After a very wet late summer and fall last year, Phytophthora root rot probably had a chance to spread within low areas of ginseng gardens. Combined with the wet conditions we are currently experiencing, management of Phytophthora leaf blight and root rot will be very important. Traditionally, growers applied Ridomil as the first application of the season. Now, many growers apply Orondis Gold when tops are just beginning to emerge. Since Orondis Gold contains Ridomil as part of the blend, these can be considered the same product for resistance management purposes. Since the labels of Ridomil and Orondis Gold prohibit exceeding a total of 3 applications of metalaxyl-M per season, Orondis Gold should completely replace a Ridomil application and they should not be applied in succession.

It is also good to keep in mind that Phytophthora cactorum, the causal agent of Phytophthora root rot in ginseng, has been shown to have some resistance to Ridomil in Ontario. Research conducted at AAFC in Delhi showed a large percentage of Phytophthora isolated from Ontario fields was resistant to Ridomil over 10 years ago. If growers are experiencing Phytophthora issues in their fields despite repeated Ridomil applications, then resistance may be an issue and it is important to switch to other products to get the disease under control.

Growers now have access to several other products that are very effective for controlling Phytophthora leaf blight and/or root rot and should be included in the management program for resistance management purposes. It is important to recognize that both Phytophthora root rot AND Phytophthora leaf blight are major threats during top emergence and leaf expansion. Although many of the products registered for Phytophthora on ginseng will control the disease in both roots and tops, it is often not possible to control both at the same time. This is because a lower water volume spray that provides good leaf coverage will not leach to the root zone, and a higher water volume application that is washed in with rainfall or irrigation will be washed off the leaves and provide no protection against the leaf blight. Separate applications may be required to protect both phases of the disease once tops have fully emerged.

The only exception is if the product is systemic or moves up the plant from the roots to the top in the xylem. The only registered products with reported mobility within the plant are Ridomil and Presidio, which will move up the xylem from roots to leaves if they are absorbed by the roots and the Group 33 products Aliette and Phostrol which are truly mobile and will move from the leaves to the roots. However, resistance to Ridomil is known to occur and it may not provide any protection from either form of the disease if resistance strains are present in the field. Aliette and Phostrol, while systemic, have not shown as much efficacy as other fungicides in management of Phytophthora. They are a good option as part of the product rotation but are not the best options when disease risk is high. They also have the limitation of only being effective if the leaves have begun to unfold so it can be taken into the plant and will only provide protectant activity with no control of disease in rotted areas of a plant. Also, remember that Presidio must be tank mixed with another product registered for Phytophthora control on ginseng. Check with the company to determine tank-mix compatibility before applying.

A listing of the products registered for control or suppression of Phytophthora in ginseng are provided in Table 1 along with their reported mobility within plants. A product listed as mobile, will move from the point of contact to other areas of the plant. A product listed as xylem-mobile, will only move towards the leaves. If they are absorbed by the roots they will move up towards the leaves but not the other way around. A product listed as translaminar will move through leaves and will protect the underside of leaves if sprayed on the upper surface but will have no other mobility within the plant. A product that is a contact fungicide and is immobile must cover all leaf and other plant surfaces because they will not move within the plant.

Table 1. Listing of Products Registered for Control or Suppression of Phytophthora in Ginseng and Their Reported Mobility within the Plant

References:

Beckerman, Janna. 2018. Fungicide mobility for nursery, greenhouse and landscape professionals. Purdue University Cooperative Extension.