Watch for Botrytis, Anthracnose, Phytophthora, and Stem Blight in Wake of Freeze
This winter’s freeze event of January 31 – February 2, 2026 caused significant damage across the state to open blooms, developing fruit, and plants, creating opportunities for disease to develop. The applicable diseases include botrytis, anthracnose fruit rot, Phytophthora root rot, and Botryosphaeria stem blight. The first three will be briefly addressed, while the focus will primarily be on stem blight. The below article was initially published days after the freeze events and has been updated with seasonal progress to date.
Botrytis and Anthracnose
Grower estimates of direct fruit losses varied widely from 10 to near 100% on farms across the state. The most immediate threat to the remaining crop was botrytis blossom blight and gray mold caused by Botrytis cinerea. Damaged blueberry buds and floral tissues are more susceptible to infection after low temperature injury but fortunately, dry conditions limited secondary losses reported. Some occurrences of gray felty sporulation were observed on attached, brown petals (corollas) in the days following freeze damage. During wet humid conditions, these spores can multiply and spread to other flower clusters and eventually onto green fruit. Where floral buds and green fruit survived, and where fungicide applications included captan to target blossom blight growers have reported good results. Some growers reported a lack of availability of botrytis products including Elevate and Captevate, even though those products still show active labels.
As this is being written, I’ve had a handful of pictures of cold-injured fruit with symptoms of anthracnose ripe rot, but fewer than I expected, probably thanks to the dry mild conditions across the state. I think it’s still likely that we are going to see increased anthracnose ripe rot pressure in the early season in central and north central Florida. Switch and Omega are two good products to include in the rotation to help prevent anthracnose ripe rot and other fruit rots. Applications made after the threat of freeze expired seem to have limited widespread, severe losses. Most growers have a normal preharvest spray routine that coupled with the Blueberry Advisory System, should continue to help reduce potential losses. More info on anthracnose management can be found at https://edis.ifas.ufl.edu/publication/PP337.
Root Rot Disease
For root rot diseases such as Phytophthora, we recommended that if growers had already applied Ridomil, Orondis, or another mefenoxam fungicide in the weeks preceding the freeze, their roots should have been protected from the limited Phytophthora infection threat during the periods of extended soil saturation associated with freeze protection. Neither Phytophthora nor Ralstonia were likely to infect new plants since temperatures were below 50°F, and I didn’t hear of any severe outbreaks of those in the weeks following. It’s still likely that plants in areas of fields where these are known to be active may be less resilient to the stresses of finishing the remaining crop and may show increased mortality compared to otherwise healthier plants moving forward.
If mefenoxam was not used in the past few weeks, consider waiting until after harvest and the hedge to make a banded app to beds or through drip irrigation. This would correspond to increased daily rain chances and a new flush of canes and roots. For foliar sprays of phytes, plants need healthy leaves and active growth to absorb these products for effective Phytophthora root rot management. Where phyte apps were planned and where healthy leaves remain (evergreen production) these apps can still be made as long as all the PHI and REI windows are observed for the products selected. Be extra cautious with green fruit on the bush to prevent quality issues or further stress.
Botryosphaeria Stem Blight
Botryosphaeria stem blight is the most common and damaging fungal vascular disease on southern highbush blueberry in the southern US, causing stem and cane dieback and reductions in yield. Advanced stages of this disease may cause premature plant death, which results in significant replanting costs for growers. Biotic or abiotic stresses from a variety of sources can make plants more susceptible to infection by stem blight pathogens.
External symptoms of stem blight on individual canes occur when the leaves no longer get enough water. Wilt and yellowing are quickly followed by reddening and death of attached leaves that take on a dry appearance. Although the fungus infected the plant weeks or even months earlier, the wilting process occurs rapidly, with leaves remaining on the dead canes. This rapid blight of one or a few canes on plants is sometimes referred to as “flagging.” The flagging of dead canes on a few scattered plants down a row is often the first stem blight symptom identified by growers.
To confirm stem blight as the cause of cane death, growers can look for the characteristic internal wood discoloration symptoms in cross sections of the cane (Figure 1). The pecan-brown discoloration is typically observed in pie-piece shapes where the fungus is in the xylem (Figure 2). Eventually, the pathogen colonizes crown tissue, spreading into multiple canes and major roots, and the plant will die.
Figure 1. Stem blight internal discoloration.
Credit: P. Harmon, UF/IFAS
Figure 2. Stem blight internal discoloration
Credit: P. Harmon, UF/IFAS
Stem blight poses a significant disease threat post freeze injury. Increases in stem blight have been noted throughout the years following cold injury, partially due to cane breakage caused by heavy ice loads building on plants during freeze protection. Plants that have fallen over due to ice load and/or wind should be repositioned and supported upright as soon as possible. If roots are exposed, they should be covered with soil/bark to prevent desiccation and additional stress. Damaged buds and canes can be colonized by the stem blight pathogens throughout the spring and summer. Infections can move into the crowns of some plants, or access the crown directly through breakage, killing them later in the year or even in the following seasons. Pruning out damaged canes has been shown to reduce stem blight severity in research conducted in North Carolina. Consider applying a fungicide such as captan immediately following pruning events to help protect pruning wounds from becoming infected. This would apply to hedging after crop loss as well as normal post-harvest pruning.
When plants succumb to stem blight, remove as much of the dead plant as possible from the field prior to replanting. Some growers may want to cut off the old plants at the base and till the remaining crown and root tissue into the bed before replanting. This practice is not recommended. Plants to be replaced should be entirely removed, including the roots, and then till in what little remains before forming the beds. The removed plant material should be burned, composted, or buried. Do not leave cuttings and dead plants in the row middles for extended periods, chip dead plants for use as mulch, or create dead plant piles upwind of production fields.
Another key practice to help minimize stem blight is good irrigation management. Drought stress is strongly associated with stem blight. Additional information on stem blight can be found at https://ask.ifas.ufl.edu/publication/PP347.
Increased stem blight risks will persist though this season and next, so growers should continue to be vigilant in scouting, rogueing, and managing stress.
Early Pruning
Growers (and some insurance adjusters) were tempted to wait and see if some of the small amount of remaining fruit could be salvaged from this crop cycle before hedging plants back, where nearly complete crop loss occurred. However, for plants that incurred severe freeze injury, this was a judgement call growers had to make to try to avoid increased disease severity and additional plant losses. Once these blooms and fruit have been killed and only a small amount of less developed bloom remained, our suggestion was for focus to be on preserving plant health for next season’s potential production. Any small amount of fruit that was allowed to remain and develop on severely damaged bushes could have put additional carbohydrate stress on the plants. Also in this situation, from an economic perspective. There were many economic considerations around price, labor availability, etc. that had to be carefully considered as well. The IFAS Blueberry team made a concerted effort to help growers and allied partners understand the risks and benefits of these decisions.
Growers deciding if and when to hedge a crop should be aware that pruning too early may have resulted in the development of new floral buds that would not be economically viable, may delay the plant’s recovery from the freeze damage, and could require a second pruning to remove unwanted reproductive growth. Consider consulting with UF/IFAS Extension and Horticultural Sciences faculty for additional input in this regard.
References
https://content.ces.ncsu.edu/blueberry-freeze-damage-and-protection-measures
Phil Brannen, UGA blog article on response to freeze injury in blueberry from 2017 with stem blight reference. https://site.caes.uga.edu/blueberry/2017/03/blueberry-cold-damage-information-and-action-items/
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.
CREDITS:
DR. PHIL HARMON, Professor, Plant Pathology, UF/IFAS
& DOUG PHILLIPS, Blueberry Extension Coordinator, UF/IFAS
