By M. Putnam
Cause Thought to be the fungus Rhexocercosporidium panacis on Canadian-grown American ginseng (Panax quinquifolius). Rusted root is a problem widely distributed throughout ginseng-growing areas, and has been a problem in cultivated ginseng for over 100 years. The disease can reduce the yield, quality, and market value of ginseng roots. In China, Ilyonectria mors-panacis is thought to be the cause on P. notoginseng. The cause of rusted root has previously been attributed to species of Fusarium, Ramularia and Cylindrocarpon (= Ilyonecria for many of the species) in the US and Canada. Rhexocercosporidium panacis requires a selective medium for growth on isolation plates. This is a superficial disease distinct from disappearing root rot.
Growth of Rhexocercosporidium panacis in culture peaks at 15°C to 20°C, although the fungus will continue to grow and produce symptoms at 4ºC. Temperatures of soil under mulch during the summer are not likely to inhibit growth.
Symptoms Roots of all ages appear susceptible, but those older than one year are more commonly affected. Symptoms begin as aggregates of pinpoint, raised, rust-colored spots on the surface of the root, often near the crown. As the disease progresses, these aggregates expand into lesions that spread across the root but remain superficial, penetrating only a few cell layers into the periderm. Affected tissues may rupture and become colonized by other fungi. Affected periderm may also slough off, leaving shallow indentations in the roots. Affected plants usually do not produce obvious symptoms above ground, and symptoms may only be noticed upon digging.
Cultural control There is no known control for this disease.
Reference Jiang, Y., Jin, H., Li, X., Yan, D., Zhang, Y., Li, M., Gao, J., Lu, B., Chen, C. and Jiang, Y. 2023. Development of a novel real-time quantitative PCR method for detection of Ilyonectria robusta, the predominant species causing ginseng rusty root rot. Plant Disease, 107:1680-1689.
Reeleder, R.D. 2007. Rhexocercosporidium panacis sp. nov., a new anamorphic species causing rusted root of ginseng (panacis [sic] quinquefolius). Mycologia 99:91-98.