As I am sure growers are aware, there is a significant amount of rain in the forecast over the next few days. This added to already saturated ground could cause drainage issues and ponding in ginseng gardens. For many growers, this will be the most significant wet period in a few years and new growers may be less familiar with the issues that can arise under wet conditions.
The biggest risk in wet conditions is Phytophthora root rot and foliar blight. Phytophthora cactorum, the causal agent of the disease, has an optimal temperature for growth of 25oC, but can develop at temperatures as low as 2oC. While temperatures are not currently ideal for disease development, the prolonged period of rain will more than make up for the lower than optimal temperatures, resulting in a high risk of disease.
Phytophthora cactorum is a water mould (oomycete) and produces sporangia (spore sacs) (Figure 1) that contain swimming spores called zoospores. When the ground is saturated, these spores can swim through the soil or surface water to infect new roots. Any water containing zoospores that is splashed onto the leaves or stems of ginseng can infect these tissues. When the disease develops on above-ground tissues, the pathogen will produce more sporangia that can be spread by wind to new areas of the garden or to new fields many kilometres away. The new infected tissues can than produce more spores that can be washed down to the soil and cause a new area of root infection. Under ideal conditions, this process can result in widespread disease if the foliage is not protected with a fungicide.
Disease can also spread when infested water or soil is moved through the garden by machinery or workers. It is best to stay out of the garden when the ground is saturated. Adding straw, wood chips or gravel over continually saturated soil in trenches can reduce this spread.
Prolonged rain can also wash off any fungicides growers have applied for protection prior to the rain. When the rain stops, it will be important to apply another fungicide for Phytophthora. It should be kept in mind that mid-May seems to be the peak time for foliar Phytophthora infections based on previous years.
Phytophthora leaf blight causes individual leaflets, prongs or the entire top to droop straight down with a water-soaked appearance (Figure 2). Patches usually develop in low areas of the garden (Figure 3), and often spread down the rows as the garden ages do to movement of spores in surface water or on machinery (Figure 4).
Alternaria panax, the causal agent of Alternaria leaf and stem blight of ginseng, has an optimal temperature range of 15-25oC. While there may still be some development at these low temperatures and protection from Alternaria is still important, the rainfall at these low temperatures is unlikely to increase the risk above normal conditions for this time of year. However, protection from Alternaria will be needed before temperatures warm up because any protection applied prior to the rain will be washed off.
Botrytis cinerea, the causal agent of Botrytis blight of ginseng, has an optimal temperature around 18oC. Conidia (asexual spores) of the fungus germinate between 8 and 30oC. With temperatures hovering around 8oC and then gradually falling during this wet period, the risk of Botrytis will be low over this wet period. The risk will likely increase with the increasing temperatures next week, especially if there is any frost damage later in the weekend.
There is a risk of frost later this weekend, especially for Sunday night and Monday night. Forecasts for Delhi currently range from -2oC to 1oC for the low Sunday night and -3oC to 1oC for Monday night (as of Friday, May 5 at 10:00 am). Dewpoint forecasts, which give an indication of how low temperatures could get at the ground level under calm and clear conditions, go as low as -5oC. For comparison, dewpoints on May 23, 2015 ranged from -8oC to -12oC due to much drier conditions at the time. Continue to watch the low temperature, windspeed, cloud cover and dewpoint forecasts over the next few days. For a summary of the various forecasting sites and which ones provide relevant information for forecasting frost events click here.