Rainfall has been highly variable over the past few days. Based on CoCoRaHS observations (https://maps.cocorahs.org/) and local rain gauges, rainfall ranged from 30+ mm from a line north of about Langton to Simcoe to 0 mm closer to Lake Erie, including Port Rowan to Port Dover. Rainfall was also lower in the Strathroy area, but this area had higher rainfall on the weekend. Monitor soil moisture closely if you do not have rain gauges and/or soil moisture monitors in each of your field locations, as irrigation may be necessary in some fields.

Phytophthora

In the last heavy rainfall on May 25, which resulted in around 48 straight hours of leaf wetness, Phytophthora apparently began infecting leaves in some fields (Figure 1). In the very sunny weather that followed, the lesions dried up, and the disease did not progress. It is likely that the pathogen is no longer active in these dried-up lesions, but it is probably still active in the soil.  With the return of rainfall, the disease may become active again. It usually takes two consecutive periods of saturated soils or prolonged leaf wetness for outbreaks to occur. In the first period, the pathogen becomes active, sporulates, and may infect nearby tissues. In the second period, the disease severity can explode as the pathogen multiplies. While fungicides are important for disease control whenever it is wet, they are most critical in the first period to prevent initial infection. Once there is a major spreading event, it is much more difficult to control the disease.

Figure 1. A Phytophthora leaf blight lesion that dried up after the initial infection.

Leaf Folders

Leaf folders are in the late instar stages and will soon start pupating within their leaf folds. This is often when the damage is most obvious because more leaves are pulled into their leaf folds. However, it is probably too late at this stage to get them under control.  Adults will start flying in late June or early July and eggs will be laid on the underside of ginseng leaves. The new generation of caterpillars will emerge by mid- to late-July. In areas of the field with significant damage now, insecticides can be applied once the new caterpillars emerge. However, based on observations last year, the new generation is often very staggered in emergence, with eggs hatching over a 6-week period. Multiple applications over that period to get good control. While there are no insecticides registered for this pest in ginseng, insecticides applied for leafroller or cutworm control likely provide control of leaf folders if applied at the right stage.

Scouting is critical to determine if sprays are economically warranted. Some natural control is provided by parasitic wasps. If you open up leaf folds over the next few weeks, you may see small, silky white pupae next to the leaf folder caterpillar. These are the pupae of parasitic wasps that developed within the leaf folder caterpillar, killing the caterpillar. Some insecticides will also kill these beneficial wasps, so it is important to only use insecticides when necessary.  

The Cumulative Impact of Plant Stress

Did you have lower-than-expected yields last year that could only be attributed to “heat stress”? It may be that you were dealing with a complex of issues that reduced yield, rather than heat alone. We often think of each disease and disorder of ginseng as separate issues, but they can often work together to cause more damage than any individual issue. Our research over the past few years is starting to link some of these issues together. For example, we now know that Cylindrocarpon (now called Ilyonectria) is frequently found in healthy roots and appears to wait for a trigger to cause disease. That trigger could be physical damage to the roots or another plant stress, such as herbicide injury. Likewise, an existing issue, like a herbicide injury or frost damage, can lead to increased heat stress symptoms. Any of these stresses will increase the risk of Alternaria leaf blight.

Excess fertility may lead to root dieback in dry years as salts accumulate in the root zone. This can occur when growers attempt to control the yellowing caused by excess rain falling off the shade (drip-line chlorosis) through additional fertilizer applications, which likely leads to high concentrations of these fertilizers in the areas between the drip lines. These fertilizers can cause moisture in the roots to be pulled into the soil by diffusion, causing the root tips to die back. As a result, the leaves may not get enough water in the heat of the day and would close their stomata (leaf pores that allow for air exchange) to prevent further moisture loss, even if the soil moisture levels are adequate. When the stomata are closed, very little photosynthesis can occur, and root growth stops until cooler conditions return. These wounded root tips could be more prone to Pythium infection, leading to further root dieback.

The goal of ginseng growers should be to minimize the stresses that could lead to further issues. While each issue on its own may appear to have a minor impact, it may lead to further issues in the future. Some key stresses that the growers have some control over include:

1. Reducing glyphosate injury in the spring. In recent years, glyphosate injury has increased, which leads to leaf distortion and cupping (Figure 2). This occurs when glyphosate is either applied too late in the spring when the ginseng tops are already emerging into the straw and can be directly contacted by the spray, or when it is applied to wet straw in the fall or spring and gets absorbed into the straw. In that case, the glyphosate can stay active for an extended period, and the plants can get a sub-lethal dose when they grow through the straw.
2. Re-thinking fertilizer application timing, source, amount and frequency. Given the high value of ginseng, there is a temptation to over-fertilize as insurance, so there is sufficient fertility even under extremely wet conditions. This fertilizer-as-insurance approach only works under the assumption that there are no negative consequences of the high fertilizer rates. As mentioned above, this may not be the case. Excess fertilizer in dry years may lead to increased heat stress and lower yields. Consider adjusting fertilizer rates and timing based on weather conditions (i.e., less nitrogen in dry conditions). Use lower rates of controlled-release fertilizers rather than high rates of conventional products to ensure a consistent supply of nutrients while reducing the amount in the soil at any one time. Use products with a lower Salt Index. Apply P and K only when a soil test indicates there is a need. The Ontario recommendation tables for P and K are provided in Tables 3 and 4.
3. Ensuring good ventilation during hot periods to allow heat to escape.
4. Installing soil moisture monitors to ensure the crop is irrigated at the right time, and before any moisture stress begins.

Figure 2. Leaf distortion and cupping due to glyphosate injury during emergence.

Table 1. Phosphorus Requirements for Ginseng on Mineral Soils. HR = High Response, MR = Medium Response, LR = Low Response, RR = Rare Response, NR = No Response

Soil Phosphorus (P) (0.5 M sodium bicarbonate) mg P/L of soil (ppm P)	Phosphate (P2O5) Required kg/ha: New Gardens	Phosphate (P2O5) Required kg/ha: Established Gardens
0–3	270 HR	80 HR
4–5	260 HR	60 HR
6–7	250 HR	50 HR
8–9	240 HR	40 HR
10–12	230 HR	30 MR
13–15	220 HR	20 MR
16–20	200 HR	0 LR
21–25	170 HR	0 LR
26–30	140 MR	0 RR
31–40	110 MR	0 RR
41–50	80 MR	0 RR
51–60	50 LR	0 RR
61–80	0 RR	0 NR
80 +	0 RR	0 NR

Table 2: Potassium Requirements for Ginseng on Mineral Soils. HR = High Response, MR = Medium Response, LR = Low Response, RR = Rare Response, NR = No Response

Soil Potassium (K) (normal ammonium acetate) mg K/L of soil (ppm K)	Potash (K2O) Required kg/ha
0–15	230 HR
16–30	220 HR
31–45	200 HR
46–60	180 HR
61–80	140 HR
81–100	100 HR
101–120	70 MR
121–150	50 MR
151–180	40 MR
181–210	0 LR
211–250	0 RR
250 +	0 NR

For more information on fertilizer sources, salt index, and optimizing fertilizer application, consult OMAFA Publication 611 – Soil Fertility Handbook.