Microbiological and molecular characterization of plant disease suppressive compost
Bambo, Thabang Lazarus
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Composts of different composition, age and maturity were evaluated for their phytotoxic impact on germination and biomass of lettuce (Lactuca sativa) and radish (Raphanus sativus) seedlings. The suppressive ability of composts on two isolates of Rhizoctonia solani (Kühn) was further evaluated in the glasshouse. The study also included a growth medium consisting of finely ground soil from disused termitaria which are reported to have high microbial diversity. Six batches of compost suppressed at least one isolate of R. solani, while nine batches, including the termitaria soil, failed to suppress the isolates. Correlations were observed in six batches of composts where two batches suppressed one or both R. solani isolates and were not phytotoxic to the plants, while four batches were strongly phytotoxic and failed to suppress the two isolates. Other composts gave variable results in terms of relationship between phytotoxicity and pathogenicity. The microbial diversity of the composts measured by means of various biochemical and molecular analyses gave variable results in terms of consistently predicting the phytotoxicity and disease suppressiveness of the composts. The impact of genetic variation of pathogens on compost’s inherent disease suppressiveness was studied by testing the suppressiveness of various compost batches to seven isolates of R. solani on radish seedlings. Various biochemical and molecular analyses were employed to evaluate the chemical and biological characteristics of composts that possibly have a relationship with their respective disease suppressive ability. The nutritional composition and fungal population densities were also studied in this regard. There was a highly significant variation between R. solani isolates and their interaction with compost amendments. The chemical and biological attributes of the composts could however not explain the variation of the isolates’ response to the diseases suppressive ability of the compost. Some isolates were suppressed by compost while others were not, which indicates that pathogen genotype plays an important role in the disease suppressive ability of certain composts. Research on the suppression of Alternaria raphani blight of radish seedlings was tested in the glasshouse. Results confirmed that certain composts have the ability to induce systemic acquired resistance (SAR) in plants. Radish plants with reduced Alternaria blight symptoms exhibited higher levels of pathogenesis-related (PR) proteins compared to plants with severe symptoms. However, the levels of PR proteins ß-1,3-glucanases and peroxidases in radish plants, grown on certain composts, rapidly increased after inoculation with the pathogen, suggesting that the compost primed the plant and enabled it to induce PR proteins upon pathogen challenge. Soil with or without salicylic acid (SA) applied to leaves was also studied and neither of the treatments was successful in suppressing Alternaria blight. However, SA sprayed onto the leaves did induce defence responses that were not significantly different from that in pathogen challenged plants. The results of the present study confirm that there is potential for the use of compost in the management of soilborne and foliar pathogens. The challenge that remains, however, is that plants are exposed to a broad range of pathogen genotypes in their lifetime and compost that can suppress a broad range of pathogens is therefore ideal. The addition of effective antagonistic microorganisms to compost that is able to inhibit a broader range of pathogens will enhance its potential for disease suppression. These practices should therefore be addressed in order to optimize the preparation and use of composts for disease suppression.