Exploring the potential of spent mushroom substrate extract from pleurotus spp. for the management of leaf blight of amaranthus (Amaranthus tricolor L.)

Abstract

The study entitled “Exploring the potential of spent mushroom substrate extract from Pleurotus spp. for the management of leaf blight of amaranthus (Amaranthus tricolor L.)” was conducted at Department of Plant Pathology, College of Agriculture, Vellayani during 2023-2025 with the objectives of evaluation of the efficacy of spent mushroom substrate (SMS) extract for the suppression of the leaf blight of amaranthus; and detection of associated biomolecule(s). The pathogen was isolated from the diseased amaranthus leaf and its pathogenicity was proved by Koch’s postulates. Cultural and morphological studies of the pathogen on potato dextrose agar medium revealed hyaline to brown, septate mycelia with right-angled branching and formation of white to brown irregular sclerotia after eight days of inoculation. Molecular characterization using LSU primers (LROR and LR7) validated the isolate as Rhizoctonia solani with accession number (PX411474). Pure culture of Pleurotus florida, P. ostreatus and P. sajor-caju were obtained from ICAR- DMR, Solan. Mushrooms were cultivated in paddy straw. Spent mushroom substrate, the by-product after cultivation (45 days), was collected from these three species of oyster mushroom. The spent substrates were shade dried, powdered and used for further studies. The nutrient analysis revealed that all SMS samples were rich in nutrients. Macronutrient analysis indicated that SMS of P. ostreatus contained the highest nitrogen (1.77%), potassium (1.40 ppm), and calcium (2.1%) contents, while SMS of P. florida had the highest phosphorus (0.16%) and magnesium (1.26%) levels, while the sulphur (0.61%) content was more for paddy straw (control). Among the micronutrients, boron concentration was highest in SMS of P. ostreatus (82 ppm), while zinc (Zn), iron (Fe), and copper (Cu) concentrations were similar among all the treatments. These findings suggest that SMS has substantial nutrient value and can serve as a potential organic fertilizer. Water extracts of SMS (1:3 w/v) was prepared from each Pleurotus spp. after shaking the powdered SMS for 24-48 h in an incubator cum shaker, filtered and

centrifuged at 10000 rpm for 10 minutes. The supernatant obtained was utilised as the crude extract which was evaluated against R. solani by poisoned food technique at different concentrations (250 ppm, 500 ppm, and 1000 ppm). Significant antifungal activity was observed in all the treatments. When compared to the control, at 250 ppm concentration of the extract, the radial growth of R. solani ranged from 0.58 to 1.82 cm in all the treated plates. The water extract of SMS (P. florida) treated plates showed complete mycelial inhibition (100%) of R. solani at 500 ppm and 1000 ppm concentrations, while the water extract of SMS (P. ostreatus) exhibited complete suppression of mycelial growth at 1000 ppm. The water extract of SMS (P. sajor- caju) was less effective with an inhibition percentage of 53 even at 1000 ppm concentration. Mycelial characters of R. solani in the treated plates was irregular and sparse compared to the dense, zonations in control. These results indicated that the SMS of Pleurotus spp., particularly P. florida, possesses potent antifungal compounds capable of inhibiting R. solani growth. A pot culture experiment was laid out in completely randomized design (CRD) to assess the efficacy of extracts of SMS, with 13 treatments and 3 replications, including foliar and soil applications of SMS extracts (0.2%), chemical (mancozeb @0.2%), biological control (Pseudomonas fluorescens @20 g/L) check, inoculated and absolute controls. Among different treatments, the foliar application and soil drenching with SMS extract (P. florida -T7) recorded the lowest Percent Disease Index (PDI) and lesion size, followed by foliar application and soil drenching of SMS extract of P. ostreatus (T8). The highest PDI (56.5%) was observed in the inoculated control (T12). Plants treated with SMS extracts delayed the symptom development and reduction in lesion size. In addition to disease suppression, significant enhancement was observed in the growth and yield attributes of the plants. SMS treated plants exhibited increased shoot and root length, higher fresh and dry biomass, compared to the untreated controls. Enhanced shoot and root biomass, and increased overall yield, were noted in SMS treated plants, particularly in T4 (soil drenching of P. ostreatus extract) and T7 (foliar spray + soil drenching of SMS extract (P. florida). The total yield was the maximum in T4 (soil drenching of P. ostreatus- 82 g/pot), whereas the highest plant height (47.5 cm) was observed in the case of T5 (foliar

spray of SMS extract of P. sajor -caju). Biochemical assays carried out at 0,1,3,5,7 days after inoculation of the pathogen revealed significant enhancement in defense related enzymes viz., peroxidase (PO), polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) in the SMS treated amaranthus plants after the inoculation of the pathogen. The highest activities of PO and PPO were recorded in T7, while PAL activity peaked in the treatment T8. Gas Chromatography- Mass Spectrometry (GC-MS/MS) analysis of ethyl acetate extracts of SMS revealed the presence of various classes of bioactive metabolites including phenols, fatty acids, sterols, terpenoids and esters. Common bioactive compounds detected in the three Pleurotus spp. included phenolic compounds, terpenes, fatty acids and carboxylic acids, with reported antimicrobial, antioxidant, and anti-inflammatory properties. The present study demonstrated that SMS extracts of Pleurotus spp., particularly P. florida, possess strong antifungal potential against R. solani causing leaf blight of amaranthus. The extracts were rich in essential nutrients and bioactive compounds capable of suppressing pathogen growth, inducing plant defense responses, and promoting overall plant growth and vigor. Therefore, SMS extracts represent a promising, eco-friendly alternative to synthetic fungicides for the management of leaf blight of amaranthus.