It should come as no surprise to ginseng growers that it has been exceptionally wet this year since the beginning of June and especially this fall. Don’t take my word for it. Figure 1 shows a map of precipitation relative to normal for the past 90 days. The entire ginseng production area is indicated at “exceptionally high” and falls within the 98th to 100th percentile. This means that precipitation this far above normal for that period of time would only be expected to occur in a maximum of 2 in every 100 years. Most of the ginseng production area is also in the exceptionally wet zone for the past 180 days (Figure 2).
Figure 1. Precipitation percentile map for central Canada for the 90-day period ending on October 25 (From AAFC – https://www.agr.gc.ca/DW-GS/current-actuelles.jspx?lang=eng&jsEnabled=true).
Figure 2. Precipitation percentile map for central Canada for the 180-day period ending on October 25.
The biggest concern for the crop with these wet conditions is Phytophthora root rot, which can progress in the soil even if the tops have died down. Phytophthora cactorum, the cause of Phytophthora root rot in ginseng, is active at soil temperatures between 4°C and 24°C with an optimum of around 15-20°C. Due to the warm October weather, soil temperatures are likely still above 10°C in most areas. While this is lower than optimal for the disease, the disease is still active. If we have another mild late fall and winter, disease could continue to progress for several more weeks.
It is very important to continue to consider disease management until soil temperatures get below 5°C. Apply fungicides before a rain to wash them into the root zone before they dry onto the straw. There is more flexibility this time of year because fungicides will not dry on the straw as rapidly as mid-summer, but any drying may drastically reduce the efficacy of some fungicides.
In comparison to Phytophthora cactorum, Cylindrocarpon destructans (now called Ilyonectria mors-panacis) is active at lower temperatures. The optimum for Cylindrocarpon is between 10°C and 15°C, with some activity much lower than that. That means that Cylindrocarpon is probably within its optimal range now and activity will only slowly diminish over the next two months.
The fungicides available for management of Cylindrocarpon (fludioxonil (Scholar/Switch), and captan (Captan/Maestro/Supra Captan)) are only registered for suppression (not control) of Cylindrocarpon root rot in ginseng. Experience from our trials suggests that these products need to be washed in immediately after application to avoid binding to the straw and are best applied immediately before or during a rain.
Fungicides also become less active at lower temperatures. If they are applied before the rain this week, then additional fungicide applications are probably not necessary and may not be active anyway as temperatures get colder. Avoid applying fungicides on cold mornings, especially when frost has occurred.