Cost of root disease on white clover growth in New Zealand dairy pastures

The cost to clover growth of soil-borne root disease was measured in ten New Zealand dairy pasture soils. The average clover growth rate after soil pasteurisation was 28.5%, but range from 64% increase (Whataroa soil) to a decrease of 11.9% (Ruakura soil). The economic cost of reduced clover growth was determined using Farmax modelling (dairy pro). In Southland and Canterbury, clover root disease is estimated to cost $750 and $715 ha-1 year-1 respectively, and in the Waikato region $1506 ha-1 year-1. DNA-based testing of the soils detected the presence of diverse fungal, Oomycete and nematode pathogen populations. A significant linear relationship was found between reduction in clover growth and group f Pythium spp. (P=0.0177). The DNA-based assay indicated that Aphanomyces trifolii, a root pathogen of subterranean clover, maybe present in the dairy-pasture soils. As this pathogen is currently not recognised as present in New Zealand, a definitive determination of its presence is required. Based on the high economic costs, the control of soil-borne root pathogens in New Zealand pastures is a direct means to increase profitability.