A recent trip to the mountains of North Carolina provided an opportunity to see wild-harvested ginseng roots and talk to wild-simulated ginseng producers. Root disease in general is very rare in wild-simulated ginseng production, and there is no evidence of typical replant disease symptoms in forest ginseng stands. So why is ginseng growing in its native environment not affected by replant disease? The answer could help focus research efforts into replant disease in the field.
Figure 1. Ginseng growing in the wild is usually widely spaced and often shows no symptoms of root disease.
All of the major fungal diseases of ginseng found in Ontario are known to be native to North America. Furthermore, ginseng seed has been sold around North America for many years including North Carolina and other areas with wild-simulated production. Any seed-borne diseases such as Cylindrocarpon, Rhizoctonia and Fusarium would have been moved to these areas with the seed, if it wasn’t already present in the forest there. Movement of fungal spores is inhibited in a forest environment because of minimal air movement down to the forest floor, a lack of wind-blown soil containing spores, and minimal surface movement of rain water. This may help to explain a reduced amount of disease in the forest, but not the fact that entire patches of 15 to 20 year old roots have no disease at all.
Ginseng stands in the forest are very thin. There may be 30 – 45 cm spacing between plants. This could limit root to root growth of fungi and prevent disease outbreaks that wipe out portions of the stand. Growth of new plants around the base of existing plants appears to be inhibited, since seedlings are often found near the base of mature plants, but often die off soon after. One potential area of research into replant disease is the role ginsenosides and other root exudates play in inhibiting growth of subsequent ginseng plants. This may be a survival feature that prevents disease in the existing plants. The tight plant spacing in the field environment encourages disease development. It is possible that replant disease and other root diseases are present in the wild and kill off individual plants entirely, which are then missed during harvest. This is more likely to occur in seedlings, since harvesters should occasionally find half-diseased roots if mature plants are affected. It is also possible that the typical root rots associated with replant disease in the field are not the cause of the problem, but a result of weakened plants inhibited by the previous crop. In the forest, a young plant weakened by any factor would likely die off, even without infection by soil-borne pathogens. Thus, replant disease could be occurring in the forest, but without the typical symptoms found in the field.
The forest floor environment promotes the development of a complex web of soil microorganisms feeding on decaying plant matter. These organisms may inhibit the development of pathogenic organisms or out-compete them for resources. One potential area of research into replant disease is to encourage beneficial organisms in the soil before seeding, which can then colonize the seed and out-compete any pathogenic organisms that come soon after. This may be especially important immediately after fumigation when the soil is virtually sterile.
The solution to replant disease may be found in the forest, but extensive research is necessary. Growers attempting to replant ginseng are encouraged to maintain records on all inputs into the garden and notes on where and when disease symptoms begin to show up. Assembling this information over time may provide additional clues on the causes and solutions to replant disease. Research efforts into replant disease are currently underway with the establishment of a fumigation trial on a replanted site this summer. Preliminary results will be available from the seedling garden in 2014.