Epidemiology of cylindrocarpon root rot and rusty root rot on American ginseng (Panax quinquefolius L.).

摘要

Ginseng (Panax quinquefolius L.), a member of the family Araliaceae, is cultivated in British Columbia and Ontario, Canada for export to Asia. The dried root is valued for its medicinal properties and is widely used in traditional Chinese medicine. Cylindrocarpon root rot, caused by Cylindrocarpon destructans (Zins) Scholten, and rusty root, a disorder of unknown cause(s), are two factors that limit ginseng cultivation globally. Epidemiological and other information on these problems is lacking and no control measures are available. In this study, 80 isolates of C. destructans were recovered from diseased roots. These isolates were classified as highly virulent (HV) (49%) and weakly virulent (WV) (51%), and showed differences in colony morphology, temperature optimum for growth, extracellular enzyme and spore production. Histopathological studies revealed direct hyphal penetration of ginseng roots by a HV isolate. In artificially infested soil, two-year-old ginseng roots were most susceptible to Cylindrocarpon root rot and severity reaches to the peak with a moderate inoculum density (3.45 x 102 CFU/g soil). Disease severity was higher at 20°C compared to 15 and 25°C near field capacity, with a significant interaction between soil moisture and temperature. Disease severity on hydroponically-maintained ginseng roots was significantly (P 0.0001) enhanced when Hoagland's solution was amended with Fe. Mycelial growth in broth culture, conidial germ tube length, secondary branching of germ tubes, spore production and PPO production by C. destructans was found to be enhanced by Fe. Studies on biochemical changes taking place in rusty root indicated rusty areas to be comprised of the epidermis and underlying six to eight cell layers, which accumulated phenolic compounds. Affected epidermal cells also accumulated Fe, Al, Si, Mg and other cations compared to adjoining healthy cells as determined by SEM-EDX and ICP-AES. Ginseng callus cultures treated with either chitosan (1%) or Fusarium mycelial fragments (MF) showed an increased accumulation of phenolic compounds and defense enzymes. Based on these results, rusty root symptoms on ginseng are proposed to result from an induction of host defense responses, especially phenolic production. This induction is likely due to attempted invasion by as-yet uncharacterized chitin-containing soil fungi. Subsequent oxidation of phenolic compounds and sequestration of metal ions, in particular Fe, appear to be largely responsible for the symptoms observed.