TESTING A PLANT BIOASSAY TO ASSESS THE CAPACITY OF SOILS TO SUPPRESS THE ACTIVITY OF SOIL ORGANISMS ASSOCIATED WITH YIELD DECLINE OF SUGARCANE

A plant bioassay was developed to test a soil’s capacity to suppress the activity of detrimental soil organisms associated with yield decline (YD). The bioassay utilised the diseased roots of sugarcane (cv. Q117) plants growing in soil that had been under continuous cane monoculture for more than 20 years, as the source of soil organisms associated with YD. A small quantity of diseased roots (2% w/w) was added to fumigated sand together with 10% (w/w) of the test soil in terra cotta pots. Suppression was measured as the capacity of the added test soil to block the detrimental effect of soil organisms associated with YD on the growth of sugarcane plants planted into the pots containing the sand, diseased roots and soil mixture. Soils tested for suppression included a soil that had been under a pasture for 7 years, soils that had been under a soybean break for 12 months, soil from under a rainforest, and soils that had been treated with a range of different organic amendments. The bioassay indicated that soil that had been under the pasture break had increased suppressiveness towards soil organisms associated with yield decline compared to a soil that had been under continuous cane monoculture. There was little difference between soils that had been under a soybean break for one year and the soil that had been under continuous cane monoculture, while a rainforest soil was found to have less suppression than the continuous cane soil. Incorporation of organic amendments to a sugarcane soil (e.g. sawdust, cane trash, grass hay, and lucerne hay), at a rate equivalent to 20 t carbon/ha, had only a minor effect on suppression.