Following recent freezes in the Georgia blueberry production region, the threat exists for subsequent damage from fungal pathogens. On tissues already damaged by freezes, two fungal pathogens of particular concern are Botrytis and Botryosphaeria. In addition, root zone saturation from overhead freeze protection/subsequent rainfall can lead to issues with Phytophthora root rot.
The fungus Botrytis cinerea causes Botrytis flower blight and fruit rot on blueberry. Cold-damaged blooms, shoots, and fruit can readily be infected by Botrytis spores. The optimum temperature for Botrytis infection is 59-68°F, and the optimum for spore germination is 68°F and above. Warm temperatures above 70°F, coupled with moisture (rainfall, prolonged dews, or overhead irrigation), provide ideal conditions for Botrytis development.
Based upon our recent temperatures and expected temperature/moisture conditions in the coming days, fungicide applications targeting Botrytis are recommended where freeze damage has occurred. There are several excellent Botrytis materials on the market: Pristine, Switch, Elevate, and CaptEvate. Captan has Botrytis activity, but it is not as efficacious as these newer products. Since Pristine also provides mummy berry and rot control, it is often recommended for the first applications. Switch is also excellent, and Botrytis resistance is not likely with one of the components in this fungicide. Apply no more than two applications of Pristine before switching to another product with a different mode of action – any of the others. Removing dead/damaged tissue through pruning, where feasible, can also reduce Botrytis issues.
In wet field conditions, aerial applications may be necessary to apply fungicides in a timely fashion. As always, follow all label directions. Of the materials recommended for Botrytis control, Pristine, Switch, and some formulations of Captan are labelled for aerial application. CaptEvate, Elevate, and other formulations of Captan are not labelled for aerial application. See the various labels for specific instructions, rates, other recommendations, and precautions.
Though freeze damage is immediate, there is a secondary danger of significant infection and disease development by Botryosphaeria fungi. Botryosphaeria fungi can move into and invade cold-injured blueberry shoots, where they can eventually cause stem blight symptoms. These symptoms consist of a dieback that moves down canes to the crown. Injured stems are colonized early, and disease incidence increases with time and temperature. This issue is of particular concern for southern highbush varieties, but rabbiteyes can also experience a similar issue. Some fungicide applications made for Botrytis control (e.g. Pristine) may suppress initial Botryosphaeria infections, but producers should watch plants carefully for Botryosphaeria-related stem diebacks in the spring and summer. Pushing plants with excessive nitrogen in the spring may also further exacerbate the Botryosphaeria situation. Pruning out dead tissue earlier, rather than later, has been shown to reduce disease, and removing cold-injured stems on young bushes may be worth a special effort. Where feasible, it is recommended that cold-injured shoots higher up on older canes be pruned back to healthy green tissue. Application of a broad-spectrum fungicide, such as Pristine, after each day of pruning is further recommended to prevent additional infections of pruning cuts. None of the fungicides are exceptional for control of Botryosphaeria, and crown infections are difficult or next to impossible to address.
With overhead freeze protection comes a tremendous amount of bark/ground wetting. This can lead to saturation of the root zones, potentially creating an environment amenable for root rot development. Though there is no certainty that root rot will become a problem, chemical control measures may be warranted in these situations. For Phytophthora root rot, there are two groups of products for use in blueberries – mefenoxam and the phosphonates. Ridomil Gold SL (mefenoxam) contains a systemic compound, and root uptake is critical to activity. This material is applied for root uptake; see the label for application instructions. If there is enough foliage for good uptake and activity, then phosphonate-type products (ProPhyt, Agri-Fos, K-Phite, etc.) can be used to help stave off root rots during the early spring. When foliar applied, these phosphonate products have systemic activity, and this systemic activity will suppress root rot. 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.