Ginseng Crop Update
Alternaria remains the biggest challenge for most growers. The problem stems from freeze damage/stunting in May and ideal weather conditions for Alternaria – hot and dry with long dew periods. With insufficient rains over the past few weeks, except in localized areas with thunderstorms, it will be very important to ensure fields are irrigated adequately to reduce stress on the plants as much as possible. If you have overhead irrigation, it is best to irrigate early in the morning or late in the evening, so you are not extending the leaf wetness period and increasing infections with Alternaria.
The current dry weather conditions will greatly reduce the risk of both foliar and root Phytophthora infections. The frequency of sprays for Phytophthora can be reduced under these conditions to the longest intervals indicated on the label, unless you have hotspots of root disease and have had more frequent rains in thunderstorms or had to overhead irrigate.
Cylindrocarpon progress will also be very slow under these warm and dry conditions. Cylindrocarpon prefers cooler soil temperatures in spring and fall. Even though seedlings do not typically show symptoms, recent research suggests that it might already be present in seedlings, infecting small feeder roots and then moving into the main taproot in subsequent years. Management should begin in seedlings, but progress will be very slow in the summer and there is no rush to get control measures on.
Recent research also suggests that some fungicides bind like glue to the straw if they dry onto the surface and will not move any further. With the spray volumes growers typically use, the sprays can dry onto the straw within minutes of application. Although some products will not bind to the straw, it is best to manage every spray for root disease as if it binds to the straw unless the company can tell you otherwise. This means applying the products when the straw is already wet from rain or dew and then applying overhead irrigation or ensuring it rains as soon as possible after spraying.
There have not been any reports of unusual insect activity over the past few weeks. Leafrollers are typically the main insect issue at this time of year.
Pre-Plant Management of Nematodes
Most growers are concerned about parasitic nematodes in ginseng. However, very few growers do anything about them except for fumigation. Managing nematodes should begin long before fumigation and continue throughout the year of seeding, since there is currently nothing that can be done to control them once the seedlings emerge.
Step 1 – Test for Nematodes
If you do not know what you are dealing with, then it is very hard to control it. Soil testing for nematodes should be a common practice for ginseng growers, but rarely is used. Testing for nematodes is very similar to testing for nutrients and the procedures can be found at the following link: http://www.omafra.gov.on.ca/english/crops/facts/06-099.htm. Both the University of Guelph Pest Diagnostic Clinic and A&L Laboratories provide nematode sample analysis.
There are two main plant parasitic nematodes that are known to damage ginseng: root lesion nematode and northern root-knot nematode. We do not have any thresholds for nematodes in ginseng, but experience suggests that any amount of root lesion nematode in a soil test can be problematic. Ginseng can tolerate a small amount of root knot nematode without serious damage, but higher amounts (e.g. >100 nematodes per kg of soil) can cause economic damage, mainly through degradation of marketability of the roots. Root knot nematodes usually occur in patches in the field, whereas root lesion nematodes are a bit wider spread.
The timing of nematode sampling is critical. Nematodes go deep in the soil in the winter and come back up in the spring. In the summer they may be entirely within the roots and not captured in a soil test. It is best to sample during moderate soil temperatures in May or October when the nematodes are moving through the soil.
Step 2 – Nematode-Suppressive Cover Crops
Fumigation practices are not perfect, and it can be assumed that some nematodes will survive fumigation. If the soil test indicates the presence of parasitic nematodes, consider the use of a nematode-suppressive cover crop in the year before seeding or the spring of the seeding year. Canadian Forage Pearl Millet is non-host for parasitic nematodes and can suppress the populations in the soil in the year prior to fumigation. To provide the best control, it should be kept weed-free, and then mowed periodically throughout the summer to prevent it from going to seed. Various mustard cover crops high in glucosinolates can also be grown in the early spring and incorporated as a biofumigant to control nematodes.
Step 3 – Proper Fumigation
Nematodes can survive within crop debris where fumigants cannot penetrate. Cultivate the soil well ahead of fumigation to incorporate plant debris and let it break down prior to fumigation. Multiple cultivations may be necessary.
Nematodes can survive in the soil below the fumigation zone. The fumigation zone is typically 30 to 40 cm deep (12 to 16”). When you form the beds, a small wedge of the bed former goes down below that depth and puts the untreated soil on top of the bed. Nematodes could be living in that wedge and re-infest the bed prior to seeding. Fumigate as deeply as possible and try to ensure the bed former does not penetrate as much into the untreated zone.
Ensure a good seal through either a tarp or a power roller on soil with proper moisture content. This will ensure the fumigant stays in the soil for as long as possible and the surface layers are properly fumigated.
Step 4 – Weed Management
Whatever nematodes survive fumigation will try to find alternative hosts to live on. If you allow weeds to grow on the land between fumigation and seeding, the nematodes can survive and multiply before ginseng is in the ground. Keep fields free of weeds during this period.
Step 5 – Nurse Crop/Volunteer Rye Management
Many growers either seed a nurse crop or allow seeds surviving in the straw to grow to anchor the straw over the first fall and winter. Rye is a known host for root lesion nematodes, and this nurse crop can allow populations to build prior to ginseng germination. Wheat appears to be less of a host, so consider wheat as an alternative straw or nurse crop if root lesion nematode populations persist. Alternative straws such as oats or miscanthus may also work as non-hosts for root lesion nematodes, but their use as a straw alternative for ginseng has not been sufficiently researched.
In the spring, killing of the nurse/volunteer crop with herbicides appears to cause the root lesion nematodes to move over to the ginseng, since they have nothing else to feed on. This is when they cause most of their damage. Ginseng is not a suitable host for root lesion nematodes and research indicates that populations of root lesion nematodes decrease rapidly in the seedling year, and they are virtually non-existent in older gardens. Management after ginseng germination is not currently an option, since there are no post-planting nematicides registered yet, and would not be effective beyond the first couple of weeks of germination.
Ginseng is a host for root-knot nematodes, but economic damage is rarer and usually only occurs in patches. Control of root-knot nematodes is more difficult since they can more easily survive fumigation within plant debris and populations can then build during the life of the garden.
Even if you do anything properly, you may still have symptoms on roots. Why? Because most of the symptoms growers attribute to nematodes is from other causes. Most of the time it is the general “rusty root” that appears to be caused by a wide range of factors including soil fungi, weather conditions, moisture and soil characteristics. Do not always assume that what you are dealing with is nematode damage. Conduct a soil test to determine if there are any nematodes present before you attribute the damage to nematodes and manage the problem by controlling for the wrong factor.