This is the time of year ginseng growers prepare new land for fumigation. Based on the numerous trials OGGA and OMAFRA have conducted over the past 8 years on fumigation, proper preparation and sealing of the fumigant into the soil appears essential for efficacy of the fumigants against nematodes and soil-borne pathogens. Here are some best practices to think about while you are getting ready for fumigation.
1. Land Preparation
To seal the soil properly with either tarps or rolling, it is important to cultivate the soil deeply and thoroughly before fumigation to allow the fumigant to penetrate as far as possible from the injection shanks. This should be at least as deep as the injection shanks. The fumigant will move very slowly through hard-packed soil. If the soil is only cultivated to a shallow depth, the fumigant will move more slowly through the packed soil below this depth and may not fully penetrate between the shanks. This will leave a refuge of pathogens in the soil that can quickly recolonize the soil after fumigation.
It is equally important to ensure that any vegetation or organic debris in the soil is mostly broken down prior to fumigation. The soil may need to be cultivated several times before fumigation to fully break down larger debris such as corn stubble. Debris in the soil can break the seal of the soil and allow the fumigant to escape too quickly. Fumigants also cannot penetrate organic debris very quickly and this can be a refuge for soil pathogens to survive the fumigation.
2. Soil Moisture
Regardless of how well you prepare the soil ahead of fumigation, your efforts can be ruined by applying the fumigant under the wrong soil moisture level. The metam-sodium labels require that soil moisture in the top 15 cm (6”) of soil to be 60-80% of available water capacity of the soil. Chloropicrin labels require equal to or greater than 50% available water capacity.
Available water capacity is the amount of water available to plants in the soil. If the available water capacity is 100%, then the soil is at field capacity, which is the amount of water the soil can hold after excess water has drained out of the soil. So, ideally the soil should be thoroughly watered before fumigation, but there should be enough time for any excess water to drain out of the soil PLUS some additional time to allow the soil to dry slightly so it is not at field capacity. Applying fumigants when it is too wet will not allow them to move through the soil fast enough. On the other hand, applying them when it is too dry will allow the fumigant to move rapidly out of the soil before there is enough time to kill soil pathogens. This can also result in drift out of the field if the soil is not sealed with a tarp. Even if the fumigant is tarped, dry conditions will result in the fumigant rapidly moving from the shanks to the surface of the soil and efficacy lower in the soil profile will likely be poor.
3. Weather Conditions
Most ginseng growers rely on custom applicators to fumigate their fields. Applicators are required to apply fumigants only under specific temperature, wind speed and other weather conditions. Fumigation under less than ideal conditions can cause drift issues, especially when there is a temperature inversion in the forecast (i.e. calm, clear night). Drift of fumigants can pose a danger for people and pets in neighbouring residences, businesses, pathways and other publicly accessed areas near the field. If drift of a fumigant occurs, it also means that the fumigant did not stay in the soil long enough to be effective. Read the fumigant label and ensure that applicators are following all listed precautions.
4. Other Considerations
Choice of fumigant also is an important consideration. The best fumigant to use depends on what nematode and disease pressures are present in the field, along with cost considerations. Metam-sodium is known to control root lesion nematodes better than chloropicrin. However, if metam-sodium is not sealed properly, it may still have less efficacy against nematodes than chloropicrin, considering chloropicrin is sealed with a tarp. Chloropicrin works better against soil fungi and water moulds, but this also requires attention to the soil conditions specified on the label. A good place to start in choosing a nematode is to take a soil sample and send it in for nematode analysis. This can give you an idea if plant parasitic nematodes like root lesion nematode will be a problem in the field. Click the following link for more info on sampling for nematodes: https://onspecialtycrops.wordpress.com/2018/06/01/sampling-for-plant-parasitic-nematodes-in-ginseng/.
The fumigant label contains important information on how to prepare soil for fumigation, procedures and precautions for fumigation, and post-fumigation practices. Even if a custom applicator is fumigating for you, it is important for all ginseng growers to read the label thoroughly before application. The grower is ultimately responsible for ensuring the label is followed correctly.
Signage is also an important consideration to protect the public from accidental exposure. Labels require that there be signage BOTH at the edge of the application area (i.e. the area actually treated with the fumigant) AND the edge of the buffer zone (i.e. the area of at least 8 m (25 ft) from which people must be excluded during and after the application). While the application zone signs must be posted at every entrance to the field, the buffer zones must be posted along every possible access point into the field (e.g. roadside without a fence, walking path, residence etc.). Since the buffer zone is a minimum of 8 m (25 ft) wide extending out in all directions from the edge of the application zone, a field with proper signage should always have a buffer zone sign at least 8 m (25 ft) outside of the application zone sign.