Effects of Organic Waste Amendments, Herbicides, and Weed Hosts on Pythium Root Rot of Sugarcane.

Abstract

Three objectives that could lead to improved understanding and management of Pythium root rot of sugarcane were addressed. First, the potential of amending soil with different organic wastes to suppress root rot was evaluated, and the biological and chemical properties of wastes that may contribute to disease suppression were identified. Secondly, herbicide interactions with root rot affecting sugarcane growth were assessed. Finally, the role of weeds in disease epidemiology was investigated. Greenhouse and laboratory studies indicated soil amendment with some organic wastes offers a sustainable disease management option through biological control. Amendment with different wastes had variable effects. Sewage sludge, sugar mill filterpress cake, and cotton gin trash compost enhanced sugarcane growth by suppressing root rot. Bark composts enhanced plant growth and suppressed disease when added in either nonsterile or sterilized forms. Municipal yard waste compost generally had no effect on disease severity and plant growth. Municipal solid waste compost decreased plant growth. Sewage sludge, filterpress cake, and cotton gin trash, had the highest microbial activity levels; generally contained higher N, P, K, and Mg, levels and had lower C:N ratios than other wastes. Bark composts had high microbial activity, lower N, and higher Ca, S, and C:N ratios. Municipal solid waste contained high salt concentrations. Six herbicides had different effects on sugarcane growth, Pythium arrhenomanes, and disease severity in greenhouse and laboratory experiments. Some herbicides inhibited mycelial growth of P. arrhenomanes in vitro but did not reduce disease severity. Herbicides that were not phytotoxic generally did not increase disease severity or reduce plant growth. Herbicides that were phytotoxic always resulted in increased disease severity, but it was uncertain to what extent plant growth reductions were caused by enhanced disease or direct herbicide injury. Greenhouse and field studies indicated grasses common in sugarcane fields serve as alternative hosts for P. arrhenomanes. They could thus provide inoculum during the growing season or a means of survival during winter and the fallow period.