Endophytic fungi associated with cabbage in New Zealand and their potential for biological control : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Agricultural crops such as cabbage (Brassica oleracea var. capitata) are vulnerable to a plethora of pests and diseases. Endophytic fungi are increasingly used in biological control against these pests and diseases as agricultural producers attempt to reduce their environmental footprint. In this study, putative endophytic fungi were isolated from cabbage grown in three regions in New Zealand in order to identify isolates with potential to assist crop production. Isolation method was optimised in an exploratory study, and subsequently large scale sampling took place, here a culture-independent method was also used to detect fungi associated with cabbage. Spatial patterns in fungal communities in various plant tissues and geographical regions were investigated based on data obtained by culture-dependent and culture-independent methods. Selected fungal isolates were tested in bioassays to determine their potential for biological control of a fungal disease caused by Sclerotinia sclerotiorum and an insect pest of cabbage - Diamondback moth (Plutella xylostella). Over 100 species were cultured, but high-throughput sequencing suggested the number of fungal species associated with cabbage tissues could be over 300. Distinct fungal communities were found in two scales of spatial distribution - various host plant tissues and geographical regions. Increased mortality of Diamondback moth larvae feeding on cabbage leaves treated with spore suspensions of several isolates of Lecanicillium sp. was observed. None of the tested isolates used as seed inoculants significantly reduced disease symptoms caused by Sclerotinia sclerotiorum in a bioassay on cabbage seedlings. In this study several fungal isolates were identified as promising biological control agents of an insect pest of cabbage. However, these isolates are members of a large consortium of fungi. The patterns uncovered in the fungal communities underline the importance of understanding the microbial communities associated with agricultural crops.