Fungal Diversity in the Wounded Tissues of Gyrinops walla Trees and the Surrounding

Abstract

Gyrinops walla (Gaertn) of family Thymelaeaceae, is believed to be endemic to Sri Lanka and is mainly found in the lowland wet zone where the elevation is lower than 1,000 m and the annual rainfall is between 2,000 to 3,000 mm. This species is known for producing a valuable resin called agarwood in its trunk after being infected by pathogens, mainly fungi. Over 20 capable fungal species have been identified to stimulate agarwood formation in the trees of the agarwood- producing genus, Aquilaria. However, since such information is lacking for G. walla, this study aimed to determine the fungal taxa that induce agarwood formation in G. walla and to compare the fungal diversity between G. walla tissues, surrounding trees, and the soil. Samples were taken from four locations in the wet zone, Olugala, Morapitiya, Weragala, and Elpitiya, and one location in the intermediate zone, Kirimetimulla. Collected naturally wounded tissues from G. walla trees of those locations were surface sterilized before culturing. In addition, the stem samples were taken from the surrounding trees at 5 m radius from the sampled G. walla trees. Soil samples were collected at each location at 2.5 m distance along the north south directions of sampled G. walla trees which is from the sub-surface. A standard dilution series up to 1:10,000 was prepared for soil culturing. All the fungal colonies emerging from initial soil, stem cultures were isolated as pure cultures in separate PDA plates. Based on the colony morphology and growth parameters, all the pure cultures were categorized into 13 fungal isolates. Morphological and microscopic characteristics were used to identify the fungal isolates. Out of the 13, six isolates were identified as Aspergillus sp. (1, 2, 3), Rhizopus sp., Trichoderma sp., and Mucor sp. The unidentified isolates, due to the lack of reproductive structures, were named as morphotypes 5, 7, 8, 9, 11, 12 and 13. After identification, their presence and frequency of occurrence in different sample types i.e. resinous tissues and nonresinous tissues of G. walla, surrounding tree tissues and soil were evaluated in all five locations. For the comparison, diagrams were built. Thereby, predominant fungal isolates in each of the sample types were identified. Based on the occurrence of fungal isolates in each location, Olugala had the greatest fungal diversity, while Morapitiya had the lowest. Morphotype 07 was identified as the dominant isolate in resinous tissues of G. walla due to the presence of 15 isolates. Rhizopus sp. was also found as a common fungal isolate in the wounded tissues of all five locations. Trichoderma sp. was more abundant in the surrounding, while Aspergillus sp (1) was dominant in soil. Fungal isolates in resinous tissues were also present either in the non-resinous tissues of G. walla, surrounding tree tissues, or soil. Therefore, inocula are believed to enter the wounded tissues from nearby trees or from the soil, and spores from water, wind or carriers like insects. 

Keywords: Gyrinops walla, Fungal isolates, Surrounding trees, Soil