Given the major stresses experienced by the ginseng crop this year, including early frost damage, hot and dry conditions, and high Alternaria pressures, many gardens are senescing early this year. There are a few things to consider for proper management of these gardens:
- How much green remains in the canopy?
At this time of year, it is natural for plants to start showing signs of stress. The amount of root growth will be proportional to what percentage of the canopy is still green. Photosynthesis is not as efficient in older leaves and light levels are significantly declining compared to June, so the potential for root growth each day is much less even for a healthy canopy at this time of year. Once the canopy is less than half green, root growth potential may be a quarter of what it was at peak growth in June. Photosynthesis will continue to decline every day. The cost of managing the canopy for another month may be higher than the value of the additional root growth in some gardens. The decision to continue to manage the garden for that root growth will depend on the disease levels in the garden.
- What is the foliar disease pressure?
Most growers have had to deal with high disease pressures this year, especially Alternaria blight. If the tops have minimal green leaves remaining and disease pressures are still high, you may want to consider pulling back the shade early to kill the tops, and as a result, stop further Alternaria development. This can help to reduce the inoculum for next year if the garden is not going to be harvested and reduce costs for fungicide applications. Once the tops have died down, a broad-spectrum contact fungicide should be applied to kill any remaining inoculum on the dead leaves and stems. No further fungicide applications will be needed for foliar disease control.
- What is the root disease pressure?
Root diseases were less of an issue this year, except for isolated areas that had excessive rains. More widespread heavy rains have occurred over the past week, which may increase disease risk again. Even if the shades are pulled back, root diseases can still spread through the garden right up to freeze up in November or December. Although the foliar spread of Phytophthora will be stopped, spread can occur through the field with the movement of machinery and workers or movement of surface water. Top up the straw to reduce splashing of soil in heavy rains and reduce the movement of disease through the garden. Continue spraying for root diseases and irrigating them in if required, especially if disease is already present in the garden. If the shade is pulled back, it will be very difficult to know how root disease is progressing through the garden, so err on the side of caution and assume a high risk of disease if the soil is moist.
- What other management practices are required?
Weed pressures will likely increase if tops senesce early and/or the shades are pulled back. Continued weed management will be required.
In areas that have not had significant rains or if a dry period occurs in September, water stress can still occur if the tops remain and they are at least partially green. Irrigation may still be necessary at this time, especially if the garden is to be harvested this year. Dry and hot conditions at harvest appear to be related to post-harvest issues with red vein disorder. However, if the shade is pulled back due to early senescence the plants will no longer transpire once the tops are dead and will not require irrigation. Keep the irrigation system active though to ensure you can wash fungicides into the root zone if required, since there is plenty of time for root diseases to develop and spread before winter.
Fertilization at this time or into the fall is not recommended. It is unlikely to help the tops recover if they are already significantly stressed, would waste time and money, and could affect the hardiness of the plants over the winter.
