Ginseng Crop Update – Phytophthora and Weed Management – August 30, 2023

Due to the heavy rains, especially in the northern and western portions of the ginseng-growing region, Phytophthora is the biggest issue in ginseng now. Most other pest issues have been relatively minor or highly localized this year. Now is also a good time to remind growers of some problem weeds to look out for.

How Wet Has It Been?

Much of the ginseng growing area is in the Exceptionally High 95th to 100th percentile above normal for precipitation over the past 60 days (Figure 1). Much of Norfolk is in the lower end of this range, but the Strathroy area, Oxford County and the extreme north and south of the area have had higher amounts. This equates to over 300 mm of rain in these areas, with the bulk of that occurring in July into early August. That is roughly double normal rainfall for this period. Even more problematic than the total rainfall for disease management is that the rain came in many separate events.

Figure 1. 60-day precipitation percentiles for the period ending August 28.

Managing Phytophthora

As a water mould, Phytophthora is promoted by wet conditions. Its swimming spores can move anywhere water moves and be splashed onto foliage to create a foliar outbreak. Persistent and repeated wet conditions can lead to major outbreaks because each wet period allows for new infections that then create new inoculum to be spread further in the next wet period. Foliar infections are a result of insufficient fungicide applications or coverage and allow for airborne spores to be spread great distances to infect new areas of a garden or new gardens. There are three main strategies to control Phytophthora, many of which ginseng growers are well aware of:

  1. Prevent water from pooling or remove excess water from the garden as quickly as possible.
  2. Manage foliar infections at all costs to prevent spread
  3. Choose a rotation of very effective products to manage the spread of the root disease.

Managing Water is the primary focus of most growers starting at the garden establishment stage and does not need to be explained further.

Managing Foliar Infections is critical for preventing spread of the disease. This involves 4 steps:

  1. Remove standing water. Standing water containing spores can be easily splashed onto the leaves and start a foliar infection.
  2. Cover exposed soil. Saturated soil can also contain the spores, and if exposed to the rain, will be splashed more easily onto the leaves. Exposed soil can also be picked up more easily on boots or equipment. Ensure a thick straw layer both on the beds and in the trenches.
  3. Avoid moving through gardens when wet. Workers and machinery can easily pick up spores or infested soil and spread them through a garden and to new gardens.
  4. Apply effective pest control products targeted to phytophthora leaf blight. For most products unless they are xylem (upwardly) mobile, this will require a lower water volume and good coverage of all above ground surfaces of the plant. It may not be possible to control both the foliar and root rot phase of the disease at the same time. A reminder that while the phosphorous-based products (Aliette, Phostrol) help build plant defenses and can be effective systemic fungicides in protecting against Phytophthora, they will not provide any curative action if Phytophthora is already active on a plant.

Finally, Managing Root Rot involves a rotation of effective products applied to the soil, which will require a higher water volume followed by a rain or irrigation before the spray has a chance to dry onto the straw. A reminder that research has shown that there is a high amount of resistance to metalaxyl-M (Ridomil, half of Orondis Gold) in the Phytophthora cactorum strains present in Ontario. While this product may have some efficacy against pythium, it should not be relied on in times of high risk for Phytophthora. Also, consider metalaxyl-M as useless for resistance management against Phytophthora, so a proper rotation will include at least two other actives/products with no single product used twice in a row.

Phytophthora Products

Products registered for use against Phytophthora on ginseng are listed here “Ontario Crop Protection Hub – Phytophthora”. Some products are registered just for the leaf blight phase of the disease, some are registered just for the root rot phase, and some for both. Be sure to confirm that your target phase of the disease is listed on the product label.

Weed Management

One of the biggest weed issues over the past few years has been scentless chamomile. It can grow very effectively in cool temperatures from late fall through spring and can be very difficult to pull out by hand due to its extensive fibrous root system. Seeds of scentless chamomile can be introduced into new fields in the straw, so it is important to manage the weed beginning in the rye fields, or source straw from farms that do not have the weed present. The timing of herbicide applications is also critical for managing this weed. Early spring glyphosate applications are probably the best option for managing the weed within the ginseng crop. For more information refer to “Control of Scentless Chamomile in Ginseng”.

Just as a heads up, palmer amaranth has been found in a corn field in Wellington County. This is a new invasive pigweed species that is concerning because it can be resistant to multiple herbicides and very difficult to control once established. Control is easiest when it is identified early and can be removed by hand. While there is very little risk to ginseng growers at this time, you can help in the surveillance for this invasive weed. For more information, photos, and where to a report a new siting, visit the article “Palmer Amaranth Found in Ontario” on FieldCropNews.

About Sean Westerveld

Ginseng and Medicinal Herbs Specialist, OMAFRA
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