Phaeophycean seaweed extracts(PSWE): exploration of its antifungal and bio-elicitor properties against major fungal diseases of black pepper

Abstract

The present study entitled “Phaeophycean seaweed extracts (PSWE): Exploration of its antifungal and bio-elicitor properties against major fungal diseases of black pepper” was conducted in the Department of Plant Pathology, College of Agriculture, Padannakkad during 2021-2023. Plant samples of black pepper showing typical symptoms of foot rot and anthracnose were collected, and identified through cultural, morphological, and molecular characterizations, as Phytophthora capsici in case of foot rot and Colletotrichum sp. in case of anthracnose. Marine brown algal samples were collected, and the extracts were prepared through four different methods, including acid, alkali, solvent and hot water extraction. The antifungal activity of the phaeophycean seaweed extract (PSWE) at different concentrations was tested using the poisoned food technique (PFT). Hot water extract at 5000 ppm (PSWE (HW 5000)) was recorded as the highest inhibition (100%) of mycelial growth against P. capsici. In case of Colletotrichum sp., hot water extract at 5000 ppm exhibited 20 per cent inhibition. Further, the stability of the active principle in PSWE against high temperature and exposure to sunlight was tested using PFT and agar well diffusion assay (AWDA) against P. capsici. It was found that the active principle was degraded by the high temperature at (121℃ for 20 min.) and sunlight (min. temp. 28℃ and max. temp. 37℃). In PFT, zero per cent inhibition was recorded in high temperature, and 24.76 per cent inhibition was observed in sunlight-exposed PSWE. 17.11 per cent of mycelial inhibition was recorded when treated with high temperature, and 36.6 per cent of inhibition was recorded when exposed to sunlight. The protein and non-protein fractions of the extract were separated using the chloroform: methanol method and tested for antifungal activity. The protein fraction of the extract contributed to 40 per cent of mycelial inhibition through PFT. The molecular weight of the protein fraction was found to be more than three kDa, and the protein fraction with less than three kDa size recorded 42.5 per cent of mycelial inhibition. The size of protein fractions was analysed through SDS-PAGE as 75 kDa to 150 kDa. PSWE (HW 5000) was selected for a pot culture experiment with 12 treatments using the variety Panniyur 1 against foot rot pathogen P. capsici. The treatments were applied, and seven days after inoculation, the least per cent lesion development (PLD) of 33.33 per cent, as well as the least lesion size (3.42 cm), was recorded in PSWE (HW 5000) treatment with pre-inoculation of the pathogen through soil drenching and foliar spray. The highest PLD, 73.33 per cent, and lesion size (5.77 cm) was recorded in PSWE HW (5000) with post-inoculation of the pathogen via soil drenching. Biochemical basis of defense induction in the host plant by PSWE (HW5000) was analysed through the expression of defense-related enzymes. Application of PSWE (HW 5000) through soil drenching and foliar application with pre-inoculation of the pathogen (T5) and same treatment without pathogen inoculation (T9) expressed higher activity of defense-related enzymes such as polyphenol oxidase, peroxidase, superoxide dismutase and catalase. The presence of reactive oxygen species (ROS) was assessed using nitroblue tetrazolium (NBT) and diaminobenzidine (DAB) staining techniques. The build-up of superoxide ions and H2O2 were observed in the leaves of pathogeninoculated plants by their corresponding colour development. In both staining techniques, T5 and T6 exhibited reduced stain intensity compared to plants in control. It was found that PSWE (HW 5000) has noticeable growth promotion activity in black pepper. The growth promotion was highest in the application of PSWE (HW 5000) via soil drenching and foliar application (T9), followed by soil drenching (T7) of PSWE (HW 5000) and soil drenching and foliar application with pathogen inoculated plants (T5). The number of leaves (36.44), shoot and root length (165 and 21.83 cm), shoot and root biomass in fresh weight basis (47.66 and 14.33 g) and in dry weight basis (15.85 and 5.25 g) was higher in T9. The present study revealed that the PSWE (HW 5000) has effective inhibition activity against P. capsici and Colletotrichum sp., and it is effective in managing foot rot disease in black pepper under controlled conditions through the induction of host defense enzymes. Moreover, it enhances plant growth in black pepper. The signaling pathways associated with defense actions and growth promotion mechanisms have to be investigated.