Assessing the prevalence, diversity, and damage potential of plant-parasitic nematodes on maize (Zea mays L.) : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Maize cultivation faces challenges from plant-parasitic nematodes (PPNs), which can significantly reduce yields. Despite their importance, PPNs are often underestimated in agricultural ecosystems. Research in New Zealand and Sri Lanka reported the presence of PPNs in maize fields. However, comprehensive data is still lacking. Environmental factors such as soil pH, moisture content, nutrient levels, and rainfall could influence PPN population densities and pathogenicity. Hence, understanding PPN dynamics and environmental interactions is crucial for sustainable production. This study aimed to; i) examine the prevalence, abundance, and distribution of PPNs associated with maize cultivation in Sri Lanka and New Zealand, ii) identify the most abundant PPN species and their distribution in New Zealand maize fields, iii) investigate the relationship between soil edaphic and climatic characteristics with PPN abundance in New Zealand maize fields, and iv) screen the susceptibility of commercially available maize hybrids in New Zealand against the most abundant PPN genus under glasshouse conditions. A survey conducted in Sri Lanka's semi-arid regions with major maize cultivation areas found that over 92% of maize fields tested positive for PPNs. The dominant species was Pratylenchus spp., with a mean population density of 2020 kg-1 of soil. Sampling at different maize growth stages revealed that Pratylenchus spp. populations were below 1000 kg-1 of soil at the seedling stage but increased by 2 to 10-fold at harvest. In New Zealand, a total of 384 soil samples were collected from 25 sites from both the North (NI) and South Island (SI) of New Zealand from maize growing regions. The major PPNs identified by morphology were Pratylenchus (91%), Helicotylenchus (38%), Meloidogyne (14%), Tylenchus (14%), Heterodera (8%), Paratylenchus (3%), and Criconemella (1%). Pratylenchus spp. were widespread across all surveyed regions. In most sampled sites from both islands, Pratylenchus spp. exceeded 1000 kg-1 of soil, which is recognised as the economic threshold set by nematologists worldwide. Five Pratylenchus spp. in New Zealand maize fields: P. crenatus, P. neglectus, P. thornei, P. penetrans, and P. pratensis, were identified based on morphology and the sequencing of DNA-PCR products extracted from single nematode using Sanger sequencing. Among these, P. crenatus and P. neglectus were predominant across maize-growing regions in New Zealand. The soil analysis revealed that tested soil properties varied between the sampled regions, while PPN abundance and diversity exhibited a regional dissimilarity. The relationship between PPN composition and soil physicochemical properties of New Zealand maize fields analysed using the canonical correspondence analysis (CCA). Results showed that soil moisture, pH, soil P, and percentages of sand, clay, N, and C significantly influenced PPN abundance and distribution. However, the influence of soil parameters on the PPN population varied within the regions, where soil moisture content was the main driver for the PPN population abundance within all three regions. A year-round sampling conducted at one of the sites in the Canterbury region of SI revealed the influence of seasonal and cropping factors on PPN abundance. This indicated a 5-fold population increase of Pratylenchus spp. from crop establishment to maturity during a season. Also, CCA on seasonal variations on soil physicochemical, climatic factors, and PPN population abundance in a field revealed that the soil moisture, volumetric water content, bulk density, sand%, N and C with the climatic factors; rainfall, relative humidity, and sunshine hours had a significant relationship with PPN population dynamics. Three nematode extraction methods for maize roots were tested. Centrifugal sugar flotation yielded the highest nematode count but with low clarity for microscopic observation. The Baermann funnel had 35.9% extraction efficiency with high clarity. The Whitehead tray had 60.8% efficiency with moderate clarity compared to centrifugal sugar flotation. The Whitehead tray is a good compromise for clarity and count, while sugar flotation is efficient and faster processing of samples for Pratylenchus nematodes, adding another layer of information for future studies. The outcomes from the glasshouse experiments using 15 commercial maize hybrids and a mixed field population of Pratylenchus spp. showed that most tested maize hybrids were susceptible to Pratylenchus spp. and demonstrated varying degrees of susceptibility. Another glasshouse experiment was conducted to determine the pathogenicity and reproduction of Pratylenchus on three selected maize hybrids based on the results from the previous trials. The results demonstrated significant reductions in root dry weight across hybrids, with losses ranging from 33% to 42%, and shoot weight losses varied among hybrids 10 to 22% compared to the controls 60 days after planting. This is the first comprehensive study on the PPN community association with maize in Sri Lanka and New Zealand. These findings emphasised the potential impact of PPNs, particularly Pratylenchus spp., on maize yield in both countries, highlighting the urgent need for further research to develop effective PPN management tactics adapted to local conditions.