The broad spectrum fungicide mancozeb was applied to monocultured sugarcane soils affected by the growth constraint known as sugarcane yield decline. The constraint is associated with the long-term monoculture of sugarcane and has been shown to decrease sugarcane growth by 20% in commercial crops in Queensland. The fungicide was shown to decrease root colonization by soil fungi, particularly dematiaceous sterile fungi. Higher doses of mancozeb (up to 400 mg kg−1) sometimes led to increased populations of Penicillium spp. The fungicide greatly improved plant growth and root health when applied at doses of 100 mg kg−1 and above. Results suggest that the role of dematiaceous sterile fungi in sugarcane yield decline should be further examined.