Changes in nematode populations on sugarcane following fallow, fumigation and crop rotation, and implications for the role of nematodes in yield decline

A multi-disciplinary research program has been established in Australia to identify the causes of sugarcane yield decline (defined as 'the loss in productive capacity of sugarcane-growing soils under long-term monoculture'). In one part of that program, five experiments were set up in which the monoculture was broken for 12-42 months by maintaining a bare fallow using herbicides, by growing a grass/legume pasture or by planting consecutive crops, most of which were legumes. Other plots were maintained in continuous sugarcane during the break period and were either fumigated with methyl bromide immediately before replanting to sugarcane, or left untreated. Nematode populations were monitored in the following sugarcane crop. These data showed that for the first 6 months after planting, fumigation, bare fallow and crop generally reduced populations of Pratylenchus zeae Graham in comparison to continuous sugarcane. Pasture had a similar effect but it was only apparent at, or soon after, planting. Fumigation and bare fallow initially decreased populations of most other plant-parasitic nematodes, but some ectoparasitic species [e.g. Tylenchorhynchus annulatus (Cassidy) Golden and Paratylenchus colbrani (Raski)] returned to relatively high population densities within 6-12 months following these treatments at some sites. Pasture generally increased populations of free-living nematodes in comparison to continuous sugarcane whereas a decrease was sometimes observed following fumigation and bare fallow. All treatments increased the yield of the plant plus first ratoon crop by 20-30% compared with continuous sugarcane, indicating that soil factors affected by fumigation and break crops were having widespread and significant effects on sugar production in Australia. A reduction in nematode populations may have contributed to the yield responses at some sites but it is impossible to be conclusive because treatments also affected many other soil physical, chemical and biological properties.