Although the past week was not nearly as cold as the same period last year, there has been an extended period of well below normal temperatures. When this has occurred during plant emergence in the past, development of the tops has been affected. This is different than direct freeze damage, which causes obvious symptoms including swollen, split and kinked stems and glassy leaves. Direct freeze damage has been relatively minor over the past week due to a more humid air mass than previous cold snaps and milder frosts as a result.
Cold temperature injury often appears as cupped leaves on emerging plants (Figure 1) and stunted and distorted leaves once the canopy fully develops (Figures 2). Sometimes the canopy appears healthy, but just not as tall as it would normally be. It is likely that this more subtle damage can still weaken plants and make them more susceptible to disease, especially Alternaria. Considering the severe issues with Alternaria in 2020 following similar cold damage, it is essential to protect plants before symptoms appear and scout plants regularly to catch new infections before they get out of hand. Botrytis could also be an issue in fields with damage from frost.
Figure 1. Cupped leaves on emerging plants as a result of cold temperature injury.
Figure 2. Stunted and distorted leaves as a result of prolonged cold temperatures above freezing.
It is also important to think about the mobility of the fungicides you are using, especially during emergence and expansion of the tops. Contact fungicides will only protect where they land on the plant and not any new growth. As the leaf expands, unprotected areas will develop when using contact fungicides. Systemic fungicides that are xylem mobile and translaminar will penetrate the plant and move upwards in the stems and leaves. These will provide some protection of new growth, although they will get diluted over time.
Once the disease is identified in the garden, the fungus will be growing within the affected plants. Contact fungicides usually do not provide curative action, since they are only on the outer surfaces of the leaves. You will need products that are at least locally systemic and/or translaminar to kill the fungus within the plant.
For more information on mobility of fungicides on ginseng consult the Ginseng Fungicide Webinar summary that was distributed to OGGA members and webinar participants. For a copy of the summary you can also contact me at firstname.lastname@example.org.