Microbial consortium against major fungal diseases of jackfruit (Attocarpus heterophyllus)

Abstract

Jackfruit (Artocarpus heterophyllus Lam.) belonging to the family Moraceae is considered as a high value crop due to its multiple uses and is gaining popularity in large scale commercial agriculture. However, the rise of fungal and bacterial diseases poses a significant challenge to jackfruit cultivation, particularly in newly introduced, high yielding varieties that exhibit increased susceptibility compared to traditional types. The increased plant disease tolerance of indigenous jack varieties may be due to the influence of root exudates at the rhizosphere region that harbours a diverse community of microorganisms. Hence, this study was taken upto characterize the beneficial microbes from the rhizosphere soil of indigenous jack (Artocarpus heterophyllus), in vitro evaluation against major fungal pathogens of jackfruit; and preparation of microbial consortium. Diseased leaves, stem and fruits were collected from Thrissur and Ernakulam districts and fungal pathogens were isolated from these infected plant parts. Two fungal pathogens EK1 and TK2 with high disease severity and virulence were selected for this study and pathogenicity was proved by following Koch’s postulates. The isolate EK1 produced water soaked lesions on leaves and symptoms of stem rot with sporulating hyphae. Pure white, fluffy, rope like mycelia on the jackfruits with white and brown coloured sclerotia was observed in the case of isolate TK2. Cultural, morphological and molecular characterization of these isolates were done. Molecular characterization through sequencing of ITS region confirmed the identity of isolates as Phytophthora colocasiae and Agroathelia rolfsii. Purposive sampling survey was conducted in the three districts of Kerala, viz.,Thrissur (AEU 10), Palakkad (AEU 10 and 23), and Malappuram (AEU 11). The older jackfruit trees which are high yielding and healthy were selected for the collection of rhizosphere soil. Nine samples were obtained from nine locations of the three districts viz., Madakkathara, Panancherry, Kannara of Thrissur, Sankaramangalam, Pattenchery, Pattambi of Palakkad and Anakkayam, Kizhuparamba, Munduparamba of Malappuram districts. A total of 88 rhizosphere microbes were isolated, in which 37 isolates were fungi and 51 isolates were bacteria. In vitro screening was carried out to assess the antagonistic potential of these microbial isolates against P. colocasiae and A. rolfsii. One fungal and bacterial isolates showing highest inhibition were selected from each location. The fungal isolate MMF1 showed highest per cent inhibition against both the pathogens. It inhibited P. colocasiae by 76.30 per cent and A. rolfsii by 58.15 per cent. The bacterial isolate TMB4, was found to inhibit P. colocasiae by 100 per cent and A.rolfsii by 76.30 per cent. The selected nine fungal and bacterial isolates were subjected to cultural, morphological and biochemical characterization. The most promising fungal and bacterial isolates were identified as Trichoderma asperellum and Ochrobactrum sp. respectively by molecular characterization. Different biocontrol attributes of the selected fungal and bacterial isolates were evaluated. Hyperparasitism, production of siderophore, ammonia, antibiotics, volatile and non-volatile metabolites, HCN, enzymes like β-l,3-glucanase and cellulase production were studied and different isolates exhibited varying results. Compatibility studies were carried out among the selected fungal and bacterial isolates. Microbial consortia were prepared with compatible bacterial isolates, compatible fungal isolates and with both compatible fungal and bacterial isolates. The in vitro evaluation of the three microbial consortia against the pathogens were done along with KAU reference culture Pseudomonas fluorescens (PN 026), Trichoderma asperellum (KAU strain), and Plant Growth Promoting Microbes (PGPM). Treatment with compatible fungal isolates viz., PPF2, MAF1, MKF2 and MMF1 (T2) exerted a highest inhibition of 100 per cent against P. colocasiae and 84.40 per cent against A. rolfsii which was on par with T3 (consortium with compatible fungal and bacterial isolates) and T5 (Trichoderma asperellum KAU strain) in the case of P. colocasiae. The least inhibition of 68.89 per cent against P. colocasiae was recorded with PGPM (T6) followed by T1 (consortium with bacterial isolates). In the case of A. rolfsii, T1 exerted a least inhibition of 60.74 per cent followed by T6 with an inhibition of 73.33 per cent. Four fungal isolates in the best treatment (T2) were characterized at molecular level. MKF2 and MMF1 were identified as Trichoderma asperellum where as PPF2 and MAF1 were identified as Trichoderma harzianum. The future studies can be conducted at field level to evaluate the efficiency of the biocontrol agents at natural conditions. Their ability to improve plant health and disease resistance can also be validated to develop suitable formulations for the sustainable jackfruit cultivation.