Biological control of the soilborne pathogen Rhizoctonia solani in Asian cabbage : a thesis submitted in partial fulfilment of the requirements for the Degree of Master of Horticultural Science at Lincoln University

Abstract

Brassicas are common vegetable crop in most parts of the world, including Papua New Guinea (PNG). However, pathogens can affect vegetable brassica production. Rhizoctonia solani is a serious fungal soil-borne pathogen that causes damping off and bottom rot of emerged seedlings. It attacks the hypocotyl or lower stem tissue of the plant and causes wire stem. In severe infections the stem separates from the root and dies. Asian cabbage (Pak Choi) is widely grown in PNG, but the effects of R. solani on its production, or methods for control to the pathogen are not known. This research programme investigated the efficacy of biological control methods to manage R. solani in Asian cabbage. Two New Zealand strains of R. solani (043-1 and 043-4) were evaluated for their virulence on Asian cabbage, with strain 043-4 being identified as more virulent and used in subsequent experiments. Four Trichoderma isolates (T. atroviride LU132, T. hamatum LU785, T. harzianum LU1347, and T. polysporum LU1358) from the Lincoln University culture collection were formulated into a seed coating and tested for their biocontrol potential in comparison with that of the seed treatment fungicide Apron XL using soil naturally infested with R. solani. T. hamatum LU785 significantly improved seedling emergence and survival, while T. atroviride LU132 enhanced plant growth parameters in a study. A further trial compared the effectiveness of T. hamatum LU785 and a ground mustard (Brassica juncea) biofumigation applied as seed treatments. Both treatments increased seedling emergence in the presence of the pathogen and increased early plant growth. Additionally, the biofumigation properties of glucosinolates from Brassicaceae species were assessed in the glasshouse. The incorporation of Frozen cauliflower tissues into the growing medium provided better control of the pathogen and allowed greater plant growth than fresh or frozen tissues of other brassica species. This study has identified the potential for using Trichoderma spp. and biofumigation to reduce the impact of R. solani on the production of Asian cabbage, but these results require confirmation in field studies.