Study on phoretic and mycangial fungi of Ambrosia Beetles and their aggressiveness associated with Tree Bean decline.
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Date
2022-11
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College of Post Graduate Studies in Agricultural Sciences, Central Agricultural University - Imphal
Abstract
Parkia timoriana, popularly known as tree bean is an underutilised yet highly potential crop in north eastern region of India. Since, last two decades, widespread occurrence of sudden death and decline of matured tree bean were reported from Manipur, Mizoram, Meghalaya, India. It has been reported that the tree bean decline disease is caused by Fusarium spp. mainly (F. euwallaceae, F. ambrosium, F. solani) fungal symbionts of ambrosia beetles, resulted significant reduction in pods production. Ambrosia beetles belongs to weevil subfamilies; Scolytinae and Platypodinae and survive in symbiosis with fungal symbionts. Ambrosia beetles carries their nutritional mutualistic fungi in specialized structures called mycangia. Fungal propagules are also dispersed phoretically on the beetle’s exoskeleton. Phoretic transmission of symbiotic fungi, Fusarium euwallaceae of the Euwallacea sp. of ambrosia beetles potentially contribute to the infection in tree bean. In this research, six different species of ambrosia beetles infesting the tree bean decline were identified using morphology and molecular, viz; Euwallacea interjectus, Euwallacea similis, Euwallacea fornicatus, Euplatypus parallellus, Xylosandrus crassiusculu and Xyleborinus sp. Identification was carried out by amplifying the mitochondrial gene COI region, Cytochrome Oxidase I, using the primer pair LCO1490 and HCO2198. Fungal isolation from each species (mycangia and phoretic) of the ambrosia beetle was done by serial dilution, to determine the abundance of symbionts and other species. F. euwallaceae recorded the highest frequency in mycangia and recovered from the beetles, E. interjectus, E. similis and E. Fornicates followed by different species such as Fusarium ambrosium, Fusarium solani. Other fungi, Nectria pseudotricha, Clonostachys rosea Lasiodiplodia theobromae, and L. pseudotheobromae that was also recovered from external isolation of fungi from the beetles. Average CFUs per beetle ranged from 14.55 to 32.05 and CFU ranged from 1 to 112. The highest CFUs (34.4/beetle) in F. euwallaceae recovered from Euwallacea interjectus, followed by E. fornicates (33.55/beetle) in phoretic whereas in internal, E. similis recorded the highest (32.05/beetle). Comparing between internal and external isolation of fungi, it was observed that frequency of recovery of fungi from external isolation was found greater than mycangial fungi. In another experiment, pathogenicity test was assayed on 2 years old tree bean seedlings to check the aggressiveness using four fungal isolates viz. Fusarium solani, Lasiodiplodia theobromae F. euwallaceae, and F. ambrosium under net house condition. Agar plug (5mm) and spore suspension technique (2×106 conidia/ml) was used to check the aggressiveness of the selected pathogens. Lesion was developed in all treatments except the control with the highest lesion length of 10.48±1.08 cm using agar plug and in spore suspension, the highest lesion length recorded was 6.88±0.57cm. All the isolates showed statistically similar aggressiveness to the host. In stem detached method, the highest lesion length recorded was 7.04±0.28 cm. In this present study, it can be concluded that different ambrosia beetles infesting tree bean and carry their symbiont fungi, especially F. euwallaceae and symbionts were found pathogenic. In future, further studies may be conducted on diversity of ambrosia beetles, symbiotic fungi and their host ranges and susceptibility in this region and at instance, identification of resistance tree beans germplasms against this pathogens and pest may also be explored.