The ginseng crop is progressing normally this year after several years of spring cold temperature and frost damage. This should reduce pressures of foliar diseases. As temperatures warm, the added stress could lead to higher Alternaria pressures over the next few weeks. Since there is minimal damage to ginseng tops and drier weather is in the forecast, the risk of Botrytis should be lower going forward.
Four-lined plant bug damage has been reported in some fields (Figure 1). Look for circular lesions on the leaves that are evenly sized. The insects suck the insides from the leaf leaving only the epidermis showing and then move on to another section of leaf. This gives the lesions a glassy appearance. Late instar nymphs (Figure 2) or adults (Figure 3) of four-lined plant bug may be present on the leaves but tend to quickly hide under the leaves as you approach the plant. This pest rarely reaches damaging levels, so insecticides are likely not required to control them. It is just important not to confuse the damage for a disease, so unnecessary fungicide sprays are avoided. Pounce/Perm Up are registered for control of four-lined plant bug in ginseng if a control is warranted. This pest only has one generation per year. Adults emerge in mid- to late-June, mate, and lay eggs in slits in the sides of stems. They feed for a few weeks before dying off for the year by early- to mid-July.
Figure 1. Spotting on ginseng leaves caused by four-lined plant bug nymphs or adults. (Photo courtesy Amy Shi, OGGA)
Figure 2. A late instar nymph of four-lined plant bug. Nymphs start out a vibrant red but become duller in colour with larger black wing pads as they mature.
Figure 3. Adult four-lined plant bugs along with their damage on a mint plant.
Symptoms of root diseases (other than Phytophthora) will often become evident during the first heat waves of summer. Root damage leads to fewer roots to take up water and the added moisture stress on hot afternoons leads to wilting and discolouration of the leaves. Sudden wilting or discolouration of leaves, usually in older gardens, is often caused by one of the following diseases:
- Cylindrocarpon root rot: Look for discolouration (red or yellow) followed by wilting on hot afternoons. The root is likely substantially rotted by the time these symptoms appear.
- Rhizoctonia crown rot: Look for sudden wilting caused by damage to the lower stem within the straw or the crown of the plant. This usually occurs in expanding circles
- Verticillium wilt: Often occurs on fields with a history of crops affected by Verticillium such as the Solanaceous crops (e.g., potato, eggplant, tomato, pepper) or cucurbits like watermelon. Isolated plants will suddenly discolour and wilt but there will not be obvious damage to the outside of the roots. Cutting across the taproot will often reveal a dark ring in the center of the root, which is caused by the fungus clogging the xylem vessels in the vascular system.
Stunting of ginseng plants is usually caused by either cold damage during ginseng emergence or diseases and disorders that have more subtle effects on the roots such as Pythium root dieback, root-knot nematode, or severe rusty root.
Aphids are also being reported in relatively low numbers. It is important to monitor populations, but sprays are not usually warranted unless they reach levels that lead to obvious damage of some plants (e.g. twisting of leaves and stems or flower/berry abortion).
Nitrogen Fertility of Ginseng
Most ginseng growers are probably over-applying nitrogen (N). Research has shown that the ginseng crop requires about 40 kg/ha (36 lb/ac) per year to reach optimal yields. This is based on trials conducted in both a research garden at the AAFC Delhi Research Station and a commercial ginseng field. Since the trials were conducted over 15 years ago, improvements in crop management and higher yields may warrant slightly higher rates but is unlikely that ginseng benefits from anything above 80 kg/ha (72 lb/ac).
The average field-grown horticultural crop grown in full sun requires around 80-120 kg/ha (72-110 lb/ac). Due to the shade requirements, ginseng is a very slow-growing crop compared to most field crops. Carrots, for example, which are relatively closely related to ginseng and a similar root crop, can yield up to 90,000 kg/ha (80,000 lb/ac) fresh in a single year. Ginseng, by comparison, with a very high yield of 6,700 kg/ha dry (6,000 lb/ac) would equate to about 25,000 kg/ha fresh (22,000 lb/ac) after four years of growth. Most of that growth occurs in years 3 and 4. Ginseng may have a lower moisture content than that of carrots, but that probably means that carrot growth is still around 5 times faster than that of the oldest ginseng gardens.
I conducted extensive research on N fertility of carrots for my graduate work at the University of Guelph and found that carrots grown on muck soils were able to get enough N from the soil without the need for added fertilizer. On sandy soils, carrots benefited from no higher than 110 kg/ha (100 lb/ac) per year. If carrot yield is maximized by 110 kg/ha per year, then it is very unlikely ginseng benefits from that high a N rate considering its much lower growth rate.
There is a perception out there that N is quickly leached from the soil after every heavy rain on sandy soils. While N is mobile in the soil, it is relatively rare to get a rainfall event during the growing season that results in a substantial loss of N from the rooting zone. Most of the leaching of N occurs in the fall, winter and early spring. On sandy soils, it takes about 25 mm (1 in.) of water to infiltrate a foot into the soil when the soil is dry. In soils that are already moist it may infiltrate twice as deep. The rooting zone of most mature crops is approximately in the top 90 cm (3 ft) of the soil profile. So, a soil with moderate soil moisture would require 37.5 mm (1.5 in.) to infiltrate 90 cm (3 ft) deep, and anything higher than that in a single event may begin leaching some N below the rooting zone. However, as the soil near the surface begins to dry, some of the moisture from down below the root zone, along with soluble N, will begin to move back up the soil profile due to capillary action, reducing the overall losses somewhat. Once the soil dries for a few days, it will take another heavy rain event to begin leaching N out of the rooting zone. So, it takes very large amounts of rain or repeated moderate rain events within a short period to leach N out of the root zone. Research on cabbage, a crop with one of the highest N requirements of any crop, has shown only minor benefits of splitting N between a preplant application and a sidedress applied several weeks into the growing season, meaning that most of the N applied pre-plant is still available late in the season, even on lighter soils.
The one difference with ginseng is the drip lines from the shade. Where the water pours off the shade cloth, considerably higher rates of rainfall can be experienced in strips through the field. This may lead to leaching of N out of the soil in those regions, leading to drip-line chlorosis. Higher rainfall in the drip lines means that the rest of the field actually gets less rainfall than experienced outside of the garden and less leaching. More frequent N applications may help avoid the N deficiencies experienced in the drip lines, but that does not warrant increasing the overall amount of N applied per acre per year. Applying the N needs of those sections would likely lead to excessive N being applied to the rest of the field and this could lower yield due to N toxicity. It is common in many crops to see yields decline if N rates go above 50-100% over the optimal rate. Improving the fertility of the 10% of the field affected by drip-line chlorosis could cause damage to the remaining 90% of the crop. It is important to remember that a dark green canopy does not necessarily translate to a larger root.
Resources for Ginseng Growers and Consultants
- Ontario Crop Protection Hub: The Hub replaces the printed copes of OMAFRA crop protection publications such as the Crop Protection Guide for Ginseng. It provides a listing of all the products registered for use on ginseng along with rates, efficacy ratings and use precautions. Visit Ontario.ca/cropprotection and click on “Ginseng Crop Protection”, then choose your crop stage (e.g., post ginseng emergence) and a list of all products registered on ginseng will appear. You can narrow the list by selecting specific ginseng pests using the drop-down menu to the right of the listing. Herbicide options can be found by clicking on “weed control search” on the main page and selecting ginseng from the crop list.
- Funding Available to Test Herbicide Resistant Weeds: Funding is available for producers, agri-business, consultants and extension personnel to submit weed samples to be tested for herbicide resistance again this year. Sample collection kits with sampling procedures can be obtained from firstname.lastname@example.org. These tests only require a small amount of leaf tissue from the suspected resistance weeds. DNA is extracted from the leaf tissue to determine if there is a molecular change where the herbicide acts to kill the weed, making the weed resistant.