Spring 2022 has been considerably colder than Spring 2021 and that will give growers more flexibility to prepare for crop emergence and avoid shade collapses due to snow. Using weather data from the Environment Canada Weather Station in Delhi and the current 7-day forecast, I calculated the growing degree days (GDD) for this year compared to last year up to April 20. I used a base of 5°C, which is commonly used for cool season crops adapted to our climate, like ginseng. Growing degree days are calculated by subtracting the base temperature (5°C) from the mean temperature each day and adding them up, ignoring any days with negative values.
Based on this calculation and the current forecast, by April 20 we will have accumulated around 58 GDD this year. Last year by April 20 we had accumulated 143 GDD. We reached 58 GDD on April 5 in 2021, so 15 days earlier than is likely this year. Keep in mind that a warm spell in late April can easily result in 10 or more GDD per day, so it is possible for the crop to catch up quickly if that occurs, considering it was quite cool in late April 2021.
The growing degree day calculation only accounts for air temperatures, while soil temperatures are key for determining crop emergence for ginseng, so this is just an estimate. Considering this past winter was much colder than the previous winter, it is likely that the soil temperatures are also well behind 2021’s values.
This means that growers should not be in a rush to pull the shade cloth over the crop, considering the risks of a late snowfall collapsing the shade. Growers have an extra two weeks compared to last year, and it is likely that shade cloth will not be needed on older gardens until around May 1 or later, depending on late-April temperatures. It also means growers have time to apply glyphosate for weed control before the crop emerges, but that should be done soon to avoid any impacts on the crop as it emerges into the straw in the coming weeks.
Spring Disease Management
Fall, 2021 was extremely wet, after a mostly wet summer. There were reports of diseases, mostly phytophthora, showing up postharvest following the heavy rains in early October. This suggests that infection of roots had begun in the field at the time of harvest. Growers should be prepared for significant root disease pressures this spring, especially if soils remain wet. Phytophthora can release spores within the soil at fairly low soil temperatures (5-10°C). While disease progress will be slow while soils remain cool, it may be more difficult to control the disease once soil temperatures warm and disease progresses rapidly.
Given the high risk that roots are already infected with Phytophthora, growers may want to begin fungicide applications before ginseng tops emerge. This will require a high-water volume application immediately followed by a rain to wash the product into the soil. Given the current forecast, there is some flexibility, but the application is best done before the next mild period (i.e., several days of maximum temperatures > 15-18°C). There are several fungicide options for control of root rot in ginseng. Recent research suggests there is a high prevalence of resistance to metalaxyl-M (Ridomil, half of Orondis) in Phytophthora in Ontario ginseng fields. The product may have some utility when applied early in the season to prevent disease and could still be effective for management of Pythium, but under a high-risk situation in which roots may already be infected by Phytophthora, Ridomil is not the best choice.
Other root diseases may also have benefited from wet conditions in the fall, but cold temperatures over the winter probably slowed any progression. Monitor fields closely for development of other root diseases like Cylindrocarpon. The risk of foliar diseases depends mostly on spring temperatures during emergence, so there is not much added risk this year.