Accuracy of methods for estimating the size of Thanatephorus cucumeris populations in soil

Abstract. The accuracy of assays based on galactosidase and the enzyme-linked immunosorbent assay specific to Thanatephorus cucumeris were compared with techniques based on soil dilution plating and baiting in sterilized field soil. Although soil dilution plating is reasonably quantitative, it requires substantial time, material and labour. Plant baits gave inconsistent results in the estimation of T. cucumeris populations in the soil. Enzyme-linked immunosorbent assay (ELISA) using monoclonal antibodies is suitable for detecting the presence of a range of anastomosis groups (AGs) of 71 cucumeris in soil samples, but more quantitative applications seem to be limited to a very narrow range of concentrations of the fungus (0–10 μg/g). Monoclonal antibody ELISA could be used if the soil samples are routinely further diluted, provided the range of concentrations is uniformly low. An assay of β-galactosidase permits estimation of a more adequate range of concentrations (0–500 μg/g) and may be used in defined experiments using uninoculated soil samples.