Efficacy of clay-based formulated Serratia in reducing inoculum of Fusarium oxysporum f. sp. cubense tropical race 4

The bacterial isolate Serratia marcescens, isolated from roots of wild bananas, has shown beneficial properties in previous studies when associated with banana plants, promoting plant growth in plantlets both in the glasshouse and the field, and suppressing Fusarium wilt incidence, albeit only in the glasshouse. The loss of control efficacy in the field was attributed to low inoculum levels of the bacterium in the soil. Therefore, this study aimed to develop a suitable formulation for Serratia to enhance its viability and efficacy in field applications. A comparison between sunlight-exposed and non-exposed formulations was also made. The clay-based materials bentonite and kaolin were selected as carrier materials as they are naturally abundant in the environment. To each clay, non-fat skimmed milk (NFSM) and sucrose were added as enrichment materials and para-aminobenzoic acid (PABA) was added for its assumed UV-protectant properties, in various combinations. Preliminary laboratory experimental results revealed that formulation with bentonite was slightly more efficient in inhibiting Fusarium oxysporum f. sp. cubense tropical race 4 (Foc TR4) compared to kaolin, with 4.06 and 4.28 log10 CFU/ml, respectively. This was attributed to the higher viable cell count of Serratia recovered, especially from formulations enriched with NFSM and sucrose. Bentonite formulations also produced higher viable cell count for both sunlight-exposed and non-exposed treatments. PABA on the contrary was found to have antimicrobial properties towards both Serratia and Foc R4. In addition, the UV-protectant effect was absent in all formulations with PABA resulting in low viable cell counts of Serratia. Therefore, for future applications, it is suggested to omit PABA from the bentonite formulation, while NFSM and sucrose levels can be further optimised for economical reasons. This biotechnological formulation is also highly cost effective as the total cost for formulations with combinations of bentonite, NFSM and sucrose was less than USD10 per 100 g.