Tablet formulation of plant growth promoting microbes (PGPM) for the management of foot rot of black pepper

Abstract

Plant pathogens and pests pose significant challenges to agricultural productivity. While synthetic agrochemicals provide rapid and reliable solutions, their excessive use has led to environmental and health concerns, driving the search for more sustainable alternatives. Biological control, an eco-friendly and cost-effective strategy, employs beneficial organisms such as plant growth promoting microbes (PGPM) to combat plant pathogens through diverse mechanisms. However, conventional formulations of biocontrol agents, including talc and liquidbased products, have limitations such as short shelf life, bulkiness, transportation difficulties, and contamination risks. Thus, the current study entitled ‘Tablet formulation of plant growth promoting microbes (PGPM) for the management of foot rot of black pepper’ is undertaken during the period from 2022-24 at Department of Plant Pathology, College of Agriculture, Vellanikkara to develop tablet formulation of plant growth promoting microbes (PGPM) and evaluate its biocontrol efficacy under in vivo conditions. The PGPM used in this study included Bacillus cereus (BPB 16) as the bacterial isolate and Trichoderma asperellum (BPT 8) and Trichoderma harzianum (CT 30) as the fungal isolates. These were characterized through cultural, morphological, and molecular methods. Media optimization was carried out by culturing the isolates in basal media - Nutrient Broth (NB) for bacteria and Potato Dextrose Broth (PDB) for fungi, supplemented with additives such as sugar sources [mannitol, trehalose], wetting agents [polyvinyl pyrrolidone (PVP), poly ethylene glycol (PEG)], adhesives [glycerol, carboxy methyl cellulose (CMC)], and surfactant (tween-80). Nine treatment combinations were tested, with population density recorded from 15 days after inoculation (DAI) to six months after inoculation (6 MAI). The treatments T4 (NB + mannitol, PEG, CMC, Tween-80) for Bacillus cereus, T3 (PDB + mannitol, PEG, glycerol, Tween-80) for Trichoderma asperellum, and T6 (PDB + trehalose, PEG, glycerol, Tween-80) for Trichoderma harzianum demonstrated the most stable population densities over six months varying from 24.67 x 1012to 18 x 108cfu ml-1, 11 x 1012 to 9.67 x 108cfu ml-1 and 40.50 x 1012 to 11.33 x 108cfu ml-1respectively. The best additive combinations for each microbe were selected for preparation of tablet formulation of PGPM. The carrier material used for preparation of tablet was talc and these tablet formulations were optimized by adjusting the carrier material's moisture content to 5 per cent and 8 per cent. Stability tests revealed that 5 per cent moisture tablets stored under refrigerated conditions had the highest population density (15.50 × 10⁸ cfu ml⁻¹), followed by those stored at ambient conditions (12.33 × 10⁸ cfu ml⁻¹) after three months of storage (3 MAS). While standardizing the recommended dosage of the formulation, it was noticed that one tablet dissolved in 100 litres of water produced population densities of 7 × 10⁶ cfu ml⁻¹ and 4 × 10⁶ cfu ml⁻¹ for 5 per cent and 8 per cent moisture tablets, respectively. The biocontrol efficacy and plant growth promotion of the PGPM tablets were assessed in a pot culture experiment against Phytophthora sp. causing foot rot disease in black pepper variety, Panniyur-1. Tablet formulations of PGPM, conventional liquid and talc formulations of Pseudomonas and Trichoderma, talc formulation of PGPM, copper oxychloride as chemical check and an absolute control were the treatments laid out in the experiment. The polybags were challenge inoculated with the pathogen after 15 days of first treatment application and on symptom appearance, treatments were applied thrice as soil and foliar application at 10 days interval. Plant growth parameters such as height, number of leaves, number of nodes of black pepper cuttings as well as disease incidence, and severity were recorded. On analysis of the biometric parameters, it was noticed that plants treated with PGPM tablet formulations exhibited the highest plant height, number of nodes and leaves and was comparable with the talc based formulation of PGPM. Likewise, observations on disease incidence and severity revealed that the tablet formulation of PGPM was superior among all treatments followed by the liquid formulation of Trichoderma and these results were comparable to the chemical check, copper oxy chloride (0.25 %). Observations on enumeration of viable count of fungi, bacteria and actinomycetes in the soil were recorded before and after treatment application by serial dilution and plating technique and it was noticed that the tablet formulation of PGPM consistently showed the highest soil microbial count. It was further noticed that the bacterial population was comparatively higher in all treatments compared to the population of fungi and actinomycetes. The study has successfully developed and standardized a protocol for producing tablet formulation of plant growth promoting microbes (PGPM) that exhibit both stability and efficacy. These tablets demonstrated the ability to retain high microbial population densities under refrigerated and ambient storage conditions for a minimum period of three months. Furthermore, the formulation has been optimized to be user-friendly, with a recommended application rate of one gram dissolved in 100 liters of water, ensuring an effective microbial concentration for practical use. To ensure wider applicability and adoption, multi-locational field trials should be conducted in different crops against various diseases to validate the findings and once validated, the PGPM tablet formulation holds the potential for commercialization as a sustainable biocontrol solution, offering farmers an ecofriendly alternative to chemical pesticides for managing plant diseases and promoting crop health.