Valorization of Spent Mushroom Substrate from Pleurotus spp.: Nutrient Profiling and Biocontrol Potential against Plant Pathogens

Spent mushroom substrate (SMS), the primary by-product of mushroom cultivation, is often considered as an agricultural waste despite its potential value. The study presents a comprehensive analysis of the nutrient composition of various SMS samples to assess their suitability as sustainable soil amendments, along with their effectiveness in controlling plant pathogens under in vitro conditions. A combination of analytical techniques was employed, including Kjeldahl digestion for nitrogen determination, di-acid digestion for macro- and micronutrient analysis, and hot water extraction for boron estimation. The nutrients analyzed included nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), iron (Fe), zinc (Zn), copper (Cu), and boron (B). Three species of oyster mushroom viz: Pleurotus florida, P. ostreatus, and P. sajor-caju were used in the present study. The results revealed that all SMS samples were rich in essential plant nutrients, with significant variability observed across different treatments. specifically, the SMS from P. ostreatus (T2) demonstrated significantly higher levels of nitrogen, potassium, calcium, and sulphur, while the SMS from P. florida (T1) had the highest levels of phosphorus and magnesium. in contrast, micronutrient levels varied less significantly, though P. ostreatus (T2) still showed the highest levels of boron, iron, and copper. Additionally, an antagonistic bacterium isolated from SMS of P. ostreatus was identified and confirmed as Burkholderia gladioli through 16S rRNA sequencing. In vitro dual culture assays demonstrated that this bacterium produced inhibition against the plant pathogens Rhizoctonia solani (62.2%) and Colletotrichum gloeosporioides (76.6%), highlighting its potential for biological control. The research validated SMS as a promising and sustainable alternative to traditional fertilizers, effectively valorizing agricultural waste as well as a potent biocontrol agent.