Most ginseng growing areas have had heavy rain over the past week, with more in the forecast tonight. The risk of foliar Phytophthora is very high and will remain high. There are three products registered for control of Phytophthora on ginseng: Aliette, Ridomil and Maestro. Aliette helps the plant defend against Phytophthora and is systemic in its action, meaning that application to the foliage will provide protection of the roots. However, Aliette will not control infections already in place and does not provide complete preventative control. It also does not control Phytophthora in the soil. Ridomil as a granular formulation will provide protection of the roots, but will have no effect on foliar infections. Maestro will provide control of any Phytophthora that it contacts. However, control of root rot will depend on it being washed into the root zone. Timing application before a moderate rain will help move the product down the soil profile.
Heavy rains at this time of year can be used to identify wet areas in gardens to be planted this year. Growers may need to consider additional drainage in these areas, considering that additional heavy rainfalls are very likely over the life of a ginseng garden.
While the repeated rainfalls are affecting the application of fumigants, once applied, the moist soil will help to seal the fumigant in the soil if tarps are not used. Excessive rain shortly after fumigation could wash some of the active ingredients out of the soil, but light to moderate rainfall after fumigation will help to seal the fumigant into the soil.
The wet conditions will also increase the risk of Alternaria and Botrytis. Fungicide programs should target all three foliar diseases and be applied on the low end of the recommended spray interval (ie. 7 days if 7 to 14 days is recommended) until conditions become less conducive to disease development. Slugs may also build up under these conditions. Growers should also watch out for cutworms, which are being reported in high numbers in some other crops. Pythium is also favoured by moist conditions and has also been reported in ginseng gardens. Fungicides used to control Phytophthora will also have some effect on Pythium.
Heavy rains can also leach nitrogen out of the root zone, because nitrogen is highly mobile in the soil. The recommended nitrogen rate for ginseng over the whole growing season is 40 kg/ha (35 lb/acre) based on research conducted by AAFC and OMAFRA. While ginseng gets most of its nitrogen from the break down of organic matter from manure in the soil, a small application (e.g. 10-20 kg/ha) after a heavy rainfall will help to recover the available nitrogen forms. The other macronutrients, phosphorus and potassium are bound to soil and organic matter particles, and will not leach out of the soil in heavy rains. Thus, preplant adjustment of phosphorus and potassium levels is extremely important for these and other immobile nutrients.