Heavy rains last week have increased root rot pressures in most of the ginseng growing area. Phytophthora is the biggest issue under these conditions. Leaves at this stage of the season are less succulent and the risk of foliar Phytophthora is reduced when this occurs. However, leaves can still be affected under very humid and wet conditions. At this time of year the stems are more woody and unlikely to show the same symptoms of collapsed leaflets as occur in May. Look for water-soaked sections of the leaves, rather than whole tops drooping.
There have been several reports of Pythium root rot in the past two weeks. Pythium appears as swollen and brown root tips, a proliferation of lateral roots, and a general weakening of the plant. Pythium is favoured by warm and moist but not saturated soil. It is likely that the hot conditions the past two years have increased Pythium activity, and the moist conditions this spring has increased root damage. Secondary fungi and bacteria may also be rotting previously damaged tissues. Both Pythium and Phytophthora are water moulds (oomycetes). Pest control products and strategies that reduce Phytophthora root rot will also have some effect on Pythium. Maestro 80DF is the only in-season product registered for the control of Pythium root rot in ginseng.
It is important to remember the role fertility plays in helping plants defend against pathogens. Under-fertilized plants are weakened and may not have the necessary nutrients to produce the chemicals and structures involved in plant defense. Heavy rains can leach nitrogen in particular out of the soil, leaving leaves pale green and weakening plants. This is likely the primary cause of drip-line chlorosis. Diseases like Alternaria and Botrytis, which are generally weak pathogens (despite what the evidence shows in ginseng), can be favoured by lower fertility. This is partly the reason Alternaria pressures are higher under the drip-lines (leaf wetness also plays a role). However, excess fertility can lead to succulent leaves, stems and roots with thin cell walls. These structures are easier for pathogens to penetrate. The recommended nitrogen application rate for ginseng is 40 kg/ha applied every year of production. After heavy rains, growers may need to exceed this rate, but 10 to 20 kg/ha additional nitrogen may be sufficient to replace the nitrogen lost to leaching.
Damage from root lesion nematode is also being reported in seedling gardens. Root lesion nematode damage appears as constrictions along the taproot with a rusty netting. Research over the past two years has led to the conclusion that root lesion nematodes damage the root in the seedling year, when they have nothing else to feed on, and then die off. We received further evidence of that from a replant research site this year. Ginseng had been harvested off the site last year. Nematode counts this spring before any treatments were applied showed no root lesion nematodes in 27 of 28 samples, and very low levels in the final sample. It is rare to find a coarse sandy soil in Norfolk County that has no root lesion nematodes. It is unknown what happens to these roots after the nematodes die off, but research results this summer will provide some answers to that question.
Regardless of when the damage occurs, it will be important to reduce root lesion nematode populations before planting ginseng. Cover crops may need to be used prior to fumigation to reduce populations, since fumigation itself is rarely 100% effective. The Midwest Cover Crop Council has produced a Cover Crop Decision Tool that can help growers identify the cover crops that best fit their goals. The tool is available at http://www.mccc.msu.edu/selectorINTRO.html. The field crops decision tool is complete. Growers can enter their county and up to three goals for their cover crop program and the decision tool will provide suggestions that could meet those goals. Nematode control is not one of the options for goals, but information is provided in the information sheets for each cover crop on its ability to control nematodes. For ginseng growers, the primary goals could include soil building (organic matter), quick growth if grown in the seeding year, and weed fighter. Once the goals are entered, an information sheet is available on each cover crop with further details.