Frost

In the last crop update, I commented on how much colder the temperatures were at Delhi than the forecast the night before. Since then, there have been several nights with temperatures several degrees Celsius colder at Delhi than at any nearby station and compared to the forecast. Environment Canada blamed it on a sensor issue, but the same thing kept happening after they replaced a temperature sensor. We have confirmed with dataloggers that the temperature readings are accurate; it is the air that is colder than it should be. We now have data to suggest that the ginseng shades surrounding the Delhi weather station are causing the temperatures to drop on calm and clear nights.

The exact dynamics of the effect are unknown, but many acres of black shade cloth are likely radiating heat into space, causing the air nearby to cool. Somehow, this is leading to a pool of cold air that is a few degrees below the dew point of the surrounding air, something that normally does not occur. This effect is likely to be more pronounced in sheltered gardens on flat ground or in low areas. Gardens out in the open probably have fewer issues. These dynamics have been happening for years, but this is the first real confirmation of the effect. Once we figure out how it is occurring, there may be strategies to reduce the damage.

With frost and cold temperature damage in gardens, and all the cool and moist weather we have been experiencing, disease risks are very high. Monitor gardens closely over the next few weeks for all ginseng diseases. Alternaria is starting to emerge on stems, and severity will increase with warmer temperatures forecast for next week. Botrytis is always a risk on damaging stems and leaves after frost events. The wet weather also increases Phytophthora risk.

Rhizoctonia

In the last decade or more, we have not seen much Rhizoctonia, probably due to the efficacy of azoxystrobin against the disease. However, active Rhizoctonia has been found in recent days in older gardens, causing rot of the lower stem within the straw and the crown of the root. This causes the top to arch over and collapse. Rhizoctonia normally requires a combination of moist conditions and good aeration. Normally, that occurs in the upper soil layers near the crown, but the recent wet, cool weather has likely caused it to grow within the straw layer, causing more damage to the lower stem.

The symptoms of Rhizoctonia include expanding circles of collapsing plants that are up to 1 m (3 ft) in diameter (Figure 1). Damage to the lower stem or crown causes the stem to arch over and then collapse. Symptoms at the crown or lower stem begin as orangish-brown discolouration followed by a soft rot (Figure 2). For more information on Rhizoctonia, consult the Rhizoctonia Pest Profile on Ontario Crop IPM. To determine if you have active Rhizoctonia in your garden, try this Rhizoctonia Quick Test.

Figure 1. A circle of missing plants caused by Rhizoctonia crown rot.

Figure 2. A soft rot of the lower stem with orangish-brown discolouration due to Rhizoctonia.

Azoxystrobin products (Quadris, Azoshy, Emissarius) are most effective for Rhizoctonia control but are only registered for use at seeding and in the spring of the seedling year. For older gardens, the best options are fludioxonil or fluazinam products (Switch/Button, Scholar, and Allegro/Vantana), but research shows they only provide moderate control. They will need to be applied at a high water volume, followed by a small amount of rainfall or irrigation to wash them into the straw and upper soil layers.

Leaf Folders

New leaf folder caterpillars continue to emerge in our research garden, so another insecticide spray may be warranted in areas where scouting indicates new leaf folds developing. This means that egg hatch must be occurring over an extended period. Earlier insecticide sprays seemed to be effective at reducing populations, but these will not affect any eggs that hatched after the insecticide was applied.

Based on our monitoring efforts, it looks like the caterpillars periodically leave their roll and then create a new one nearby. This does provide an opportunity for control with insecticides. Rotate insecticides for optimal control. In addition to the registered products – Delegate, Success/Entrust, Dipel 2X DF – some products applied for cutworms may also control leaf folders. For example, Coragen is registered for control of both cutworms and European corn borer in ginseng, and leaf folders are in the same family as European corn borer. When applied for labeled pests, Coragen may also provide control of leaf folders.

Ginseng IPM Workshop Reminder

A reminder that our annual Ginseng IPM Workshop for growers, scouts, and industry representatives is scheduled for Thursday, June 5 at 1:00 pm at the OGGA research garden. Learn how to scout for and identify the major insects, diseases, and disorders of ginseng. For more information and a registration link, click here.