Although some cooler weather has temporarily arrived, we are currently experiencing a hot stretch that may continue again next week. Many areas have also been very dry, with only 43 mm of rain having fallen at the Delhi Weather Station since May 23. Normal for this 2-month period would be around 140 mm. On the other hand, there are a few areas that have received heavy rains in thunderstorms and may have had above normal rainfall during the same period.

It appears that temperatures at the Environment Canada Delhi station continue to be influenced by the ginseng shade nearby. Based on comparisons with the Environment Canada London weather station over the past 10 years as opposed to this year, the ginseng shades seem to be adding on average about 1-1.5°C during the day to the nearby air temperatures and making it about 2°C colder at night. On windy or cloudy days there is no difference, but there is a large difference on calm and clear days or nights. Bottom line: Do not trust the Environment Canada temperature data for Delhi. It has been hot, but not as hot as the daytime weather records show. Based on average daily temperatures, we are on pace for the third warmest July of the past 15 years after 2020 and 2011.

Heat Stress, Foliar Disease and Spraying

While the Delhi data may not accurately reflect air temperatures, it does reflect what the ginseng crop experiences. It has long been known that temperatures in the ginseng canopy can be up to 6°C warmer than the ambient air temperature on calm, sunny days. This puts a lot of stress on the ginseng plants, including:

1. Making leaves close their stomata (pores used for air exchange) to preserve water, effectively shutting down photosynthesis until temperatures cool. This limits root growth during heat waves. Ensuring adequate soil moisture can relieve this somewhat, but not entirely. Raising side shades and adding vents in the shade at any hilltops can also reduce heat stress.
2. Causing direct damage to sections of the leaves as the hot temperatures and solar radiation heat the dried-out leaves to the point that the cells die (Figure 1). Damage appears as papery sections of the most exposed leaves, including along the midrib or at the leaf tip. Adequate irrigation can also reduce this damage.
3. Leading to Botrytis leaf blight as the damaged leaf tissues are invaded by the pathogen, which then spreads to healthy portions of the leaf from there (Figure 2).
4. Making plants more susceptible to Alternaria leaf blight, which can affect any plants under stress.

Figure 1. Various papery symptoms on leaves as a result of heat stress (with some Botrytis infection).

Figure 2. Botrytis blight progressing on heat-stressed leaves and a berry cluster.

The resulting foliar disease problems necessitate regular fungicide applications. Unfortunately, it is often a rule of thumb not to apply pesticides when temperatures exceed 25°C. This is because the spray droplets may evaporate before hitting the target, the droplets on the leaves may dry too rapidly to spread out on the leaf, and for certain products, the plant may take up more of the pesticide, leading to phytotoxicity and leaf damage. When you add in the additional heat found in ginseng gardens, this makes it difficult to find a time to spray during a heat wave because temperatures in the ginseng canopy during the day may exceed 25°C by 8 am and continue well above that until after midnight. It is important not to add to the stress of plants by spraying when it is too hot. It is often better to wait for the right conditions rather than add to the stressors in the garden. Fortunately, the shade also appears to cause nighttime temperatures to be cooler in the garden, so there is a window to spray very early in the morning on most days.

Leaf Folders

Based on our leaf folder monitoring, leaf folder caterpillars are no longer active, and adults began emerging a couple of weeks ago. There may still be a few pupae in the leaf folds. We are on the lookout now for a second generation that would start with egg laying. There are no records of what the eggs of this species look like, but we will monitor for eggs for a potential second generation. If we cannot identify the eggs, keep an eye out for new leaf folds at the tips of leaves, and let us know if you find any. We need to build a better picture of the biology of this pest so we can control it better in the future.

Other Diseases

Rhizoctonia progression appears to have slowed in recent weeks, likely due to hotter and drier weather. It will be important to continue monitoring for this disease, because it may continue again once wetter and cooler weather returns.

Verticillium wilt is active in ginseng gardens. Verticillium is a fungal disease that invades the roots and then clogs the vascular system, reducing water and nutrient flow to the leaves. As a result, the first symptom is wilting and discolouration of the tops (Figure 3). The root and stem can appear healthy from the outside, but cutting across the root often reveals a darkened ring within the root (Figure 4). It often affects sporadic plants in the field. Fields with a recent history of solanaceous crops like potato and eggplant often have more issues with this disease. There is nothing that can be done to control Verticillium wilt in ginseng. Fungicides offer minimal control of Verticillium in other crops, and there are no products registered for Verticillium management in ginseng.

Figure 3. Discolouration of leaves due to Verticillium wilt.

Figure 4. A darkened ring within the ginseng taproot due to Verticillium wilt.

Foliar Fertilization in Heat

Just like with pesticides, foliar fertilizers become ineffective or even damaging if applied in hot weather. They may dry on the leaf surface too quickly to be taken up into the leaf when the air is hot or dry. Conversely, they are also not taken up if the air is near saturation. They are best applied when the relative humidity is 60-80%. Hot temperatures can also make everything more active if the humidity is at the right level. This can lead to excessive uptake of micronutrients, which can burn the leaves. Foliar fertilizers are not effective enough to take the risk of applying them in hot weather. Depending on the nutrient, consider granular application followed by overhead irrigation or rainfall, since roots are far more effective at nutrient uptake than the leaves. Mixing foliar fertilizers with fungicides is also riskier in hot temperatures due to the higher activity of both in the mix, which could lead to unusual interactions.