Quantification of the effects of fumigation and nematicide treatments on the early shoot and root growth of sugarcane in a yield decline soil

In a glasshouse experiment, a comparison was made of the early shoot and root growth of sugarcane (Q 117) in a soil known to contain soil organisms associated with yield decline, and soil that was fumigated with methyl bromide or was treated with the nematicide, aldicarb. Sequential measurements were made over a 4-month period on root growth (both sett and shoot roots), shoot growth, colonisation of sett and shoot roots by the lesion nematode and mycorrhizal fungi, and populations of rhizosphere bacteria and fungi. Plants grown in the fumigated soil developed a more rapidly growing primary shoot, a larger sett root system, and initiated secondary shoots and shoot roots earlier than plants growing in untreated soil. The sett and shoot roots of these plants were also whiter and healthier compared to those of plants in the untreated soil. Aldicarb controlled lesion nematodes and enabled plants to develop a seU root system equivalent in weight to that of plants in the fumigated soil. However, between 38 and 52 days after planting and coinciding with the initiation of secondary shoots and shoot roots, growth slowed in the plants in the aldicarb-treated soil compared to those in the fumigated soil, and subsequent growth was similar to that of plants in the untreated soil. The failure of plants in aldicarb-treated soil to maintain optimal growth appeared to be due to factors other than nematodes. These factors may have included increased infection of shoot roots by pathogenic fungi or lower populations of beneficial bacteria in the rhizosphere. Control of lesion nematodes is thus important for the development of the sett root system, but a healthy soil environment, without other deleterious soil organisms is critical at the time of shoot root initiation and is a prerequisite for subsequent optimum plant growth.