With the warm temperatures this week, older ginseng gardens are quickly emerging. A few seedlings are also beginning to emerge in the warmest areas. Ginseng at this stage of development is highly prone to all three foliar diseases: Alternaria, Botrytis and Phytophthora. Here is a reminder of what conditions favour the development of these three diseases:
Alternaria Leaf and Stem Blight:
Alternaria is favoured by mild temperatures and long dew periods. Rainy and humid weather is not necessary for Alternaria infection. A couple of long dew periods in a row are sufficient for Alternaria to infect the leaves or stems. The ginseng shade can extend the dew period, making it the ideal environment for Alternaria. Infection usually begins once the canopy begins to close, which reduces air flow around the plants and reduces drying. Alternaria can also be worse when the plant is stressed by another factor such as frost, dry conditions or wind damage.
Botrytis is only weakly pathogenic to ginseng. It often needs a wound in order to infect the plant. It is most commonly a major issue after frost damage or damage from another disease or insect pest. Once one plant is infected, the disease can spread to healthy plants in contact with the infected plant. Botrytis is favoured by very humid and warm weather and does not need rainfall for infection.
Phytophthora Leaf Blight:
Phytophthora requires extended periods of leaf wetness in order to infect the leaves. Most older ginseng gardens have wet areas containing some Phytophthora root rot. Splashing of spores onto the leaves in wet weather can lead to a foliar infection. Airborne spores are then produced that can travel to new areas on the wind. If conditions remain wet, a couple of rounds of infection can lead to a major disease outbreak across the region. Spores then fall back to the soil and lead to new root infections.
The spring so far has been abnormally dry. Unless there was a thick cover of a winter cereal in the garden that would have used the available moisture, soil moisture levels should still be adequate for ginseng. These conditions are not conducive to Phytophthora leaf blight at the moment, but Alternaria and Botrytis can still be an issue. Weather conditions can change quickly, and single thunderstorm can make conditions ideal for a major disease outbreak. The main problem with the dry conditions is getting fungicides down to the roots for Rhizoctonia, Phytophthora root rot, Pythium and Cylindrocarpon control. Growers with overhead irrigation should consider irrigating after fungicide application (e.g. 10 mm) to move the product down to the roots. Growers with drip irrigation may want to irrigate before fungicide application so the fungicide can move farther into the soil, and then apply the fungicide at a very high water volume (e.g. 4,000 L/ha) to get the fungicide through the straw. The alternative is to wait to apply the fungicide until rainfall is in the forecast, but that may result in major delays in protecting the roots.
Once the ginseng leaves begin to unfold, soil moisture should be monitored carefully to ensure adequate moisture. Moisture stress could lead to additional foliar disease issues.