This has been a challenging year for disease management in ginseng. In addition to the earlier issues with root diseases, especially Phytophthora, we are now dealing with high pressures of Alternaria (Figure 1) and Botrytis (Figure 2). It can be difficult to manage all these issues at the same time, especially when dealing with both root and foliar disease. Frequent rainy periods also make it difficult to time sprays properly and to know whether previous sprays have washed off. Here are some things to think about to ensure protection from all these diseases while not wasting money on unneeded sprays.
Figure 1. Alternaria lesion on a ginseng stem
Figure 2. A range of lesions of Botrytis on ginseng leaflets. Note the lack of a yellow halo that would be typical of Alternaria leaf lesions.
Choice of Product
Ginseng growers have a lot of fungicides available for use. If you are having difficulty controlling a particular disease, consider using something more targeted against that disease. The old products growers have been using for a long time such as chlorothalonil (Bravo, Echo) or mancozeb (Dithane, Manzate, Penncozeb) may have good efficacy against a broader range of diseases, but newer products may have even better control of a specific disease. For example, research has shown that products such as trifloxystrobin (Flint), pyrimethanil (Scala) and iprodione (Scholar, Switch) can have better efficacy against Alternaria on ginseng than chlorothalonil or mancozeb.
When the risk of multiple diseases is high at the same time, multiple products may need to be applied to ensure good protection from each of the diseases. There is no single fungicide available that works well on each of Phytophthora, Alternaria and Botrytis. Multiple applications or tank mixes may be required. Contact the chemical companies to ensure compatibility of any products you intend to tank mix. Growers should also ensure they are rotating the different fungicides to prevent the development of resistance and improve overall control. The need to rotate fungicides applies to each disease separately. Products applied for Phytophthora do not count as rotation partners for products applied for Alternaria and vice versa, because most fungicides that work against Phytophthora have no effect on Alternaria.
Water volume is an important consideration for determining whether a product is going to be effective against foliar, stem or root diseases. For good coverage of leaves and stems a water volume of around 500 to 1,000 L/ha (45-90 G/ac) is often recommended. Some labels require specific water volumes outside of this range, so it is important to always check the label first. Higher water volumes will dilute the product and cause too much of the spray to fall off the leaves. This leaves less product in contact with the leaves. Lower water volumes may not provide adequate coverage, especially of the lower stem.
On the other hand, if the target is root disease a higher water volume is desirable to ensure the product gets into the straw layer and will not dry on the leaf surfaces. This should be followed by a rainfall or overhead irrigation to wash the product into the root zone before the spray has a chance to dry onto the straw.
Most products registered for disease control on ginseng are best used preventatively before the spores of the pathogen land on the plant surfaces and before weather conditions are forecast to be favourable for infection. This often means timing the application before a wet period is forecast. It is also important not to spray too close to a rain event before the product can dry and/or be absorbed into leaf surfaces. Different products have different periods before they are rain-fast on plant surfaces. Rainfast times can range as little as 30 minutes to as long as 6 hours or longer, and a few products may never be rainfast. Check with the chemical company for specific rainfast periods. Once a product is rainfast it will not be washed off by rainfall and there is no need for re-application after the rain until the recommended spray interval has passed. Conversely, if you need control of a root rot and the product is rainfast rapidly and it dries onto the plant or straw surfaces, rainfall will not wash it into the root zone.
If you are having difficulty controlling a disease despite the choice of good products, always double check the product rate per acre. Most products allow a range of rates. Under high disease pressures, the highest rate recommended on the label is warranted. Sometimes poor control can come down to a simple mathematical error, so also double check your math!
If all of the above are done correctly and disease control is still not adequate, you likely have issues achieving good spray coverage. The lower parts of the leaves and the stems in a thick 3- or 4-year old canopy can be very difficult to reach. Unfortunately, this is also the area of the canopy where humidity is highest and Alternaria inoculum is often first released. Good spray coverage requires proper choice of nozzles, frequent calibration of spray equipment, regular replacement of nozzles, use of drop arms down the alleys, spraying under optimal conditions (i.e. windspeed), and proper water droplet size for the conditions. There are numerous resources on spray coverage on ginseng. Sprayers101.com is an excellent resource on all topics related to spraying. Type “ginseng” into the search box to find specific work on ginseng. For a related article on spraying berries see the following article: https://sprayers101.com/when-is-fungicide-coverage-critical-always/
It is impossible to control disease perfectly because there will always be unprotected surfaces in the canopy that the spray just cannot reach or periods where spraying is just not possible due to frequent rains. Sometimes weather conditions are so ideal for disease that control can be lost despite your best efforts. However, the tools we have available should provide good control if you are applying them properly.
Other Garden Concerns
Other than disease, there are the typical issues with slugs, leafrollers and some cutworms. Leafrollers are past the stage that they can be effectively controlled (Figure 1). Insecticides for both leafrollers and cutworms are best applied when they are small. Unfortunately, their damage is often very difficult to detect at this stage. Regular scouting is essential for early control of these pests.
Figure 1. Leafrollers in ginseng leaves are now too large and too protected to be controlled by fungicides. They will soon be pupating.
Frequent rains may have also leached away some nitrogen this year. However, leaching is usually not as severe as growers assume because rainfall rarely penetrates right through the root zone after about the middle of May. If there have been frequent rains with water ponding in the trenches, then it is more likely that leaching of nitrogen has occurred and reapplication of some nitrogen would be warranted.