Ginseng Crop Update – Spring Pests – May 5, 2023

The prolonged cool period over the past two weeks has delayed ginseng emergence, but plants should emerge rapidly over the next week. There has been at least some precipitation 13 of the last 14 days keeping the soil very moist over that period. A freeze to around -2°C occurred last week in most areas, but dewpoints were high, suggesting that temperatures at the ground level were not any colder. It is unlikely this caused any significant damage unless emerging older plants were exposed due to thin straw cover. Temperatures have been cool since then. In some recent springs, prolonged cold temperatures without a significant frost have caused incomplete development of the tops including distorted leaves and shorter stems, which then led to higher Alternaria problems later in the year. While it has been cool over the past week, it has not been as cool as some previous springs due to a very moist airmass that prevented night temperatures from dropping too much. It is unlikely temperatures were cold enough to cause damage, but this is something to watch out for.

Root Diseases

The risk of root diseases is relatively high, but hopefully the cooler temperatures prevented significant development. Timely fungicide applications will be necessary over the next few weeks to keep disease pressures low, especially Phytophthora. Invariably, the first reports of foliar Phytophthora occur around the middle of May, so ensure foliage is protected as well as the root. This can be done with soil applications of xylem-mobile fungicides (fungicides that move upwards in the xylem), fully systemic products like Aliette or Phostrol once leaves unfold and can absorb these products (although these do not provide 100% control on their own), or separate applications of products targeting the roots (high water volume followed by irrigation or rainfall) followed by products targeting the tops (lower water volume and good coverage of above-ground surfaces). Upward movement of fungicides in xylem-mobile products will be minimal until the leaves begin to unfold a bit and the leaves start to transpire. A mild winter may have also allowed Cylindrocarpon to progress since last fall. In areas with existing Cylindrocarpon issues, monitor for the disease and apply products for suppression.

Leaf Folders

When I first started working with ginseng 15 years ago, I was taught about the damage caused by leaf rollers in ginseng. They roll up leaves to protect themselves during the day and then come out at night to feed on nearby leaves and stems. They often chew part way through the petioles (leaf stems) and cause leaves to droop. From a distance, the damage looked like a combination of slug damage, with ragged holes in leaves, and foliar Phytophthora with drooping leaves.

Over the past few years, leaf rollers have all but disappeared, and a new insect has emerged that is very similar – the zigzag herpetogramma moth, which is a leaf folder. The difference is that these leaf folders fold up leaves and eat within the fold, seemingly never leaving the roll at night.

So, what changed to cause leaf folders to take over? Growers are probably getting better control of leaf rollers due to availability of more and better insecticides, such as those applied for cutworms. Leaf folders are protected from insecticides because they never leave the fold. As a result, they may be effectively resistant to insecticide applications, and are the only ones to survive to overwinter and lay eggs the following year in the same garden.

Based on this theory, here are some potential strategies for control (if needed):

  1. Leaf folders are patchy and have only been noted to cause economic damage in confined areas of older gardens. Control may not always be warranted.
  2. If they were bad in a garden last year, they are likely to be bad again this year if not worse. These are the areas that may need to be targeted with well-timed insecticide applications.
  3. There is not a lot known about the life cycle of this insect, but it is likely that they overwinter as adults, mate sometime in mid-May, and lay eggs. Eggs likely hatch within 2 weeks and the young larvae quickly start folding up leaves. Your only opportunity for good control would be at the very young larval stage before they fold up leaves. This will take very close observation in the field looking for:
    • Small, mottled, brown, triangular moths (1.5-2 cm width) that fly in a zigzag pattern ahead of you as you walk or drive through the garden (Figure 1). This is probably the sign of active egg laying and insecticides should be timed for one to two weeks after this observation.
      • The very first signs of leaf folding. At this stage, the young larvae are probably still folding in new leaves, so insecticides applied to the leaves may be incorporated into the leaf fold and consumed by the larvae.

Figure 1. The zigzag herpetogramma moth is brown, triangular and about 1.5-2 cm wide.

Other Pests

  • Slugs and cutworms will become active in the next week or two as temperatures warm.
  • Products to protect against Alternaria will need to be applied before the canopy begins to fully develop.

About Sean Westerveld

Ginseng and Medicinal Herbs Specialist, OMAFRA
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