Alternaria leaf and stem blight continues to be an issue in the ginseng industry this year. Much of the symptoms occurring now are due to damage earlier in the year, but active disease is still present in some gardens. In these gardens, hot and dry weather has contributed to the development of the disease despite repeated fungicide applications. A lack of control of the disease is usually due to one or more of these factors:
- The plant is stressed
- Spray coverage is poor
- The spray interval is too long for the conditions
- Improper choice of fungicide for the conditions
The severity of the disease this year is likely due to a combination of the heat and the dry conditions causing the plant to be stressed and unable to defend against the disease. The ginseng plant is adapted to a forest floor environment that is humid, calm and has moderate temperatures. Under the shade cloth, studies have shown that the air temperatures at canopy height can be up to 6oC higher than the outside air temperature on a calm, sunny day. When you increase the air temperature, you decrease the relative humidity in the air because warmer air can hold more moisture. For example, if the air temperature outside of the garden is 30oC and the relative humidity is 55%, raising the air temperature 6oC will lower the relative humidity to 39%. A lower relative humidity puts much more moisture stress on the ginseng plants, sometimes resulting in heat burn with some parts of the leaf turning papery and brown. Heat burn is mostly due to the plant drying out faster than the roots can take up the water.
Alternaria is considered a weak pathogen unless it is under perfect conditions for its development (dry weather with long dew periods and warm temperatures). It usually only becomes severe when the plant is weakened. The best way to prevent or reduce the spread of the disease is to reduce the moisture stress to the plant through irrigation and reduce the temperatures in the garden through improved air flow. Raising side shades can improve air flow through the gardens. Vents may be needed in the shade on hill tops where the heat accumulates on calm days to allow it to escape.
Reaching all of the plant surfaces in a ginseng garden with a fungicide spray can be very challenging. The most likely places to miss when spraying ginseng are the middle of the post rows, the underside of the leaves and the stems. The following two articles discuss ways to improve spray coverage in ginseng.
A Comparison of Sprayer Technology in Ginseng
The Perfect Sprayer Boom for Ginseng?
Improper Spray Interval for the Conditions
When conditions are highly favourable for the disease you should apply fungicides more frequently. The frequency of application will depend on the efficacy of the product and what is permitted on the product label. Products with longer spray intervals indicated on the label, generally last longer than those with a shorter spray interval. However, efficacy and spray interval are not always related. A product with high efficacy can sometimes not last as long on the leaf as a product with a lower efficacy. Consult the product labels for more guidance. Products may also need to be re-applied if a rainfall occurs, but keep in mind that re-application counts as a second application and factors into the number of applications permitted per year and for resistance management.
Improper Choice of Fungicide for the Conditions
If all of the above have been addressed and control is still not achieved, then the last option to adjust is the choice of fungicide. Growers have a tendency to stick to a rotation of lower cost, broad-spectrum fungicides that control multiple diseases at once. However, these may not be as effective against Alternaria as more expensive products that are more targeted towards Alternaria. Considering the potential yield loss caused by tops going down in July could be as much as 500 to 750 lb/ac for an older garden, the added expense of a highly effective product is well worth it. Consult OMAFRA Publication 837 Crop Protection Guide for Ginseng for more information on product efficacy based on research trials on ginseng.
Also, newer fungicides tend to be much more prone to the development of resistance. Poor efficacy can also be caused by repeated use of the same fungicide or fungicides in the same group as resistance develops in the pathogen. For example Scala (pyrimethanil) is in the same group (Group 9) as cyprodinil, which is one of the two fungicides in Switch. If these two fungicides are rotated back and forth, development of resistance to both cyprodinil and pyrimethanil and all Group 9 fungicides is more likely. Once resistance develops it will never go away and will spread to other farms, resulting in permanent loss of a control option. Considering the impending loss of some of the core products for ginseng due to re-evaluation by PMRA, this is something the industry needs to avoid. Always follow label directions for rotating fungicides. Also remember that two products with the same active ingredient (e.g. Bravo and Echo) are the same product under different names. They need to be treated as the same product for both resistance management purposes and the maximum number of applications permitted per year. For example, 3 applications of Bravo plus 3 applications of Echo = 6 applications – the maximum allowed per season for chlorothalonil.