Ginseng Crop Update – Alternaria Management – June 5, 2020

With ginseng plants stressed from prolonged cold and/or freeze damage in many fields, they are more susceptible to foliar diseases including Botrytis and Alternaria leaf blights. Combined with ideal warm, dry and humid conditions for Alternaria, the risk of Alternaria leaf blight is currently very high. The risk for Botrytis is still high, but once plants heal after frost damage, the risk tends to decrease slightly.

There is no secret formula for managing Alternaria. It all comes down to 4 main factors:

  1. Reduce plant stress
  2. Choose the right products
  3. Apply at the right time and interval
  4. Ensure good spray coverage

Reducing Plant Stress

A stressed plant will have reduced defenses against pathogens. Reducing stress involves proper fertility and mimicking the environment of the forest floor with modest air flow and moderate temperatures. Plants in windy locations or where heat builds up in a garden on calm, hot days will be stressed and more prone to Alternaria infection. Ideally gardens should have good air flow, but some barrier to protect the plants in very exposed locations on the edge of gardens. Most of the plant stress this year is due to freeze and cold temperature damage, and there is not much that can be done to reduce that stress.

Choosing the Right Products

If you have been spraying frequently and not getting good control, it may be time to switch to a different fungicide. Consult Publication 847 Crop Protection Guide for Ginseng for more information on products available for ginseng and their relative efficacy. There are many products registered for Alternaria control in ginseng and most should have the efficacy required to get good control if all the other factors in this article are considered.

Applying at the Right Time and Interval

Most fungicides work best when applied preventatively before the fungus infects the plant or curatively shortly after infection, before any symptoms appear. If symptoms persist, ensure the product you are using can penetrate the plant tissues to get to the fungus (e.g. translaminar, locally systemic, xylem-mobile). See this article for a list of fungicides by mobility in the plant. I have not been able to find a listing of fungicides that have curative or eradicative action, but the chemical company reps may be able to provide information on products that can get control of ongoing disease.

Ensuring Good Coverage

If all the above strategies are done properly and you still have disease persist in the field, then it is time to re-evaluate your spray coverage. The first thing to do is to evaluate where in the garden the disease is still active by mapping it out:

    1. Mostly in the post rows? The post rows are the most difficult place in the garden to get good coverage. If disease persists in this area alone, then coverage is clearly the issue and not the choice or timing of products. There are three main things that you could do to fix the problem:
      1. Ensure you have drop nozzles not only behind the tires but also in the two outer alleys. See this article on a good method to do this.
      2. There may be a zone at the post where the sprays from either direction do not overlap. Angle the last nozzle to face outwards towards the post row to extend the control area, while still having one facing downwards at the end of the boom.
      3. Use a backpack sprayer to focus on hot spots where coverage may be poor.
    2. Mostly on the stems? Alternaria that is mostly on the stems can come from localized infection around an old infested stem but can also signal poor stem coverage (Figure 1). Good stem coverage is best achieved using drop nozzles down each alley. However, with a variable canopy caused by frost damage, leaves may get in the way of good coverage of the stems even with this strategy (Figure 2). One strategy to consider is alternating the direction you travel through the field from one spray to the next. When the canopy is dense over the alleys, growers tend to want to travel the same direction for each spray to avoid tangling up the leaves and causing damage. With the stunted canopy in many gardens, and most gardens not yet fully developed, this should not be a problem. This strategy can ensure that different parts of the plant that may have been blocked by a leaf in one spray, can be protected by the next spray if it is coming from a different direction.
    3. Mostly on certain beds? If you look at the direction the spray normally travels and one section of a bed is always the worst for disease, then you have a nozzle issue in that area. Calibrate the sprayer to ensure all nozzles are working properly. See this article for how to do this properly.
    4. Throughout the garden? If disease is persisting on leaves and stems throughout the garden, and there is no consistent pattern, then you may have an overall coverage issue causes by poor nozzle choice, improper water volume or improper pressure. There is a wealth of information on this topic on Sprayers101. However, even if all of this is set properly, you may still get poor coverage because of the tangled mess of leaves in the canopy in a frost-damaged garden. Alternating the direction of travel through the field is a good strategy to try reach different parts of the canopy. Consider identifying hot spots and spraying them separately with a backpack sprayer. If none of this works, re-examine your choice of products and your timing. Also consider that some plants may be damaged beyond repair and Alternaria is attacking because the top is dying.

Figure 1. Stem lesion of Alternaria leaf blight.

Figure 2. Variable canopies in a frost-damaged garden. The second photo shows an Alternaria outbreak in this variable canopy.

In addition to Alternaria, this appears to be a bad year for cutworms. Scout regularly for damage to time products properly.

About Sean Westerveld

Ginseng and Medicinal Herbs Specialist, OMAFRA
This entry was posted in Ginseng, Ginseng Pest Management and tagged , , , , , . Bookmark the permalink.

Leave a Reply