The following is largely a synopsis of information previously provided by Gerard Krewer (UGA Professor Emeritus and consultant), Bill Cline (NC State), Dave Lockwood (Univ. of TN), Danny Stanaland (retired county agent and consultant) and Phil Brannon regarding a suggested response to cold damage on blueberries and the potential threat of Botryosphaeria canker development on damaged tissues. The extent of the damage to southern highbush blueberries may be variable throughout the state this year, but we suspect it will be somewhat universal for blueberries grown anywhere along the same latitude as Georgia. Based on initial reports and opinions of field specialists, cold damage will likely have a significant negative impact on yield this year. Without regard, freezes may soon cause tissue damage to canes, and this will need to be addressed.
Though freeze damage is immediate, there is a secondary danger of significant infection and disease development by Botryosphaeria fungi. Botryosphaeria fungi may take a while to move into cold-injured blueberry shoots, but they will likely invade them eventually to cause stem blight symptoms (mainly dieback that moves down canes to the crown). Some fungicide applications made for Botrytis control (e.g. Pristine) may also suppress initial Botryosphaeria infections, but producers should watch plants carefully for Botryosphaeria-related stem diebacks in the spring and summer.
There will be an advantage to pruning out dead tissue in late February to mid-March; there is research-based information from North Carolina (Bill Cline) to back this up. Widespread infection by Botryosphaeria dothidea following cold injury has been reported. Injured stems are colonized early, disease incidence increases with time and temperature, and the later you wait past mid-March to prune, the more disease you are likely to observe. It is generally recommended to apply a fungicide after each day of pruning to prevent additional infections of pruning cuts. Also, do not push plants with excessive nitrogen this spring, as this might further exacerbate the situation with Botryosphaeria.
Dave Lockwood (Univ. of TN) has also advised growers to delay pruning until late winter/early spring so that they can feel relatively sure that the potential for additional cold injury is past. He also advises holding off until one can easily see, based on bud swell or early shoot growth, where the strong, new growth will originate. At that time, he advises pruning back to healthy wood. Bill Cline suggests that “it is worth a special effort to remove cold-injured stems, especially on young bushes. With cold-injured basal shoots (suckers that emerge from the crown), snap them off by hand at the crown, since the brown pith often goes all the way to the crown. In controlled experiments this significantly reduced disease incidence. For cold-injured shoots higher up on older canes, prune them back to healthy green tissue.”
When pruning, producers should review the weather forecast, and attempt pruning when 3-4 days of dry weather (no overhead frost protection or irrigation as well) are predicted to follow. This will also help to reduce infections on new pruning cuts, and again, we need to consider use of fungicides after each day of pruning to prevent yet more infections.
Fungicide Applications
Should you apply a fungicide ahead of or following potential cold damage? 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 at some point, and temperatures around 70 F with moisture are generally optimal for Botrytis.
It is really a judgement call as to whether a fungicide should be applied with this freeze damage event, but if A grower wishes to apply anything when it does warm up a little, Captan or Ziram would be the best options 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 Botryosphaeria infections.
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 not likely.