Over the past week there have been confirmations of both foliar and root Phytophthora, Alternaria on stems and leaves, active grub feeding, leafrollers, and slugs. Many of these issues are not too surprising for this time of year. Foliar Phytophthora is the most concerning of these considering the rapid spread of the disease this can cause. Foliar Phytophthora was found in an area with no root infection, which suggests it was spread on the wind at least a week ago. This can be a source of new root infections in areas that you wouldn’t normally expect disease. Drier weather will limit further spread of the disease, but protection is necessary to stop disease development where it is already active.
Alternaria problems will increase with the hotter and more humid weather if the crop is not adequately protected, especially if live lesions are already present in the garden. Rhizoctonia may also be an issue in seedlings, since this disease has been found in similar crops this week and weather conditions this winter were ideal for spread of the disease. However, this is more likely if fungicides for its control were not timed properly, did not get down into the root zone due to last year’s dry weather, or were washed away by heavy rains.
Grubs have been found actively feeding in gardens. It appears that grubs can be an issue under certain weather conditions that promote egg-laying in empty fields between fumigation and seeding. Egg-laying usually occurs when adults emerge in late June but it can be earlier or later than this depending on the weather. Those fields fumigated in late June or July may have no issues with grubs because fumigation would likely provide good control. Due to the dry weather last year, adults that usually seek out green grass for egg laying may have had no good options and laid eggs in the field instead. These eggs would hatch after a few weeks and the grubs would feed on weeds or the volunteer rye or wheat that grows after seeding. They would survive the winter as grubs and feed on the ginseng seedlings in the spring when there are no other options left in the field. They are unlikely to lay eggs in older gardens, and damage to older gardens has not been seen.
Grubs feed near the crown of the seedling and eat both the roots and the stem. As they feed they slowly pull down the stem through the straw until the leaves are sucked into the straw (Fig. 1). They then move around until they find the next root. When they first start to feed, the top slightly wilts before it is pulled into the straw (Fig. 2). Eventually you will see a circle patch roughly 30 cm in diameter with some dried dead tops, a few fresh tops pulled into the straw, and perhaps one plant that is slightly wilted (Fig. 3). The best way to confirm their presence is to dig straight down from the seedlings that are just slightly wilted or recently pulled into the straw and you will find the grub just below the soil surface (Fig. 4).
The species of grub that attacks ginseng appears to be European chafer, which is confirmed by looking at the raster (thick hair) pattern on the inside (leg side when curled into a “C”) of the rear end of the grub (Fig. 5). A dissecting microscope or strong hand lens would be required for this. The raster pattern of the European chafer resembles a zipper being closed. June beetle and Japanese beetle grubs could potentially cause damage to ginseng as well, but have not been confirmed. These pests would have different egg laying timings and this may change how they can be controlled. Click here to compare the raster patterns of these other species to those of European chafer.
Grubs can be controlled with an application of Admire/Alias insecticide at seeding. However, growers should start to look for alternative non-chemical control strategies over the next few years (e.g. later fumigation, keep fields clean between fumigation and seeding), since this insecticide is under re-evaluation by PMRA and has been proposed to be removed from the market. Many growers do not use this insecticide and report no problems with grubs. More research is required to determine when and where grubs will be a problem in ginseng so growers will know if control is necessary. There are no insecticides registered for control of grubs once they are causing damage to seedlings.