Alternaria leaf and stem blight continues to be the biggest issue in ginseng at this time (Figure 1 and 2). Conditions have been ideal for development and spread of the disease. Dr. Jason Deveau provided a reminder on achieving optimal coverage from fungicides earlier this week. Look for a second article from him on achieving ideal coverage of under-leaf and stem surfaces in the coming days.
Getting optimal control from a fungicide application requires three things:
1. Optimal timing. For best protection from the disease, fungicides should be applied preventatively. This will require regularly scouting to ensure that lesions are identified immediately as they develop in the garden before they have time to spread. Products also need to be applied at the intervals specified on the label and especially before wet periods, when rainfall may interfere with spraying. After heavy rains, consider a subsequent application to re-protect surfaces where the product may have been washed off.
2. Effective products. Ginseng growers now have eight different products/actives registered for control of Alternaria and all are considered effective at controlling the disease (Dithane/Manzate/Penncozeb, Bravo/Echo, Rovral, Scala, Switch, Flint, Scholar, and Allegro). A rotation of products from different groups is required for optimal protection. Avoid applying Scholar and Switch back to back because they contain the same active. Avoid applying Scala and Switch back-to-back because they contain actives in the same group. It is also necessary to follow the rates specified on the label. Rates below the label rates will not provide adequate protection, and rates above and below the label rate may contribute to the development of resistance.
3. Coverage of all above ground surfaces. Look for more information on this in the coming days, but it is important that every above-ground surface is protected. This does not mean all surfaces need to be soaked to provide protection, but some protection is necessary. This includes the stems in beds close to the posts or at the edge of gardens. If some surfaces remain unprotected, Alternaria will have a chance to develop in these areas and produce inoculum that will spread to the rest of the garden throughout the growing season.
If foliar and root diseases are a consistent problem in your gardens, consider whether the plants are getting optimal fertility. Alternaria tends to be a weak pathogen that infects plants stressed by other factors. In a ginseng garden, the conditions are so ideal for Alternaria development that the disease can develop even on healthy tissues. However, several studies have shown that fertility can affect the severity of Alternaria diseases in other crops. Most studies show that insufficient nitrogen can stress plants and increase Alternaria severity. However, excess nitrogen fertility can promote leaf growth with weaker cell walls. This can make it easier for some pathogens to infect the plant. It can also lead to more root disease, which stresses the plant and will make the leaves and stems more susceptible to Alternaria.
The recommended rate of nitrogen for ginseng, which was established by the Ontario Soil Management Research and Services Committee and is based on Ontario research results, is 40 kg/ha (36 lb/ac) broadcast once in the spring of each growing season or split up and applied through the drip irrigation system. Excessive use of other nutrients can cause similar issues, or block the uptake of other nutrients leading to a deficiency.
Other issues present in ginseng at this time are minor leafroller damage, Phytophthora root rot, slugs and Pythium. Flower clusters are beginning to flower and it is necessary to ensure adequate soil moisture from now until berry ripening for optimal seed production, especially during hot temperatures.