There are reports of storm damage to ginseng, including damaged shade cloth in some areas and heavy rains in most areas. In addition, there have been 5 straight days of nearly continuous leaf wetness in this area, along with saturated soils. This greatly increases the disease risk. Here are some considerations for how to respond to these impacts, given where we are in the growing season:
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Shade Cloth Damage
The decision to repair shade cloth after a storm becomes more difficult the later in the season it occurs and also depends on how much labour is required to repair it. If shade cloth damage occurs in June, it is obvious that it needs to be repaired as soon as possible because a whole year’s worth of yield would be impacted. However, that decision changes as the season progresses because there is less of the season remaining and the ginseng plants start to shut down anyway. The decision to fix the shade now, rather than pull it back and repair it in the off-season, should be based on how much yield may be lost if the tops are killed by pulling back the shade. It is also impacted by labour availability and other farm practices that would be delayed if labour is focused on shade repair. Here are some estimates of potential yield loss based on solar radiation and photosynthetic efficiency as leaves age:
Solar radiation: Based on total photosynthetically active radiation (PAR) by month for Toronto, and an active canopy from June 1 to October 1, 20% of the total summer radiation would occur after September 1. This is based on the assumption that minimal photosynthesis occurs in May, when the leaves are still unfolding, or October, when leaves are near the end of their life cycle and temperatures are falling.
Photosynthetic efficiency: As leaves age, they gradually lose their ability to photosynthesize. Photosynthesis creates the sugars that are transported to the roots and converted to starch, so the rate of photosynthesis is directly related to root growth and yield. When leaves start to show fall colours of purple, yellow and red, it means that the amount of chlorophyll in the leaves is declining. This reduces how much light the leaves can utilize. If photosynthetic efficiency is near 100% in June, then studies suggest it is about 60-75% for the first half of September and 25-55% for the second half of September. Averaging this out, it means that photosynthesis is only at 50% of full efficiency for September. This also depends on the state of the plants. Some fields are already showing significant reddening, and these plants likely have very little potential to photosynthesize.
Total yield impact estimates: Taken together, 20% of total radiation x 50% efficiency = a maximum of 10% of this year’s root growth would be lost by pulling back the shade now. This is based on a healthy canopy in a normal year.
A rough estimate for a four-year ginseng crop would be 5% of yield is produced in Year 1, 15% in Year 2, and 40% each in Year 3 and 4. Losing 10% of yield in year 3 or 4 would result in an estimated loss of 4% of total root yield. The direct impact on yield for Year 1 and 2 crops would be a lot less, but smaller roots may result in smaller canopies the following year, so similar overall yield impacts may occur over the life of the garden.
Moisture Impacts
Disease risk is very high in ginseng due to the heavy rains in August and 5 straight days of wet conditions this week, although the impacts of each disease are different in September than they would be in the spring.
The foliar diseases, Alternaria and Botrytis, reduce the photosynthetic area of the plant as their main impact. They would have the same impact on yield as not repairing a shade crop if they killed the tops entirely in early September. This is just due to the loss of leaf area, since they have no direct impact on the roots. The only lingering effect that high disease in September would have is on the amount of inoculum that makes it through to the spring to start new disease outbreaks. On the other hand, foliar Phytophthora would have far more impact because it leads to root rot, and this pathogen can survive in the soil until next year. If you can’t get Phytophthora under control, it is best to pull the shade cloth to kill the tops and prevent the disease from spreading through the garden and entering the roots.
After a year of repeated fungicide applications, you may need to switch to a new fungicide for optimal disease control. Even for an effective product with no known resistance development, there can be a slight decline in efficacy after repeated use within a season. Ensure good protection against all three foliar diseases. Botrytis may be the highest risk now due to existing damage from other pests and the warm and wet conditions experienced over the past week.
Click here for estimated fungicide efficacy ratings for these three foliar diseases in the Ontario Crop Protection Hub.
