Tea (Camellia sinensis (L.) O. Kuntze) plant productivity and protection with microbial stimulants : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

While there is an increasing consumption of organic tea as a healthy beverage, tea growers are struggling to produce tea without agrochemicals. To help find solutions for this challenge, a study was conducted on bioprospecting microbes to potentially create bioprotectant products against tea pathogens in Sri Lanka and biopromotion products in organic tea cultivation in New Zealand. Blister blight, caused by Exobasidium vexans, is a major tea disease in Sri Lanka. Investigation of this obligate parasite has been limited due to a lack of successful artificial culture media. Potato Dextrose Agar medium was modified with five different tea extracts to investigate the best growth media which can facilitate dual-culture assays among indigenous endophytes and tea pathogens. Brown blight caused by Colletotrichum camelliae was also tested in this study. Potato dextrose agar ammended with either 25% fresh or dried tea leaf infusion incorporated, tea extact agar (TEA) media were successful for in-vitro cultivation of tea pathogens. During the research, Nigespora sp. was arbitrarily identified as a leaf blight pathogen on tea in Sri Lanka for the first time. Twenty indigenous fungi and seven bacterial endophytes were identified from the tea phyllosphere in Sri Lanka. More than 50% of them exhibited biocontrol potential against the economically important tea pathogens in-vitro while Trichoderma harzianum provided the best inhibition. Seven indigenous fungal and four bacterial genera were identified from soil and leaf samples from a tea plantation in New Zealand and used for biopromotion bioassays on young tea plants grown under glasshouse conditions. Indigenous Hypocrea lixii provided the most significant biostimulation, than its exogenous anamorph; T. harzianum, which was tested representing the new taxonomic concept of H. lixii and T. harzianum as two individual species (previously classified as teleomorph and anamorph of the same species). This study isolated H. lixii for the first time in New Zealand under this new taxonomy. T. harzianum was the only fungal species that was able to colonise tea roots in glasshouse experiments, but all tested indigenous and exogenous microbes showed a higher biostimulant efficacy on tea than the controls. Commercially produced, water-soluble Trichoderma atroviride powder with a 2 x 105 cfu mL-1 concentration was in-vivo tested on mature tea plants in Hamilton. A single foliar application of T. atroviride increased tea yield by up to 28% six months post-application. The first-ever microbiome study conducted for tea soil in New Zealand suggested a relationship between yield and Ascomycota presence, indicating that introduced T. atroviride stimulated the growth of the tea plant synergistically favouring its own phylum. Tea soil in New Zealand is reported to be rich with yeasts and less decaying material compared to traditional tea grown under shade trees with high organic matter content, suggesting the soil has higher simple sugars than lignocellulosic compounds which could facilitate the Trichoderma establishment and persistence, thereby artificially introduced T. atroviride might have been overshadowed by the complex resident soil microorganisms. Therefore, the study suggests field application of a multi-strain consortium of the best potential indigenous biostimulants found in the glasshouse experiments (H. lixii, Epicoccum sp., Bacillus sp., Clonostachys candelabrum, Penicillium limosum, and Penicillium arizonense) to form a more stable soil memory and effectively manipulate the resident soil microbiome in-situ, improving the desired biostimulat functions between the biostimulants, soil and plants, which is particularly more important for perennial crops like tea.