Severity of root rot in mature subterranean clover and associated fungal pathogens in the wheatbelt

Abstract. Pasture decline is considered to be a serious challenge to agricultural productivity of subterranean clover across southernAustralia. Root disease is a significant contributing factor to pasture decline. However, root disease assessments are generally carried out in the early part of the growing season and in areas predominantly sown to permanent pastures. For this reason, in spring 2004, a surveywas undertaken to determine the severity of root disease inmature subterranean clover plants in pastures located in thewheatbelt ofWesternAustralia. DNA-based soil assayswere used to estimate population density in the soil of a variety of soil-borne pathogens known to commonly occur in theMediterranean-type environments of southern Australia. The relationships between severity of disease on tap and lateral roots and root diameter, root length, nodulation, and total rainfallwere determined. The survey showed, for thefirst time, that severe root disease iswidespread in spring across the wheatbelt of Western Australia. There was a positive correlation between rainfall and tap root disease, and between tap root disease and average root diameter of the entire root system. Despite the high levels of root disease present across the sites, the DNAofmost root disease pathogens assayedwas detected in trace concentrations. OnlyPythiumClade F showed highDN