Blueberry Cold Damage Information and Action Items Mar 16, 2017 | Written by Phil Brannen With the freeze of last evening, producers are questioning what actions they should take to prevent damage from Botrytis and/or Botryosphaeria. The answers are not that simple, and it will depend on the type of blueberry and the situation encountered. I will give some immediate actions, and then I will give later actions. Immediate Actions Not to belabor the obvious, but we just had cold temperatures (<28 F) in most parts of our blueberry belt; temperatures were much lower in the northern parts of the state. Where cold-damaged blooms/shoots are observed, Botrytis might be a real issue, since damaged blossoms and buds will provide infection courts for the spores. In addition, it has been warm enough prior to the recent cold events that Botrytis development and sporulation may be more prevalent than normal for this time of year. One more point. The optimum temperature for infection of Botrytis is 59-68 F, but the optimum for spore germination is actually 68 F and above. That means we will have perfect temperatures for infection within the next few days, as temperatures around 70 F should be optimal for Botrytis; weather predictions for Alma and Homerville are indicating that highs will be ~70 F by Friday. Rainfall is not currently predicted, but if rainfall is heavy and prolonged dews (or overhead irrigation) are associated with the warm temperatures, we may have Botrytis/Botryosphaeria development. Where freeze damage has occurred, I would recommend the following: Rabbiteyes (fully damaged). My understanding is that damage is often severe and producers may simply want to maintain bushes for next year at this time. Rabbiteyes are generally less likely to have major Botryosphaeria issues, and I think the same is likely true for Botrytis as well. I would consider an application of Captan (activity against Botrytis), but that might be it. Producers will need to maintain leaves, so rust management or control of other leaf spots should be part of the plan going forward, but I would use cheap materials for leaf spot management. Rabbiteyes (in bloom). If maintaining bloom on rabbiteyes, I would recommend using Indar or Orbit or Quash or Proline for control of mummy berry (still out there), and I would suggest that one use either Captan, or Captan + Elevate or CaptEvate (Captan + Elevate comix product) tank- mixed with these DMIs. I would then follow suit with a regular spray program for the remainder of the season and going forward. Rabbiteye and southern highbush (fruiting). In this case, I would be more aggressive, and I would recommend an immediate application of Switch + Captan or Abound + Captan or Pristine + Captan. I would then follow suit with a regular spray program for the remainder of the season and going forward. Also, there are always questions regarding the tremendous amount of bark/ground wetting that occurs with overhead freeze protection. I am hopeful that this will not increase root rot diseases substantially at this time, but the root zones are likely saturated. Ridomil application through drip tape would work. If there is enough foliage for good uptake and activity of phosphonate-type products (ProPhyt or Agri-Fos or Kphite for example), I would consider foliar application of one of these materials to stave off root rots during the early spring; follow label directions and do not over-concentrate these materials in the final spray volume, as damage can occur with their use if label directions are not carefully observed. One more point, if you do encounter Botrytis, Guido Schnabel (Clemson University) will be willing to test these samples for resistance to our current fungicides. He has found that resistance is widespread in strawberries, so due to the fact that we have sprayed the same materials for years, this could be a problem for blueberry Botrytis populations as well. Samples would need to go directly go Guido Schnabel if we are to have this checked. Please let producers know that this can be accomplished this year if they are interested. Secondary Actions I have modified this from a previous post. The following is 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 myself relative 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 I 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. For rabbiteyes, the damage may be really severe as well. 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). This is particularly important for southern highbush varieties, but rabbiteyes can also experience a similar issue. 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; 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 to prune, the more disease you are likely to observe. I do generally recommend a fungicide application after each day of pruning to prevent additional infections of pruning cuts. None of the fungicides are exceptional relative control of Botryosphaeria, but Pristine and Abound would be good choices. In other systems (e.g. grape), DMI fungicides also help, but more data is available relative the strobilurins/QoIs. 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 I would 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. Reference Infection of Cold-Injured Blueberry Stems by Botryosphaeria dothidea. W. O. Cline, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 78:1010. Accepted for publication 20 June 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-1010A. I hope these suggestions will be helpful, and I wish each of you the best as you adjust to this difficult situation.