Seed treatment options for New Zealand carrot production : A thesis submitted in partial fulfilment of the requirements for the Degree of Master at Lincoln University

The New Zealand carrot industry currently uses fungicide seed coatings, rather than using some of the biological options available for disease control. The industry standard seed treatment has been in use since at least 2008. This research had the aim of comparing the current seed treatment with a pre-commercial biological (Trichoderma) seed treatment under a variety of conditions to determine the best approach. Data were gathered to show the effects of the various seed treatments on the carrots’ emergence, harvest weights, disease severity, and storage ability. A total of four experiments were conducted, two in the field and two in the glasshouse. A further experiment assessed the effects of all the pesticides applied to the crop or seed on Trichoderma conidia germination. A field trial was established in a commercial crop at Waiuku with five different seed treatments, including the combination of chemical and biological seed treatments. Unfortunately, due to covid restrictions, no data could be gathered until final carrot harvest, and harvested carrots were used in a cool store based storage trial. The Trichoderma alone treatment significantly increased yield over the industry standard seed treatment but there was no difference in storage performance among the treatments. A second field trial at Lincoln compared between chemical and Trichoderma seed treatments and untreated seed. These carrots were grown at a site known to contain soil-borne pathogens, and the crop received no pesticide applications. The aim was to investigate whether Trichoderma could provide long term disease control and increase marketable carrot yield. The carrots were also ground stored for a month after harvest to further test any long term disease suppression. However no significant differences were recorded among the treatments for any of the parameters measured. The glasshouse trials used soil from major carrot growing areas of New Zealand and looked at how chemicals and Trichoderma could help reduce the impact of the pathogens present compared to untreated seed. Additional data related to growth promotion such as foliage, taproot, and fine root weight were also recorded. Both trials tended to show that both the Trichoderma and chemical seed treatment had comparable performance in emergence and disease control and that this exceeded that of untreated seed. No growth promotion effects were recorded based on the dry weights of the foliage, taproots, or fine roots. To test Trichoderma conidia germination in the presence of pesticides, conidia were incubated at 25°C in the presence of pesticides at label rate concentrations. Glyphosate, difenoconazole, and mancozeb completely inhibited conidia germination but the seed treatment fungicides did not. Overall the results suggest that, under the conditions used, a Trichoderma seed treatment had a similar performance to the commonly used fungicide seed treatment. There was no best seed treatment in this study, but more options have been identified. The results in glasshouse conditions were not replicated in the field. The results also suggest that this strain of Trichoderma did not provide long term disease control, or promote carrot growth. The compatibility results showed that these strains of Trichoderma are sensitive to some pesticides but they are pesticides that are unlikely to come into direct contact with the conidia. The seed treatment fungicides did not affect conidial germination.
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