Following heavy rains in many areas earlier this week, many ginseng gardens were flooded, at least temporarily. With the potential for three days of wet weather this weekend the risk of Phytophthora will be greatly increased. The risk will be especially high if there are thunderstorms with heavy downpours mixed in. All Phytophthora needs to infect leaves is free water on the leaves for a period of about 24 hours, so the amount of rain is not as important as the duration of rain. Growers should ensure protectant fungicides are applied for foliar Phytophthora ahead of the rains. Foliar application should be done at a lower water volume than what is used for root rot, since you do not want the fungicide to run off of the leaves. The first signs of foliar Phytophthora are darkened, water-soaked areas on the leaves, or isolated leaves drooping straight down (Figure 1). If there are areas of exposed mud in the trenches, apply additional straw to reduce the potential of splashing spores onto leaves with machinery or rainfall.
Figure 1. Initial symptoms of Phytophthora leaf blight include darkened areas on the leaves that quickly become water-soaked.
Alternaria leaf blight is the biggest issue in gardens at this time. Areas of crop stress from any reason can be an initial focal point for the disease. Cooler weather this week and early next week will slow the spread of the disease somewhat, since ginseng will experience less stress under these conditions. However, it looks like heat will build again later next week. If Alternaria is not under control, consider switching to a more targeted Alternaria product. If it is still not under control after that is used, spray coverage may be the issue.
Various insect pests have been found in ginseng. Leafrollers are generally at a stage that is too late to achieve any significant control with insecticides. It is important to continue scouting for insect pests, but other than cutworms, foliar feeding insects do not usually cause enough damage to warrant an insecticide application, or can be controlled with a localized spray.
Ginseng Research Update
OGGA and OMAFRA continue to work towards a solution to ginseng replant disease. We continue to need the assistance of as many growers as possible to make this research successful. If you are establishing this year or planning to establish next year any replanted gardens, we would like to monitor gardens for disease development as part of our replant survey. Please contact myself or Amy Shi at OGGA if you would like to participate in the research. Also, we welcome you to bring plants with any unusual disease symptoms or any Cylindrocarpon root rot symptoms to us at the Simcoe Research Station. We are not a diagnostic lab and cannot guarantee a timely diagnosis, but these symptoms can help us learn about the pathogens in the soil and provide us with a diversity of fungal organisms for our research projects.