Other than the normal lingering Phytophthora root rot issues and occasional Alternaria escapes, few pest issues have been reported over the past few weeks. Except for some severe spring storms, weather conditions have been ideal for ginseng production up to this point with mostly dry weather and normal temperatures. Although temperatures have occasionally been hot, there have also been plenty of cooler days and nights to reduce plant stress. Temperatures are forecast to be above normal beginning this weekend, which could increase plant stress. Along with longer dew periods at this time of year, this could increase Alternaria disease pressures. Although most areas received a much-needed rain early last week, rainfall since then has been more sporadic. It is important to monitor soil moisture closely and ensure timely irrigation to reduce plant stress and the risk of Alternaria outbreaks.
Early Senescence of Tops and Root Disease
This is the time of year that growers start to notice stress/early senescence in ginseng tops as a result of less destructive root diseases. Plants often begin showing a purple/reddish colouration much earlier than normal. The damage leading to this discolouration probably occurred earlier but just didn’t show in the tops until now. Potential diseases include Pythium, Cylindrocarpon, Verticillium, and severe rusty root:
Certain species of Pythium can lead to a proliferation of fibrous roots that have fattened and brownish root tips (Figure 1). The damage restricts the uptake of water and nutrients leading to stunted tops and early senescence (Figure 2). Where Pythium is suspected, fungicides may help reduce further spread. In ginseng, captan products and several biofungicides are registered for control or suppression of Pythium. Ridomil Gold 1G applied for Phytophthora control may also suppress Pythium.
Figure 1. Fattened and brownish root tips as a results of Pythium infection.
Figure 2. A stunted plant canopy due to a loss of fibrous roots caused by Pythium infection.
Cylindrocarpon often develops over the fall, winter and spring, and reduces the number of fibrous roots. It leads to a dark brown, dry rot that moves upward from the root tips to the crown over time (Figure 3). Damage only becomes apparent in the tops as a wilting and yellow/reddish discolouration mid-summer (Figure 4). Cylindrocarpon damage to roots cannot easily be mistaken for any other disease. In most gardens, Cylindrocarpon is often minor and affects sporadic roots throughout a garden. It is in replant conditions, where previous ginseng debris or washwater has been spread, or where soil from a former garden has been deposited by erosion that it can become severe. Switch/Scholar and captan products are registered for suppression of Cylindrocarpon in ginseng. Products should be applied to wet straw and immediately followed by a rain or overhead irrigation for optimal efficacy.
Figure 3. Discoloured and collapsing tops as a result of Cylindrocarpon root rot (disappearing root rot).
Figure 4. Cylindrocarpon usually appears as a dark brown rot moving from root tips towards the taproot and then upwards towards the crown.
Verticillium, which is more common in fields planted where a solanaceous crop was previously grown (e.g., eggplant, potato, pepper, tomato), clogs the vascular system leading to wilt and discolouration of individual tops (Figure 5). Cutting across the taproot will often reveal a brown ring in the centre of the root (Figure 6). Verticillium is not normally severe enough to warrant a spray and there are no fungicides registered for control of this disease in ginseng.
Figure 5. A collapsed top due to Verticillium wilt.
Figure 6. Discolouration of the vascular system of a ginseng root due to Verticillium wilt.
Rusty root, which is a rusty discolouration of the skin of the roots due to unknown factors, can occasionally be severe enough to reduce fibrous roots and uptake of water and nutrients (Figure 7). Rusty root is potentially caused by a combination of the plant’s defense response to an unknown pathogen, saturation of roots in heavy rains or near irrigation emitters, and/or imbalances in nutrients. As a result, there are no known ways to combat the issue.
Figure 7. Severe rusty root can lead to fewer fibrous roots and early senescence of tops.
There are a few insect pests around including cutworms. For control of cutworms, keep in mind that many species of cutworm have developed resistance to permethrin products (Pounce/Perm Up). Since cutworm species affect a wide range of hosts, the cutworms attacking ginseng could have obtained resistance in other crops. If control is poor, consider switching to other products registered for cutworm control (e.g., Coragen, Exirel, Scorpio). Since Coragen and Exirel are in the same insecticide group, consider them the same product for resistance management and rotate with another group whenever possible. Keep in mind that some insecticides applied for leaf roller control (e.g., Dipel) may also reduce cutworms and can serve as a tool for preventing the further development of insecticide resistance.
Second generations of some leafroller species may appear in very localized patches. A spot spray with a backpack sprayer may be sufficient for adequate control. Sprays are best applied in the evening, since leafroller larvae come out to feed at night. Leaf folders, which are more common in ginseng fields, do not appear to have a second generation in Ontario, although growers should look out for new outbreaks, since minimal research on its biology has been conducted.