Integrating the use of a beneficial fungus in the genus Trichoderma into the production of export onions (Allium cepa) in New Zealand : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

Trichoderma biocontrol products for use in onion crops are available in New Zealand but are not part of standard practice. Onion diseases and pests are largely controlled by current chemical pesticides but the export onion producers face problems with their use. The onion industry is looking to rely less on chemicals and more on biopesticides, like beneficial Trichoderma, for pest control. The aim of this research was to test the hypothesis that Trichoderma atroviride strains, when applied as a seed coating, to onion seed, will become endophytic in the plant and increase quality and yield of bulbs for export. The project also investigated development of a method to cost-effectively measure the presence of T. atroviride in seed inoculated onion plants. A replicated field trial was conducted using two intermediate-day onion varieties (Tilbury and Waikato), grown under commercial cropping practice. Two T. atroviride mixes were applied via a commercial and a novel seed coating method, combined with registered fungicides (normal current practice). One month post-harvest, yield, complete skin numbers per bulb and unmarketable bulbs were determined. The application of Trichoderma showed the following benefits to the onion crop; across varieties in the 50-69 mm bulb size category, an increase by 0.294 (33%) and 0.283 (32%) of the mean complete skin number per bulb over the standard fungicide only treatment; and in Tilbury, in the 40-89 mm bulb size category, an increase by 4.37 t/ ha (8.5%) by one Trichoderma treatment over the standard fungicide treatment. The yield of quality bulbs (minimum one complete skin) increased in the 60-69 mm category for Tilbury by 6.29 t/ ha (45%) and in Waikato by 5.97 t/ ha (54%) over the standard fungicide treatment. These benefits differed between variety. These yield and quality benefits may be due to induced systemic resistance or growth promotion by the inoculated Trichoderma spp. although this was not determined. These benefits were seen despite full chemical usage including carbendazim, previously shown to be toxic to Trichoderma in in vitro tests. This shows a need to take Trichoderma, plant (variety genetics) and the field environment into account in trials for applicable results for growers. An isolation method was adapted to see if from T. atroviride seed inoculated plants, the inoculated strain could be differentiated from background (soil-borne) environmental Trichoderma. The method isolated Trichoderma from seedlings and roots, but not from leaves. The inoculated T. atroviride could not be distinguished from other Trichoderma by morphology and there was no significant difference in the Trichoderma levels between treatments. Trichoderma, when applied as a seed coating, significantly increased onion yield and quality over that of the standard fungicide only treatment. The use of fungicide in the crop did not appear to negatively impact Trichoderma activity. This suggests that Trichoderma could be included in an integrated pest management programme for onion production, and eventually may be able to replace, or significantly reduce, fungicide use. However, this still needs to be evaluated in further field trials.