With the sudden switch to summer-like weather and plenty of sunshine to heat up the soil, ginseng is rapidly emerging this week. More humid weather is expected over the weekend, and this will increase the risk of foliar diseases, especially Alternaria. The current forecast suggests rainfall amounts will be relatively light Sunday and Monday, which should keep the risk of foliar Phytophthora lower, but a thunderstorm accompanied by even a brief period of heavy rain could increase the risk.
A brief cool-down is in the forecast for Tuesday and Wednesday next week. Current forecasts show low temperatures of between 3 and 5oC at night (as of May 13 pm), which would not be a concern. However, long-range forecasts tend to be conservative and as the days approach, forecasted temperatures on warm days tend to get warmer and cold nights tend to get colder. For example, forecasts for yesterday were showing a high temperature of 24oC for much of the week and that switched to 28oC the night before. The actual high temperature was 29.6oC. If that kind of fluctuation goes the other way, a forecasted low of 4oC could go down to 0oC or below over time. Although the risk is currently low, growers should keep an eye on the forecast for the potential for frost and have row covers ready to deploy, especially in low-lying fields. By then, most seedlings will likely have emerged through the straw.
The prolonged warm weather this week should allow ginseng tops to unfold more normally this year compared to the last few years. If the forecast holds for early next week and frost is avoided, the brief cool-down should not affect normal canopy development. This will hopefully reduce plant stress going into the summer and reduce foliar disease pressures.
May Pest Control
The Alternaria control program should begin as soon as tops emerge and begin to unfold, especially in warm, humid weather. Once the canopy closes, the extra humidity within the canopy will allow for stem infections that can lead to patches of disease later on. Applying fungicides before the canopy fully emerges can provide better stem coverage than waiting until the risk is higher and the canopy is closed.
Botrytis risk is also high in the first humid days of the year, especially if there is any damage to the leaves from wind, sandblasting or frost. None of those events have occurred yet, but one thunderstorm or frost event can change that quickly. It is best to ensure that there is some Botrytis protection provided by the products chosen for Alternaria control. If a leaf-damaging event occurs, switch to a more targeted Botrytis product.
Foliar Phytophthora usually shows up around mid-May but will depend on a rain event to splash spores from the soil onto the leaves. Once that occurs, leaves are highly susceptible when they are unfolding. Fungicides applied for Phytophthora root disease control can provide some protection of the leaves if they are xylem-mobile, meaning they move up from the roots and stems to the leaves in the xylem. The fungicides registered for Phytophthora root rot that are xylem mobile are Orondis Gold, Ridomil and Presidio. Given the risk of Ridomil resistance and that Ridomil is already a component of Orondis Gold, it is best not to apply Ridomil on its own for Phytophthora root rot control at this time of year.
Root disease pressures are expected to be high this spring due to the wet conditions last fall. This could include Phytophthora, Cylindrocarpon and Pythium. Dig up a few roots in older gardens in random areas to monitor for root disease and apply pest control products if needed.
Now is the time to begin scouting for leaf folders in ginseng. See my previous post for potential signs of the pest. The best time to control the insect is before it forms a thick roll of leaves. If you see any of the signs of leaf folders, please report sightings to Sean Westerveld at firstname.lastname@example.org or Melanie Filotas as email@example.com so can have a better idea of the timing and life-cycle of this pest. Registered insecticides applied for leafroller control in ginseng will also likely provide some control of leaf folders, but only if they can be applied before the larvae are protected inside the roll of leaves.
Slugs and cutworms are becoming active this week. Controls for slugs should be applied as soon as possible before damage becomes obvious. Unless using baits for cutworm control, cutworms will need to be present and actively feeding for adequate control. Scout fields for tops severed off near the straw to confirm the cutworms are feeding before applying insecticides for cutworm control.
If European chafer grubs were not controlled with Admire at seeding, damage will likely begin in seedling gardens soon after they emerge. Look for tops being pulled into the straw from below and gradually expanding patches of missing plants. There are no controls for grubs in established gardens.
For a listing of pest control products registered for use on ginseng along with a summary of rates, use restrictions and precautions, consult the new Ontario Crop Protection Hub at Ontario.ca/protection.