Today is a good day to apply fungicides for Phytophthora and other diseases if they have not been applied in the last few days. Rainy periods over the past two weeks likely resulted in isolated foliar infections of Phytophthora. The extended rain forecast for this weekend could result in sporulation from these tissues and spread of the disease. Windy conditions are also in the forecast, which could result in spread of spores many kilometres from the source. If leaves are not protected, foliar disease could be widespread and spores can be washed into the soil to cause new root infections.
The heavy rains over the past few weeks along with the rain in the forecast could result in leaching of nitrogen out of the root zone, especially under the drip lines. Alternaria can be more of an issue on plant tissues with nutrient deficiencies. This would be in addition to the stress caused by frost damage to the stems. A nitrogen application may be necessary after the rains to ensure that at least the nutrient stress is reduced. Research has shown that ginseng is not a heavy feeder of nitrogen, so an application of 25 kg/ha or so of nitrogen should be sufficient to correct a deficiency unless additional heavy rains occur to leach more nitrogen. Excess nitrogen application can promote too much leaf growth and this can make plants more susceptible to other foliar diseases.
The forecast weather conditions are also ideal for the development of Botrytis. All botrytis needs to start new infections is newly damaged tissue and favourable weather. Spores of botrytis are ubiquitous (virtually everywhere) in the environment, so there is no way to get rid of inoculum from the fields. Plants continue to collapse from the stress of frost damage, so newly damaged tissue is still present. Fields should be scouted regularly to look for new infections. New infections of Botrytis are easy to identify because they usually contain grey fuzzy growth, especially in the mornings or in periods of high humidity. Older plants that have hardened stem and leaf tissues are less at risk of Botrytis because it is a relatively weak pathogen.