Varying amounts of damage from the past week’s freeze and snow events are being reported depending on the low temperatures in the area, how emerged the crop is, whether row covers were put on the crop, and how much snow was received. It appears there is low risk of additional damaging frost events, although a milder frost is in the forecast for tonight.
While the risk of disease was low over the past week due to the cold temperatures, much warmer temperatures are in the forecast starting Thursday. The dewpoints are going from a low of -14.5oC late Friday and -8oC Tuesday evening to about +14oC starting Thursday night. This is an incredible increase in the amount of moisture the air can hold, resulting in a much more conducive climate for disease development. This may be accompanied by significant rainfall both Thursday and Sunday. Given the damage to emerged plants and the forecast, the risk of Botrytis blight is extremely high (Figure 1). The risk of Alternaria and Phytophthora are always high in the spring after emergence, but not as high as the risk of Botrytis.
Many growers have row covers that have been on the crop for a week. During this time the crop could not be sprayed. Ideally the covers should come off as soon as possible and the crop sprayed before the rain in the forecast for Thursday. Any wounded tissues will need to dry up and heal somewhat before being sprayed to avoid phytotoxicity. If that is not possible, there may be another window of opportunity to spray late on Friday or on Saturday before the next round of rain. If applying products before the rain, choose any product with good efficacy against Botrytis as the first criteria, since the risk of this disease is the highest. There are many products that have good efficacy against both Botrytis and Alternaria. Separate products for Phytophthora should be applied according to your normal spring schedule.
If you are not able to spray the crop until after the rain, some limited infection may have occurred before the product was applied. In this case you will want to choose a product that has some locally systemic or penetrant activity to get inside the plant and control the fungus. See this article from Purdue for more information on fungicide mobility in the plant by active ingredient and fungicide group. Applying a contact fungicide like chlorothalonil or mancozeb after the fungus has already infected the plant will not help to control the disease that is already in the plant. Keep in mind that the plants will be rapidly expanding over the weekend and next week, resulting in fungicides being stretched out on the surface of the plant, leaving some areas unprotected. Some limited mobility in the plant will help distribute the product a bit more than purely contact fungicides. Re-application may be needed more frequently during rapid expansion.
Do not forget about other pests. Slug and cutworm activity will increase over the next week with warmer weather. The strange weather this spring may mean emergence of different pests occur at earlier or later stages of plant development than normal. Prolonged cool weather may allow some cold tolerant pests to progress, while others are dormant until the weather warms. Scout fields for all pests, even if they are not normally seen at this stage of ginseng emergence.