Although ginseng emergence was slow this year, weather conditions have been ideal over the past few weeks for emergence and development. However, it is important keep an eye out for foliar Phytophthora, which usually shows up around this time each year. Earlier this week there were extended periods of leaf wetness that may have allowed enough time for Phytophthora infection of tops. If more rains end up coming this weekend, especially heavy downpours in thunderstorms, the risk will be high for infection again.
Look for portions or whole tops suddenly drooping straight down and developing a water-soaked appearance. Tops can also die from Phytophthora root rot, but these tops would wilt down and not develop the water-soaked appearance. While it is the root rot that causes the economic damage, foliar infection is much more serious because it results in airborne inoculum that can infect the entire field and fields for kilometres downwind.
Keep in mind that there is a difference between fungicide applications for root rot and applications for foliar blight. If you are trying to protect the plant from foliar Phytophthora you need to apply the fungicide at a lower water volume and ensure thorough coverage of the tops. These applications should be the same water volumes and methods used to treat for Alternaria. If you are trying to target root rot, then the fungicide needs to be washed into the soil, if that is permitted on the label. It is best to apply these at the highest water volume possible and follow that with overhead irrigation or rainfall to move it to the root zone. Some fungicides such as Ridomil or Orondis can move up the xylem to the tops after being taken up by the roots, but this would not be as effective in preventing foliar blight as a direct foliar spray. This is because movement to and uptake by the roots is more variable than direct coverage of the tops.
For all fungicide applications, remember that drop nozzles down each row greatly improve the spray coverage on the stems and underside of leaves for older gardens with a thick canopy. Click here for more information on drop nozzles and equipment to achieve the best coverage within the canopy.