Integrated management of fruit rot of jackfruit incited by Athelia rolfsii

Abstract

The study entitled ‘Integrated management of fruit rot of jackfruit incited by Athelia rolfsii’ was conducted during 2021-23 at College of Agriculture, Vellayani and IFSRS, Karamana with an objective to assess the factors influencing growth of A. rolfsii and development of an integrated disease management strategy against fruit rot of jackfruit. A survey was conducted in four agro-ecological units of Kerala viz., AEU 4, 8, 9 and 14 covering Thiruvananthapuram, Alappuzha, Pathanamthitta, Kollam and Kottayam districts. Fruit rot affected jack fruit samples were collected from a total of eight locations i.e., 2 from each AEU during 2021-23. Six out of the eight fruit rot affected fruit samples were observed at soil level whereas the remaining two were above soil level. The disease was manifested as white, fan shaped mycelial growth with presence of cream coloured sclerotia on fruits. Gradually, discolouration and rotting were observed on the fruits. The natural incidence of the disease was observed at slightly acidic to neutral pH, at soil temperature of 24 to 34℃ and 21 to 50.40 per cent soil moisture level. Pathogen isolation was undertaken from each fruit rot affected jack fruit sample collected from the eight locations and pathogenicity of the isolates was confirmed individually. The mycelia of all the isolates appeared as white with fan shaped growth except for a cottony appearance in the case of Kunnumma isolate. All the isolates produced round shaped sclerotia with colour variations ranging from light cream to dark brown. Among all the isolates, Karamana isolate took the least days for completion of mycelial growth (3 days) and formation of sclerotia (5 days) in vitro. The pattern of growth of sclerotia appeared as scattered (Venganoor, Chavara, Alappuzha, Kunnumma and Pathanamthitta isolates), ring like at centre (Karamana and Koodal isolates) and aggregated at the centre (Kottayam isolate). The hyphae of all the isolates were septate with size ranging from 0.02 to 0.03 μm. The size of the sclerotia of the isolates ranged from 1.02 to 2.00 mm. Karamana isolate was revealed to be the most virulent isolate with 195.84 cm2 of infection area within three days of artificial inoculation on fruits. The virulent isolate of the pathogen was selected for identification. Morphological and cultural characters including presence of fan shaped mycelia and clamp connections confirmed it as A. rolfsii. Further, molecular characterization using ITS primers conclusively confirmed the fungus as A. rolfsii. Evaluation of chemicals for their antifungal potential against A. rolfsii by poisoned food technique revealed that dithane M-45 75% WP, tebuconazole 25% WG, propineb 70% WP and hexaconazole 5% EC completely inhibited the fungus at recommended and half the recommended dose. The chemicals viz., dithane M-45 75% WP, tebuconazole 25% WG and hexaconazole 5% EC were effective even at one fourth of the recommended dose. In vitro evaluation of biocontrol agents by dual culture technique revealed that Trichoderma asperellum (KAUT6) and Trichoderma viride (NBAIR) were the most effective bio agents which resulted in 80.55 and 51.77 per cent inhibition of the mycelial growth of the fungus respectively. Studies on the compatibility between effective chemicals and biocontrol agents revealed that T. asperellum (KAUT6) was completely compatible with hexaconazole 5% EC (0.025%, 0.05%, 0.1%) and dithane M-45 75% WP at recommended dose, half as well as quarter of the recommended dose. Tebuconazole 25% WG showed compatibility with T. asperellum (KAUT6) at quarter of its recommended dose only. In vitro assessment of the influence of various environmental factors on the growth of the fungus on artificially inoculated fruits was conducted. Exposure to varying light intensities revealed that alternate cycles of light and darkness promoted mycelial growth of A. rolfsii compared to continuous darkness and other varying levels of light intensity. Evaluation of influence of different levels of temperature demonstrated that high temperature levels of 37 and 40˚C inhibited the growth of the fungus when artificially inoculated on to the fruits. The pH range for optimum growth of mycelium and production of sclerotia was revealed to be within a range of 5.5 to 6.5. High soil moisture percentage was revealed to inhibit the growth of the fungus. Pre and post application of the best effective contact fungicide, bio agent and their combination on artificially inoculated jack fruits revealed that pre application of dithane M- 45% WP (0.3%) resulted in complete (100%) inhibition of the growth of the fungus on fruits upto one week period wherein the untreated inoculated fruit got completely rotten. Studies on post-application of dithane M-45% WP (0.3%) at 24 h after artificial inoculation resulted in complete (100%) inhibition of the growth of the fungus followed by post application of talc based formulation of T. asperellum (KAUT6). However, subsequent observations revealed that, beyond 72 hours after artificial inoculation, the virulent isolate of the fungus continued to grow on the fruits, revealing that neither the chemicals nor the biocontrol agents, alone or in combination, was effective in controlling the fungus on artificially inoculated fruits. Thus, the present study could identify A. rolfsii as a predominant fungal pathogen causing fruit rot of jackfruit in Kerala. The disease incidence was revealed to be influenced by the prevailing weather conditions. It was revealed that pre application of dithane M-45 75% WP (0.3%) before artificial inoculation as well as post-application of dithane M-45 75% WP (0.3%) within 72 hours of artificial inoculation were the most effective treatments followed by dithane M-45 75% WP (0.3%) combined with talc based formulation of T. asperellum (KAUT6). Further validation through multi-seasonal field trials need to be undertaken to identify the cardinal environmental factors resulting in disease incidence as well as to develop a holistic approach for the integrated management of the disease.