The below information is from Dr. Phil Brannen:
“I had a question concerning whether fungicide application(s) would be warranted ahead of or following potential cold damage over the next few days, especially cold damage to southern highbush. While cold damage can increase both Botrytis and Botryosphaeria infection and subsequent disease issues, it is hard to know whether there will be significant damage (happens mainly when plants have not gone dormant due to a warm fall). Also, fungicide application is not warranted with the current temperatures, since fungi will not readily infect when it is this cold. However, the optimum temperature for infection by Botrytis is 59-68 F, and the optimum for spore germination is actually 68 F and above. That means we will be above optimum temperatures for spore germination within the next few days, and temperatures around 70 F with moisture are generally optimal for Botrytis; weather predictions for Alma and Homerville are indicating that highs will be >70 F by Saturday in Homerville and Alma, with rain. It is really a judgement call as to whether a fungicide should be applied with this freeze damage event, but if I were to apply anything, I would go with Captan or Ziram immediately after the cold temperatures pass. These are relatively cheaper protectants, and if temperatures warm and we have moisture from dew or rainfall, they will help to prevent fungal infections. If significant cold-damage occurs this winter, producers should prune this out in the early spring to prevent mainly Botryosphaeria infections. I’m sorry to be so vague as to a recommendation at this time, but there is not a clear answer, and this situation has not been clearly researched. You can spray, and you will likely still have to prune out cold-damaged tissue, or you can not spray, and you will have to prune out cold-damaged tissue. The real question is whether a fungicide application in the next few days will reduce the amount of fungal-damaged tissue that comes in on top of the cold-damaged tissue, and the answer is maybe.”