Plant defense activators for the management of leaf blight of amaranthus (amaranthus sp.L)

Abstract

The present study entitled “Plant defense activators for the management of leaf blight of amaranthus (Amaranthus sp. L.)” was conducted in the Department of Plant Pathology, College of Agriculture, Padannakkad during 2021-2023. A purposive sampling survey was conducted in Agro Ecological Unit II (AEU II). Amaranthus plants exhibiting leaf blight symptoms were collected from the Instructional Farm, College of Agriculture, Padannakkad and also from farmers’ fields in AEU II. Among the six locations surveyed, percent disease incidence (PDI) and percent disease severity (PDS) showed a range of 32 to 71 per cent and 27 to 58 per cent respectively. Six isolates of the fungus associated with leaf blight of amaranthus were obtained and were denoted as Rs1, Rs2, Rs3, Rs4, Rs5 and Rs6. Pathogenicity was proved for all the six isolates. Among the six isolates, Rs2 developed leaf blight symptoms within two days after artificial inoculation along with highest lesion size when compared to other isolates. Hence Rs2 was selected as the most virulent isolate and used for further studies. The isolate Rs2 was identified with it’s cultural, morphological and molecular characteristics and was confirmed as Rhizoctonia solani. All the remaining isolates exhibited cultural and morphological characters exactly similar to that of isolate Rs2. In the second part of this study, the efficacy of a category of novel chemicals known as plant defense activators against R. solani was tested. In vitro screening was conducted to evaluate the antifungal activity of plant defense activators such as two seaweed extracts and chitosan at different concentrations using the poisoned food technique with 11 treatments and three replication using CRD. Ascophyllum nodosum extract (ANE) at a concentration of 7500 ppm (T2) exhibited the highest inhibition of mycelial growth of the pathogen (39.40 %). The plant defense activators were short-listed based on their efficacy under in vitro evaluation and the effectiveness of selected treatments against the disease was assessed by pot culture experiment. Two separate experiments were carried out with pre-inoculation and post-inoculation of R. solani each with eight treatments and three replication using CRD. After a week of inoculation, the least per cent lesion development (34.22 %), as well as the lesion size (2.51 cm2), was recorded in the treatment ANE (7500 ppm) (T2) with post-inoculation of the pathogen. However, based on efficacy in pot culture experiment five treatments were selected for field evaluation. A field experiment with six treatments and four replication using RBD was laid out during May- July 2023 in the Instructional Farm, College of Agriculture, Padannakkad. Natural incidence of the disease was noticed after 15 days of transplanting (DAT). Hence the treatments were given as foliar spray at 15, 30 and 45 DAT. ANE (7500 ppm) (T2) and Phaeophycean seaweed extract (PSWE) (7500 ppm) (T1) recorded least PDI of 40 per cent and 45 per cent respectively. At the same time, absolute control (T6) and turmeric powder + baking soda (5:1) (T5) recorded PDI of 60 and 55 per cent respectively. Significant reduction in number of lesions, lesion size and PDS were observed after the use of different plant defense activators. ANE (7500 ppm) (T2) was superior to all treatments in reducing the disease which recorded the least number of lesions (38.4), lesion size (2.84 cm2) and PDS 44.60 per cent. In addition to disease reduction, ANE (7500 ppm) (T2) recorded significant growth promotion in amaranthus plants. It recorded the highest number of leaves (66), shoot and root length (114.95 and 24.5 cm), shoot and root biomass on fresh weight basis (13475 and 7547.5 kgha-1) and dry weight basis (5650 and 2420 kgha-1). Further in the study, the biochemical basis of disease reduction was elucidated by estimation of defense related enzymes. Polyphenol oxidase (PPO) activity was increased in the treatment ANE (7500 ppm) (T2) from 1.46 to 1.88 ΔA/mg/min. Plants that were treated with the ANE (7500 ppm) (T2) exhibited elevated peroxidase (PO) activity of 6.78 μg g-1 fw 7 days after treatment. After pathogen infection, a significant increase in catalase (CAT) activity and elevated level of phenyl alanine ammonia lyase (PAL) activity was recorded in (T2). The present study revealed that ANE (7500 ppm) (T2) a seaweed extract has remarkable inhibitory activity against R. solani and is effective in managing leaf blight disease of amaranthus by the induction of host defense enzymes. Moreover, it enhances plant growth and biomass production in amaranthus. The signalling pathways associated with defense induction and growth promotion activity by plant defense activators have to be investigated further.