The incidence of plant-parasitic nematodes on sugarcane in Queensland, and studies on pathogenicity and associated crop losses, with particular emphasis on lesion nematode (Pratylenchus Zeae

In Queensland, sugarcane has been cropped as a monoculture for 80 years or more in most districts. In the last 30 years, plough-out and replant (no fallow) has increased,\ud as has reliance upon inorganic fertilisers, and intensive tillage to remove soil compaction. An associated decline in the productive capacity of the soil to grow sugarcane has been identified, and has been termed ‘yield decline’ (YD). Root health and sugarcane yields are increased after fallowing, crop rotation, and soil fumigants\ud (Magarey and Croft 1995; Garside et al. 2001; Meyer and Van Antwerpen 2001), implicating root pathogens in YD. However, in the past, nematode studies have been confined to testing the economics of using nematicides.\ud It was the objective of this work to explore the association between plant-parasitic nematodes and sugarcane in Queensland. Firstly, this thesis examines the incidence\ud of nematodes on field crops. The regional distribution of nematodes is reported, together with nematode populations and dynamics relating to (a) root habit, (b) root\ud distribution across the row to inter-row profile, and (c) temporal changes during the crop cycle.\ud Secondly, this thesis explores the parasitism of Queensland sugarcane by nematodes, and role in YD. The importance of sett roots, nematodes, and general YD biota on\ud early plant establishment from 0-100 days after planting is examined in field miniplots. Crop losses due to nematodes are assessed at 16 field sites using nonvolatile\ud nematicides, and the pathogenicity of Pratylenchus zeae is examined in glasshouse pots and field miniplots.\ud The lesion nematode (P. zeae) was found to be ubiquitous in sugarcane fields, and usually at higher densities than other species. The density of root-knot nematode\ud (Meloidogyne spp.) was also high in sandy soils (<20% clay), but a high proportion of other soils also contained this nematode, albeit at lower densities. The\ud ectoparasites, spiral nematode (Helicotylenchus dihystera), stubby-root nematode (Paratrichodorus minor) and stunt nematode (Tylenchorhynchus annulatus) were\ud also detected in a high number of fields (>66%). Historically, the sugar industry has perceived nematode problems to be confined to very sandy soils in south Queensland. However, plant-parasitic nematodes occur in all soils, suggesting a more widespread role in YD.\ud Within sugarcane fields, nematodes were distributed in aggregated patterns. Thus, densities of lesion nematode varied up to five-fold across short distances (1.4 m)\ud even at a constant distance (20 cm) from the sugarcane stool. Ring and spiral nematode were more aggregated than lesion nematode, perhaps due to more\ud sedentary feeding habits and greater sensitivity to edaphic gradients (eg. soil texture and moisture) across the field at the macro-distributional level. The ‘negative\ud binomial model’ was used to predict the sampling effort required to estimate mean nematode densities with degrees of precision. Mean nematode densities across the row, near row (20-30 cm from the stool), and inter-row were very similar during the crop cycle. Because high densities of\ud nematodes were regularly recovered from ‘near the row’ this zone was recommended for standard sampling. During the crop cycle, nematode densities were related to the\ud volume of the root system and its growth rate, as influenced by season. Because sugarcane develops a new root system annually, nematode densities increased and then declined each year. At planting, up to 400 lesion nematodes and up to 100 spiral nematodes/200 mL of soil were present, which was usually more than other pest species (<50 nematodes/200 mL of soil). Lesion nematode generally persisted at\ud higher densities than other pest species during the crop cycle.\ud Lesion nematode was pathogenic to sugarcane in 1.5 L pots, reducing root weight and sometimes reducing shoot biomass. In 50 L pots, this nematode caused a general blackening of roots and reduced fine root length by over 50%. Shoot biomass was generally not reduced, suggesting that YD is induced by a combination of root pathogens.\ud At planting, prior studies have related poor primary tiller emergence to poor sett root growth in field soil (Cadet and Spaull 1985; Garside et al. 2002 a; Pankhurst et al.\ud 2002). However, this study showed that buds can rely entirely upon the stem cutting to shoot and become established primary tillers. It was concluded that damaged buds, dormant buds, a poorly nurtured seed source, and poor sett root growth, all contribute to poor primary tiller establishment. Deleterious soil biota and nematodes also\ud reduced the health and volume of shoot roots, which reduced the number of secondary tillers emerging at early establishment. While the experimental sites had a\ud history of consistent fumigation responses (>80%), nematicide responses were quite variable (0-50%). Experiments in glasshouse pots confirmed that nematodes\ud contribute in part to fumigation responses in YD soils.\ud To assess crop losses, nematodes were controlled for the entire crop cycle using nonvolatile nematicides at 16 field sites. Fertile sandy loam to clay soils were chosen\ud where losses from nematodes have only been speculated on previously. While poor tillering due to serious nematodes problems is well documented in sandy soils (