irradiation or heat treatment negated R. solani suppression. Thus, we suggest that the mechanism involved in the observed suppression is biological. This hypothesis is supported by the fact that the addition of 5treated CMSW resulted in recolonization of antagonistic microorganisms and restored the compost's suppressive ability. On the other hand, heat treatment of CMSW did not eliminate its ability to suppress F. oxysporum f.sp, vasinfectum. The mechanism governing the suppression of R. solani is suggested to involve lysis of fungal hyphae, mostly by the bacterial population, as observed by microscopic examination. Some bacteria isolated from the CMSW showed chitin-degradation ability and the capacity to inhibit R. solani growth. One of these isolates, which suppressed R. solani in cotton plants in greenhouse experiments, was identified as Enterobacter agglomerans. Inoculation of heated compost with Trichoderma harzianum suppressed R. solani significantly. However, the addition of T. harzianum did not improve the suppressive ability of the mature compost, despite the fact that this compost was successfully colonized.