Root diseases, especially Phytophthora, are spreading in ginseng fields due to the heavy rains over the past three weeks. Tops collapsing from root damage can look similar to foliar disease. Tops collapsing from root disease lack the water-soaked appearance of a foliar outbreak (Figure 1). It is important to distinguish between root and foliar Phytophthora, because the risk of spread is much greater with foliar Phytophthora. If Phytophthora is confined to the roots, spread should be controlled by improving drainage, avoiding spread of inoculum by machinery and field workers, and timely application of fungicides targeting the root system. Once Phytophthora spreads to the leaves, fungicides are required immediately. The only registered fungicide available to ginseng growers at this time that will have any effect on an active foliar Phytophthora outbreak is Maestro 80 DF Fungicide. Preventative application of Ridomil and Aliette will reduce disease pressures in the field, but these products will not control foliar disease in progress.
Figure 1. Foliar symptoms of Phytophthora root rot on ginseng (above) appear as collapsed plants, but lack a water-soaked appearance.
Glyphosate resistance is becoming an important issue in Ontario, especially after repeated use in a particular area. The use of glyphosate to control weeds around the perimeter of ginseng gardens is a standard practice. Over the four-year life of a ginseng garden, this could result in the development of resistance. Glyphosate resistance has been identified in common ragweed, giant ragweed and Canada fleabane in parts of Ontario. To reduce the chances of glyphosate-resistance developing in these areas, it is a good idea to rotate with other non-selective herbicides, especially for broadleaf weed control. If you have recently sprayed these perimeter areas and weeds were not killed by glyphosate, you must rotate to another product to prevent these glyphosate-resistant weeds from becoming dominant in the field. OMAF and MRA Publication 75 – A Guide to Weed Control lists products registered for use in “Road-Sides and Non-Crop Areas” (Chapter 18). These products should not be used in the ginseng gardens and will damage or kill ginseng plants if contacted. Venture is the only herbicide registered for use in ginseng gardens during the growing season.
At this time of year insect pests may begin to show up in ginseng gardens, including leafrollers, four-lined plant bug and aphids. Insect pests rarely cause economic damage to a ginseng crop. Their presence in a ginseng garden does not necessarily warrant an insecticide application. The cost of the insecticide may exceed the lost ginseng production. In many cases the use of a backpack sprayer in a small area of the garden may be sufficient to control an outbreak, because insect pests are typically patchier than foliar diseases. Scouting will be critical for identifying these patches before economic damage occurs.