Evaluating the distribution, diversity, and abundance of plant parasitic nematodes in New Zealand kūmara growing regions : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science at Lincoln University

Abstract

Plant parasitic nematodes (PPN) significantly threaten agricultural productivity, leading to significant yield losses. In New Zealand, kūmara (Ipomoea batatas) is a crop with immense cultural, societal, and economic importance. While several ecologically important PPN species are known to impact kūmara globally, no studies have examined their distribution and impact in New Zealand. This study evaluates the distribution, abundance, and diversity of PPNs across New Zealand’s kūmara growing regions and how environmental factors influence PPN populations. Soil samples were collected from kūmara fields in Dargaville, Kaitaia, Palmerston North, and Hastings. The nematodes were extracted from soil samples using the sieving-centrifugal-flotation method, analysed in terms of diversity and abundance, and identified using morphological and molecular techniques. Geographic Information Systems (GIS) were used to map PPN distribution and analyse relationships between nematode populations and soil physiochemical and climatic factors. Nine key PPN genera were identified, including Meloidogyne, Pratylenchus, Helicotylenchus, Paratylenchus, Heterodera, Dorylaimus, Aphelenchoides, Hoplolaimus, and Criconemoides. Meloidogyne and Pratylenchus were the most prevalent in all locations and were in high abundance. Morphological identification confirmed Pratylenchus pratensis and Helicotylenchus dihystera. Molecular identification confirmed P. pratensis and M. naasi. GIS spatial analysis effectively mapped PPN distribution. However, the data revealed no significant relationships between PPN populations, soil physiochemical parameters, and the climatic factors of rainfall, air temperature, and soil moisture. However, these findings are predicted to become significant with climate change. This study provides the first evaluation of PPNs in New Zealand’s kūmara growing regions, addressing knowledge gaps by identifying key species and assessing their threat to kūmara production. While current PPN populations do not immediately threaten kūmara, climate change could alter their distribution and parasitic potential, necessitating ongoing monitoring and management. Despite these advancements, the absence of economic thresholds for PPNs in New Zealand remains a critical limitation. Future research should focus on developing threshold levels through pathogenicity trials and investigating the susceptibility of local kūmara varieties to key PPN species. As climate change intensifies, proactive management strategies and continued research will be essential to protect New Zealand’s kūmara production. This study lays the foundation for future investigations into the interactions between PPN populations, environmental factors, and sustainable crop management practices in New Zealand’s kūmara industry. These findings have significant implications for enhancing kūmara management and guiding future research directions.