Rusted root (also known as rusty root) of ginseng (Panax quinquefolius) was first described over 70 years ago, but the causal agent has not been clearly established. The disease is characterized by slightly raised reddish-brown to black root lesions of varying size. The lesions, regardless of size, remain superficial; however, peridermal tissue is ruptured and sloughed off, giving the root a scabbed appearance. Culture-independent techniques were used to demonstrate that a fungal internal transcribed spacer (ITS) region DNA fragment was strongly associated with diseased but not healthy root tissue. The fragment ( approximately 650 bp in length) was cloned. Restriction enzyme digests of cloned DNA indicated that the 650-bp fragment represented a single taxon. BLAST analysis following sequencing of the fragment found that the nearest matches in GenBank were anamorphic genera associated with discomycetes, in particular Rhexocercosporidium spp. This putative identification was supported further by isolating fungi from diseased tissue using a semiselective agar medium. With this procedure, a Rhexocercosporidium-like fungus was isolated; DNA extracted from fungal cultures and amplified using ITS oligonucleotide primers was found to be identical to similarly amplified DNA from the 650-bp bands. However, the isolates were distinct, with respect to growth rate on agar media and ITS sequence, from Rhexocercosporidium carotae, the only described species in this genus. The ability to reproduce symptoms on ginseng roots was confirmed in pathogenicity tests. Oligonucleotide primers based on ITS sequences were designed to amplify DNA of Rhexocercosporidium spp. Polymerase chain reaction assays on DNA extracted from naturally infected root tissue showed that the fungus was present in nearly all symptomatic roots but was infrequent in healthy-appearing roots. The most probable cause of rusted root of ginseng is a previously undescribed species of Rhexocercosporidium.

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 colonyforming 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.

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