Cause The soilborne fungus-like microorganism Phytophthora cactorum. Spores are produced on infected foliage and in roots. Spores are spread via water splash, surface water runoff, and movement of equipment and workers through the beds. Spores also survive up to at least a year in decayed plant material. The fungus-like microorganism can infect foliage and move down into roots, although direct root infections are more common.
Symptoms Plants of any age may be affected. Leaves that become infected are water soaked and wilt rapidly while still green. Symptoms of root rot are usually first noticed when the leaves "flag" or droop so they hang parallel with the stem, or they turn fall colors before it is fall. In very dry weather, leaves may wilt and rapidly desiccate. Under some conditions, the stem may be constricted at or near the soil line, or it may collapse altogether at the soil line and become detached from the root crown. The first indication of infection in the root is a yellowish orange or pinkish brown discoloration of the vascular tissue. Secondary roots may be discolored if present or may be lacking altogether. Primary roots of affected plants will be watersoaked, soft, and discolored pinkish brown as opposed to the white, firm appearance of a healthy root.
- Avoid planting in low areas that collect water. In heavy soils, plant in beds raised at least 6 inches to improve drainage. Tile drainage may be needed for some soils.
- Remove and destroy diseased plants and plant debris.
- Provide good air movement by not planting too densely. A good goal is 120 plants/sq yard; for example, use 90 to 100 lb seed/A if germination rate is 70%.
- Do not apply excessive mulch.
- Weed and perform other tasks in healthy portions of the beds before moving to or through the diseased portions. Clean soil from tools and shoes before moving into healthy sections to prevent moving the fungus.
- Incorporating composted tree bark in the soil before planting may help produce an environment suppressive to Phytophthora for the first year. Composted pine bark mulch was shown to result in larger ginseng roots while composted pine, hardwood, and fir bark has been shown to be suppressive to Phytophthora in other host systems.
- Dig a perimeter trench around the garden that is lower than the garden beds to drain and collect water. Be sure to provide a drainage outlet for the trench that is aimed away from the garden.
Chemical control Focus on cultural controls. Use these products to prevent, not cure, the disease. The phenylamide and phosphonate fungicides (Group 4 and P7) used to manage Phytophthora do not kill this organism. They can only prevent establishment of the organism before it gets into the plant. They can also prevent continued growth if the organism is already inside the plant thereby delaying symptoms that might have developed. Once chemical activity has subsided with time, the organism can resume growth within infected plants. Rotate fungicides that have a different mode of action for resistance management.
- Agri-Fos at 2.5 quarts/100 gal water. Do not combine with a copper-spray program for control of other diseases. Group P7 fungicide. 4-hr reentry.
- Copper-based compounds protect foliage from infection but are not be effective against root infections. Group M1 fungicides. O
- Forum at 6 fl oz/A. Do not apply within 14 days of harvest. Group 40 fungicide. 12-hr reentry.
- Fosphite at 1 to 3 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group P7 fungicide. 4-hr reentry.
- MetaStar 2E at 1.5 quarts/A as a onetime drench in the spring. Group 4 fungicide. 48-hr reentry.
- Orondis Gold 200 at 4.8 to 9.6 fl oz/A. Do not use within 14 days of harvest. Group 49 fungicide. 4-hr reentry.
- Orondis Opti at 1.75 to 2.5 pints/A. Do not use within 14 days of harvest. Group M5 + 49 fungicide. 12-hr reentry.
- Orondis Ultra at 5.5 to 8 fl oz/A. Do not use within 14 days of harvest. Group 40 + 49 fungicide. 4-hr reentry.
- OxiPhos at 2.5 to 5 quarts/A as a foliar spray. Group P7 fungicide. 4-hr reentry.
- Phostrol at 4.5 pint/ A. Group P7 fungicide. 4-hr reentry.
- ReLoad at 2.5 to 5 pints/A. Do not use with copper products. Group P7 fungicide. 12-hr reentry.
- Ridomil Gold GR at 15 lb/A before plants emerge in spring and at 10 lb/A monthly thereafter for plants at risk. Do not use continually or the fungus may become resistant. Do not use within 9 days of harvest. 48-hr reentry.
References Konsler, T.R. 1982. Some responses of American ginseng (Panax quinquefolium L.) to kind of bed mulch and to plant spacing thru four growing seasons. In: Proceedings of the Fourth National Ginseng Conference. Lexington: University of Kentucky.
Li, T.S.C., Utkhede, R.S., and Wardle, D.A. 1997. Chemical and biological control of leaf blight and root rot caused by Phytophthora cactorum in American ginseng. Canadian Journal of Plant Pathology 19:297-300.