This has been a challenging year for ginseng growers in some areas, with really only one major problem – Phytophthora due to flooded fields. While it has been necessary for some growers to pull the shades in some gardens and harvest early, growers with less severe issues will need to carefully manage fields for Phytophthora root rot well into next year.

Most growers who are pulling the shades early intend to harvest or abandon parts of the field, but if the intent is to keep the fields until next year it is important to keep in mind that pulling the shade early only stops the foliar phase of Phytophthora blight. While that does serve to slow the spread of the disease through splashing and some movement of machinery by reducing the need to manage foliar diseases, it does very little to stop the spread of the root rot phase of the disease. Phytophthora cactorum has swimming spores called zoospores that will continue to spread the disease from root to root whenever the soil is saturated.

Due to repeatedly flooded fields Phytophthora root rot is now spread everywhere in some gardens and is not confined to the low areas. In some fields the extent is obvious, but in others there are just one or a few roots infected and the overall canopy still appears healthy. The disease could spread from these isolated roots whenever wet weather returns.

It is nearly impossible to eradicate a root rot once it begins. The best you can hope for is that fungicides along with drier conditions limit further spread. Here are some strategies for minimizing Phytophthora in fields with these lingering issues:

1. Continue to apply a rotation of fungicides registered for use on the root rot phase of the disease until Phytophthora cactorum is no longer active. The pathogen has an optimal temperature of 23-25°C but can grow at temperatures as low as 2°C. Growth is likely very slow below about 10-15°C. Soil temperatures are likely at or above this range until about October 10 or later, depending on the weather.
2. Target the root rot phase of the disease by applying at a high-water volume just before or during a rain or overhead irrigation to ensure the product never dries on the straw and moves into the root zone. Most products will be completely immobilized once they dry onto a surface. Ensure you are using products that are registered for control of the root rot phase of the disease.
3. Focus on products with xylem-mobile or locally systemic action to provide some curative action on roots already infected with the pathogen. Xylem mobile products may also protect the tops at the same time, but foliar-applied products will not protect the roots. Products like Aliette and Phostrol are systemic and would provide some protection of healthy roots following a foliar spray, but these products provide no curative action because they cannot get to the parts of the root already damaged by the pathogen. For a list of products registered for Phytophthora root rot on ginseng and their efficacy rating on ginseng visit the Ontario Crop Protection Hub at Ontario.ca/cropprotection. This information along with the product’s mobility in the plant is also available on the Ginseng Pest Control Product Summary Tables distributed to OGGA members by email this spring. Contact me if you need a copy.
4. Consider starting your spray program early next spring before tops begin emerging to slow down the pathogen as soils begin to warm.

Roots in fields with repeated saturated conditions may also have higher rusty root this year, especially if fields dried out between rain events. There is some research to suggest that the most common rusty root symptom we have in Ontario may be the result of sudden increases in root growth caused by inputs of water or nutrients after a period of reduced availability, and this causes the skin to burst leading to micro-cracking of the skin and the rusty discolouration.