Phytophthora leaf blight on ginseng in a previous year.

The cold weather over the past week has slowed down ginseng emergence considerably. Emergence is now delayed to around normal for this time of year. Temperatures are forecast to remain below normal for the next two weeks. However, given the mild winter and early spring, ginseng is poised to emerge rapidly once warmer weather returns.

Between the point of emergence of the stems into the straw and full canopy closure by early June, ginseng foliage is highly susceptible to three main diseases: Alternaria, Phytophthora and Botrytis. Botrytis is mostly an issue if there is some mechanical injury such as damage from frost. Alternaria and Phytophthora are always a threat in ginseng gardens, although less of a threat to emerging seedlings since there is less inoculum in those gardens. In the next week or two, as the ginseng tops emerge into and through the straw, protection from these diseases will be required.

Complicating factors is the need to ensure roots are protected from the root rot phase of Phytophthora, while also protecting the leaves and stems from both Phytophthora and Alternaria leaf blights. There is no single fungicide or application method that can accomplish protection for all these diseases and parts of the plant at once.

Ginseng growers have a good understanding of the need for fungicides that are specific to each disease, since fungicides for Phytophthora have little or no effect on Alternaria or Botrytis and vice versa. However, there is also a need to understand that applications for foliar blights do not necessarily have any impact on root disease and vice versa. That is because the water volume required to get good coverage of the foliage is not high enough to penetrate through the straw to get to the roots. Applying the fungicide at a high water volume can cause the fungicide to run off the foliage, which results in poorer coverage. Washing the fungicide to the root with rainfall or irrigation washes it off the foliage. On the other hand, if you are successful at getting the fungicide to the root, a few fungicides will move up the xylem in the plant to get to the foliage internally.

Fungicides vary in their ability to bind to foliar surfaces and this variability also influences their ability to bind to the straw when trying to get it to the roots. If a fungicide is rain-fast on the leaves once it dries on the surface, it is likely to not bind well to the straw, and as a result, will be moved to the soil easier if applied for root disease control. That is because the leaves are mostly covered with wax layer that is hydrophobic (repels water), whereas the wax that would have covered the straw is likely degraded by the time you are applying fungicides and would have a hydrophilic surface (attracts water). This can be demonstrated by placing a water droplet on each surface. The water would ball up on the leaf surface but spread out and wet the surface of the straw. On the other hand, a fungicide that is less rain-fast and would be washed off the leaf with the next rainfall would likely bind tightly to the straw once it dries on the surface and would not be moved any further, no matter how much water you apply. Some fungicides are designed to cover the leaf surface and move into the leaf, rather than bind to the surface, which is why effective fungicides may not always be rain-fast.

When you are applying a fungicide, even if it is applied at the highest water volume you can practically apply with a sprayer, it is only enough water to go a few millimetres into the straw. When there is minimal foliage present, this can dry onto the surface in a matter of minutes depending on the weather and the moisture content of the surface layer of the straw. Consequently, for some fungicides, additional water is needed immediately after application to move the fungicide out of the straw and into the root zone. This can only be achieved by turning on overhead irrigation immediately after a section of field is sprayed, or by applying fungicides during or immediately before a rain. It also helps if the straw is already wet during spraying, which will move the fungicide a little farther into the straw and extend the length of time you have to wash it into the root zone.

With all of this in mind. Here are best practices for applying fungicides early in the season for disease management:

1. Apply fungicides for both Phytophthora and the fungal diseases (Alternaria/Botrytis) to ensure protection from all foliar diseases.
2. For protection from Phytophthora root rot, use a high-water volume for fungicide application followed by rain or irrigation to wash the product out of the straw and into the root zone. Keep in mind that this application will only provide foliar protection if the product is xylem-mobile (Table 1). For some fungicides, irrigation or rainfall must occur before the product dries onto the straw. Product suppliers should be able to provide info on which products may bind to straw. If this is unknown, err on the side of caution and assume it will bind to the straw if it dries onto the surface.
3. Use the water volume listed on the product label for foliar diseases to ensure good coverage of all above-ground surfaces. With the exception of Aliette, which is truly systemic in the plant, foliar applications will not provide root protection.
4. Since you cannot accomplish protection of all surfaces with same application, apply fungicides for root diseases first, followed by irrigation or rainfall, followed by a separate application for foliar diseases.

Remember that the product label is the law, so always apply the product according to label directions to ensure the application methods you are using are permitted for that product.

Table 1. Listing of Products Registered for Control or Suppression of Phytophthora in Ginseng and Their Reported Mobility within the Plant

Reference: Beckerman, Janna. 2018. Fungicide mobility for nursery, greenhouse and landscape professionals. Purdue University Cooperative Extension.