Heavy rains in the northern two-thirds of the growing area over the past week are by far the biggest issue in ginseng. Within the ginseng growing area bullseye for rain was the 402-401-403 corridor from Strathroy through to Brantford with 175-200 mm (7-8 in.) of rain over an 8-day period with probably higher amounts locally.
Table of Contents
Phytophthora
The risk of phytophthora is obviously high in areas that received high rainfall amounts. At this time of year, foliage seems to be less affected than during emergence when leaves are tender, but foliar infections can still occur. Ensure some protection of both the root rot and leaf blight phases of the disease. This can be done with a combination of xylem-mobile fungicides applied to the root zone and/or separate applications targeting the roots and the foliage. It is difficult to control both phases of the disease in one application.
For the list of fungicides registered for control of phytophthora root rot, leaf blight, or both, click the following link: Ginseng crop protection – Ginseng (gov.on.ca)
Nitrogen Leaching
Growers are often concerned that heavy rains will leach all the nitrogen out of the root zone. However, leaching is not as big an issue in the summer as you would think. To saturate 30 cm (1 ft.) of relatively dry soil takes 25 mm (1 in.) of rainwater for sandy soils and 37-50 mm (1.5-2 in.) for loamy soils. In other words, if you applied 25 mm of water to a dry sandy soil, the top 30 cm of soil would absorb all of the water and it would not infiltrate any further. The bulk of the roots will be in the top 60 cm (2 ft.) of soil but there are some roots deeper than that, except in seedlings. Water that goes down below 60 cm in depth can start to come back up again (along with any nutrients in it) through capillary action around the soil particles when the soil begins to dry again. That means that leaching out of the root zone will effectively begin after 50-75 mm of rain within a day or two in relatively dry sandy soils and probably 75-100 mm or more in dry sandy loams. That doesn’t mean all the nitrogen is gone with higher amounts of rain, it just means available nitrogen will start to be lost. Nitrogen in organic matter or bound as ammonium to soil particles would not be lost. It gets more complicated with repeated heavy rains over a longer period because it depends on how much the soil dries between rainfalls. For example, a wet soil that received a new 25 mm rainfall will start to leach nitrogen immediately, but if the soil dries between rain events, the rainwater has to saturate the root zone again before leaching begins. Also, really heavy rains can run off the surface and not even infiltrate the soil, and this surface runoff also does not contribute much to leaching (although it is bad for phosphorus, erosion etc.).
Nitrogen can also be lost in waterlogged soils through denitrification, the process of converting nitrates to atmospheric nitrogen by bacterial action in low-oxygen environments. Because ginseng soils are relatively sandy, denitrification is usually less of a concern, but it may be an issue in areas with the heaviest rainfall amounts if they remained saturated for extend periods.
Based on this, don’t assume you have lost all of your nitrogen. Some areas received close to 200 mm of rain over a week and would have some definite leaching concerns. However, it is best not to reapply large amounts of nitrogen all at once to replace what may have been lost. We are starting to suspect that sudden bursts of growth during the growing season due to a sudden increase in fertility or water may be a primary cause of the main rusty root symptoms found in Ontario, although his hasn’t been proven yet. It is better to apply small amounts of nitrogen (10-20 kg/ha = 9-18 lb/ac) initially and monitor the crop for additional deficiency symptoms. Alternatively, slow-release fertilizers can be used so some nitrogen is always made available, even after a heavy rain.
Leaf Folders
Leaf folders have passed the adult stage. We don’t know enough about the biology of this pest to know if there will be a second generation, but likely eggs will have been laid over the past two weeks. If there is a second generation, they should be emerging now. Look for the earliest signs of folding of the leaf edges and ensure products are applied immediately if there were high populations in the first generation. It is possible a second generation either doesn’t occur in ginseng because adults leave the field or is much lighter because of a buildup of natural enemies like parasitic wasps.
