The hot conditions over the past two weeks can obviously stress ginseng. Each hot day likely reduces yield as the plant virtually shuts down in the heat of the day. A University of Guelph study in the past showed that heat impacts the yields of cool-season crops like ginseng. The study was done on cabbage and showed that cabbage yield decreased by 1% for every day the air temperature went above 30 °C. Cabbage is regularly irrigated, so these temperature effects cannot be easily reduced by ensuring adequate soil moisture. As a multi-year crop, the yield effects of any single hot day would be less in ginseng but still significant.

Pest Updates

Adult leaf folders are flying now, which means eggs are likely being laid. Over the next two weeks, you may start to see the window-like feeding damage of the new generation of caterpillars (Figure 1). These will feed for a few weeks before overwintering in the soil or straw.

Figure 1. Window-like feeding damage from newly hatched leaf folder caterpillars.

Disease issues have been relatively normal for this time of year, although expect Alternaria damage to increase if hot and humid weather continues. Heavy rains in some areas yesterday may also increase Phytophthora risk.

Verticillium symptoms often show up in July after hot conditions. Look for discolouration and wilting of isolated plants. If you dig up the root and cut across it, you can easily see a ring of brown around the core of the root as the fungus is blocking the vascular system (Figure 2) if Verticillium is the cause. Verticillium often occurs in fields with a history of susceptible crops, such as potato, eggplant, or watermelon. There are no controls for Verticillium in ginseng, but it rarely leads to economic damage.

Figure 2. A discoloured ring in the centre of a ginseng root due to Verticillium wilt.

Diagnosing Unusual Foliar Symptoms

This is the time of year that both common and unusual symptoms begin to show up in the leaves and stems. If the cause is one of our common diseases, it is relatively easy to confirm. The only foliar diseases known to affect ginseng in the field in Ontario are Alternaria, Botrytis and Phytophthora. Most growers are very familiar with the symptoms of these diseases. Other, less common symptoms can be much harder to diagnose.

The foliar symptoms of ginseng can be divided into 5 main categories:

1. Wilting
2. Leaf spots
3. Chewing damage/holes
4. Edge and tip dieback
5. Discolouration

The potential causes can also be divided into 5 main categories

1. Weather: These are the effects of heat, wind, frost and rainfall. They are more likely to cause wilting (drought stress), edge and tip dieback (heat or windburn), or discolouration (yellowing due to nutrient and nutrient uptake impacts), but on a wider scale (across the whole field, along a whole field edge, or in an obvious low or high area)
2. Root disease: This includes mainly Phytophthora, Cylindrocarpon, nematodes and Pythium, but also more obscure diseases like Verticillium and Rhexocercosporidium. These mostly cause wilting and discolouration of the tops, and when these symptoms occur, it is important to examine the roots and stems for any issues. Root disease usually leads to small to broad patches of damage in a field or damage to isolated plants (e.g., verticillium).
3. Abiotic: This would include the effects of excess or insufficient fertilizer, pH issues, drip-line chlorosis, herbicide drift or carryover from the previous crop, and abiotic rusty root, which is likely due to uneven supply of water or nutrients during peak growth periods. Symptoms of abiotic issues are often widespread in the field (except in the case of herbicide drift), although symptoms may be worse in drip-lines from the shade, near irrigation emitters, or may differ across a bed.
4. Pests: This includes insects, vertebrates (e.g. rodents) or invertebrates (e.g. slugs). These pests are more likely to lead to obvious damage like chewing/holes and spotting, but stem damage from pests can lead to wilting or discolouration in some cases. Pests are often very patchy in the field compared to damage from other causes.
5. Spray damage: This is the direct damage caused by either incompatible pesticide mixtures, improper pesticide rates, spraying under the wrong weather conditions, or other spray errors (e.g. improper sprayer calibration, nozzle issues, herbicide residues in the tank, etc.). Spray damage most often leads to spotting, discolouration, or, in rare cases, complete defoliation. The damage is likely uniform across a field or in an obvious pattern (e.g., a strip down one bed, where the sprayer began in a field, etc.). The most common cause of spray damage is spraying when it is too hot. As a general rule, do not spray if the air temperature is above 25 °C. Last week, the low temperature on a few of those nights was around 24 °C, which means there was likely no time of day that was safe to spray.

To diagnose a new issue, examine the following to narrow down the possibilities:

1. The extent of the damage. If it is only one or two isolated plants, it is worth keeping an eye on, but probably not worth the effort to diagnose the cause.
2. The pattern in the field (as discussed above)
3. The location in the field (low areas, high areas, field edges, drip lines, etc.)
4. The spray and crop history of affected parts of the field compared to unaffected parts
5. The roots
6. The soil (i.e., take a soil nutrient test)
7. The nutrient status of the plant (i.e., take a leaf tissue nutrient sample)
8. Consult the Ginseng Module of Ontario CropIPM, which includes photos of symptoms of both common and some obscure diseases, insects and disorders.

If you still cannot figure out a cause, please contact me or the OGGA, and we may be able to help diagnose the issue or suggest the next course of action.