The risk of all foliar diseases of ginseng is very high over the next few days. Growers should ensure protection from Alternaria, Botrytis and foliar Phytophthora. Minor frost injury has occurred in a few areas and cold temperatures may have weakened the young shoots. With the rainfall last night, more rain in the forecast, and temperatures forecast to soar over the next few days, conditions will be ideal for the development and spread of disease.
Alternaria levels were already high before this weekend’s cold conditions. Windy conditions would have allowed the spores to spread, and the rainfall and humid conditions can result in new infections. Stressed plant materials are more at risk of Alternaria infection. Conditions may allow for repeated infection, spread, and re-infection in ginseng gardens.
Botrytis usually begins on injured plant materials, such as frost damage or sandblasting caused by blowing sand this past weekend. Botrytis can develop and spread rapidly with the onset of warm and humid weather. Warm and humid weather a few days after a heavy frost in 2001 resulted in a severe Botrytis outbreak. Fortunately growers have new tools to help manage Botrytis now that were not available at that time. There are a number of pest control products registered on ginseng that control both Alternaria and Botrytis. Consult the 2013 Crop Protection Guide for Ginseng (Supplement to Publication 610) for a list of registered products.
Conditions were dry preceding last night’s rainfall, and as a result, foliar Phytophthora had not been reported. However, last night’s rainfall provided a good soaking and any heavy rain in thunderstorms over the next few days could greatly increase the risk of foliar Phytophthora developing. Maestro is now registered for control of Phytophthora and is the only registered product that will provide control of Phytophthora in already damaged tissues. Use Maestro in rotation with other Phytophthora products.