It has been a very wet month in southern Ontario, with especially damp conditions over the past 7 days. There has been approximately 134 mm of rain in Delhi, 148 mm in Port Dover, 112 mm in London and 146 mm in Waterloo since June 20. Because much of the rain came in slow moving thunderstorms, some areas probably received much more rain. What is much more concerning than the volume of rain has been the duration of leaf wetness. Over the past week, it is likely that some leaves in older gardens have had almost continuous leaf wetness. This is at a time when air temperatures have barely fluctuated at all and have hovered between 18 and 25C (day and night) with very high humidity the entire period, a perfect temperature range for fungal development. More rain is forecast over the next two or three days. These are ideal conditions for the development and spread of diseases, especially Phytophthora.
To add to the perfect conditions for disease development, it has been difficult for growers to protect the crop with fungicides because of the constant saturated soils and repeated rains. Under these conditions it is important to understand how the registered fungicides act to protect the crop, so the proper fungicides are chosen for those periods when you can get the fungicide on. The proper choice of product will depend on the status of the disease in a garden.
The fungicides currently registered for management of Phytophthora root rot and/or foliar blight of ginseng are Ridomil Gold 1G, Orondis Gold, Reason, Phostrol (suppression only), Aliette, Forum (suppression only), Revus, Presidio, captan (e.g. Maestro, Captan, Supra Captan) and Echo. Of these, Orondis Gold can only be applied once per year and is recommended for spring application during emergence.
Contact fungicides for Phytophthora control include captan products and Echo (chlorothalonil). Since they do not enter the plant, they can only control what is on the surface, not what might already be inside the plant. As a result, they are best applied preventatively when conditions are conducive to disease development but before infection is confirmed within the field. They can also prevent spread of the disease to new plants within a garden. If plant tissue is already killed by the pathogen, they could help to control the pathogen on and potentially within the dead tissue, if the tissue is degraded enough to expose the pathogen to the fungicide. Captan is only registered for management of the root rot phase of the disease and Echo is only registered for management of the foliar blight phase. Research shows chlorothalonil to be only slightly effective at managing the disease and its primary efficacy is against Alternaria and Botrytis blights.
Locally-Systemic and Xylem-Mobile Products
Many fungicides can get within the plant and move at least locally to protect areas that were not contacted by the spray or upwards in the plant xylem following the path of water flow within the plant (i.e. from roots upwards and outwards). Examples of these for management of Phytophthora in ginseng are Ridomil (xylem-mobile), Reason, Forum and Presidio (xylem-mobile and translaminar), Revus (translaminar) and Orondis Gold (locally systemic, translaminar and xylem-mobile). When the disease is already present within the garden and the risk is high, these are the best options for management because they can provide protection of the areas of the plant that are hard to reach with the fungicide spray. They may also provide some curative action if the pathogen is already within the plant at the time of spraying. During rainy periods, they also offer the advantage of being absorbed into the plant, so cannot be washed off by subsequent rainfall if sufficient time has passed for them to be absorbed (1 to 2 hr rainfastness, depending on product).
Keep in mind that preliminary research shows that Phytophthora cactorum isolated from ginseng gardens has shown a high level of resistance to metalaxyl-M (Ridomil). Research is underway to determine how extensive this problem is. Until those results are available, Ridomil should be used with caution and should not be relied on alone to manage this disease.
Both Forum and Presidio must be tank-mixed with another compatible tank-mix partner also registered for management of Phytophthora in ginseng. Consult the label for details.
Fully Systemic Products
Aliette and Phostrol belong to a group of fungicides called phosphonates (FRAC group P07). The mode of action of phosphonates is still up for debate. It was originally thought that they primarily activated the plant’s own defenses, but more recent research suggests it may be a combination of suppressing the pathogen, blocking the ability of the pathogen to suppress the plant’s defenses, and/or directly activating plant defenses. They offer the advantage of being truly systemic – moving both upwards in xylem and downwards in phloem. Foliar application is sufficient to protect the whole plant. What they do not offer is a cure. Once the plant is infected, they may slow the pathogen down, but are unlikely to kill it entirely. They also cannot reach areas of the plant that are already damaged by the pathogen. They offer a good choice as part of the Phytophthora management program but are best used preventatively. For more in depth reading on phosphonates as well as similar fertilizer products, refer to this article on phosphonates for Pythium management in turf from Penn State University (https://extension.psu.edu/understanding-the-phosphonate-products).
Under these wet conditions it is important to keep in mind that the disease can be on the roots, the leaves or both. It is difficult to manage both using the same sprays unless the product is applied to the roots and is xylem-mobile. Foliar infections can occur anywhere in the garden under these conditions, so it is important to scout the fields regularly for the disease so you know what parts of the plant to target.
There is still a high risk of Alternaria in gardens, especially in gardens where the disease was already established earlier in the year. Wet and cooler conditions are less ideal for Alternaria due to reduced plant stress but, given the existing lesions and the plant stress from this spring’s cold periods, it is important to continue to assume a high risk for this disease.
The weather conditions over the past weak are especially perfect for Botrytis. However, ginseng plants appear to be less susceptible to infection with Botrytis once they are fully developed by late June. Any plants with damage from another issue like Alternaria, heat stress, wind-burn, or leafrollers are still at high risk of Botrytis development. Once Botrytis develops on dying plant tissues, it can more easily infect healthy tissues nearby. Ensure the products included in your Alternaria spray program also have good activity against Botrytis.