Quantification of Cylindrocarpon destructans f. sp panacis in soils by real-time PCR

Ginseng (Panax quinquefolius) is an important cash crop in various regions of North America, but yields are often reduced by various root pathogens. A quantitative real-time PCR (qPCR) assay for Cylindrocarpon destructans f. sp. panacis (CDP), the cause of a root rot and replant disease which discourages successive cropping of ginseng on the same site, was developed to quantify the levels of this pathogen in soils previously cropped with ginseng. DNA was extracted from 5-g samples of soil. In pasteurized soils which were re-infested with varying levels of the pathogen, qPCR estimates of pathogen DNA were significantly correlated with disease severity (r = 0.494) and with counts of colony-forming units (r = 0.620) obtained with an agar medium. In several naturally infested field soils, qPCR estimates of CDP-DNA concentration were significantly correlated with disease severity (r = 0.765) and these concentrations were estimated to range from 0 to 1.48 ng g(-1) dried soil. A principal components analysis did not show any strong relationships between soil chemistry factors and the concentration of pathogen DNA. The approach outlined here allows the quantification of current populations of CDP in soil many years after ginseng cultivation and the prediction of disease severity in future crops. The method should be generally applicable to root diseases of many crops.