The ginseng crop is progressing normally with no major disease or insect issues for this time of year across the industry. However, heavy rains have increased Phytophthora root rot issues in many fields. Continued heavy rains could lead to more significant disease spread in those fields.
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Rainfall and Soil Moisture
Prior to last night’s storms, rainfall had been highly localized in the south-eastern half of the ginseng growing area and more widespread in the northwestern half. The AAFC precipitation maps show clear differences with over 200 mm of rain around Woodstock in the past 30 days and 60-80 mm or less along the lake near Port Dover (Figure 1). Some areas received even less rain given the localized storms over the past few weeks. Given the complete lack of rain in the previous month, that means some areas were very moisture stressed, while others were very wet prior to last night. Last night’s rain added 15 to 40 mm in the region. Some areas may still be relatively dry and may benefit from additional rainfall or irrigation. As a result, monitor soil moisture and do not assume that all of your fields have received sufficient moisture, especially closer to the lake.
Figure 1. Map of accumulated precipitation over the past 30 days (as of July 26)
Phytophthora
Those fields with excessive rains are at risk of major Phytophthora outbreaks. Dry weather last year and early this year has resulted in lower Phytophthora pressures, especially in younger fields that did not go through the extreme wet period at the end of 2021. With the wetter weather there are reports of increasing Phytophthora (Figure 2). At this time of year due to regular fungicide applications and less tender leaves, root rot is more common than foliar Phytophthora. As a result, fungicide applications should focus more on root rot (i.e. higher water volume/drench applications) than the foliar phase of the disease, and can be targeted more at low areas of the field where the disease is most likely to occur. However, do not forget about the foliar protection entirely or the foliar phase could become an issue.
Figure 2. A patch of collapsed tops due to a new Phytophthora root rot outbreak.
Alternaria
Alternaria is a constant threat all season. The risk may be higher right now because of hot temperatures, high humidity and festering infections where fungicide coverage is difficult to achieve such as deep within the canopy. Older gardens usually have some live lesions at this time of year, especially within the canopy, and all it takes is ideal weather and incomplete spray coverage for the disease to spread rapidly. Watch for collapsed tops in older gardens. Stem lesions are often found within the canopy at the base of these collapsed tops (Figure 3). Don’t assume that every discoloured and wilted top is due to root disease.
Figure 3. Lesions of Alternaria leaf and stem blight on stems that caused tops to discolour and collapse.
Grubs
As I wrote about a few weeks ago, this is a good year to start thinking about how grubs will affect your production once imidacloprid is no longer an option in 2025. At this time, most growers have erected shade structures and are waiting for seeding later this summer. If you have extensive weed cover in those fields, that is ideal habitat for young grubs to survive until seeding. Controlling weeds in those fields could prevent grub problems in seedling gardens next year.
