Horticultural Update: Freezing Temperatures in the Forecast
Many apple growers in the southeastern US are bracing for a cold front. Low temperatures in the forecast are near or below critical temperatures for apple for a four-day period. At several locations, two of the four days are predicted to have low temperatures in the mid to low 20’s. If the forecast holds true, this is a serious threat to the 2021 crop.
The current stage of bud development ranges widely based on variety and location. At higher elevations, most apple varieties are at a stage of bud development between green tip and pink. Lower elevation sites and early flowering varieties have advanced bud development, and some orchards are in bloom.
For your reference, Michigan State University compiled an illustrated reference with critical temperatures of major tree fruit crops (research done by Washington State University).
These critical temperatures only serve as a guide, your specific site/situation needs to be considered. Several factors determine the potential severity of blossom mortality, including site, location, bud development stage at time of freeze event, lowest temperature observed, the duration of low-temperature exposure, cultivar, cropping history, and tree health.
What Can be Done?
In this case, unfortunately very little. The cold weather that is predicted is an advective freeze, meaning that there is a large cold air mass moving in with winds. If high winds are observed, it is very unlikely that frost control measures like sprinklers and wind machines will help, because there is cold air throughout the atmosphere. Winds may simply increase the cold from evaporative cooling (and cause more damage). Wind machines may be ineffective during this event to a lack of an inversion layer, and will mix cold air with more cold air. High winds are expected on Thursday and Friday morning at some sites, but lower wind speeds are predicted Friday evening and Saturday morning (at present, subject to change of course).
Will Promalin/Perlan Work for Frost Rescue at This Stage?
The label for Promalin/Perlan only extends to the bloom period. Unless in the bloom period, you should not use this material for frost rescue. For many high elevation sites, this is not an option on most cultivars. Lower elevation sites with advanced bud development may have this as an option. Caution is advised.
Since frost/freeze events are sporadic/unpredictable, there has been limited research related to use of Promalin for frost rescue. Following a series of frost events during the bloom period McArtney et al. (2014) demonstrated an increase in fruit set and yield in some cultivars when using Promalin. While significant increases in yield were observed with some cultivars/locations with Promalin, this equated to approximately 25% of a full crop. For some operations, 25% of a crop would be an important source of revenue in a potentially difficult year. For others, this would result in an expensive and cosmetically flawed nuisance crop.
Promalin mimics chemical signals that are present in seeds, which can result in parthenocarpic (seedless) fruit development despite death of the embryo. Parthenocarpic fruit will have poor storage potential and may be misshapen. Subleathal injury to fruit (russet, frost rings, etc.) may be present. Your specific situation (site, temperature observed, market, crop insurance, etc.) should be considered before employing this rescue treatment. This is not “one size fits all”.
Those that are interested should refer to page 83 of the 2021 Integrated Orchard Management Guide for Commercial Apples in the Southeast for application information.
Assessing Cold Damage
Once the cold-front is behind us, the presence and severity of frost/freeze damage can be estimated by dissecting flower buds and assessing viability. Researchers at Penn State University developed a method for assessing fruit bud survival to help guide future management decisions. We will work with extension agents in the region to provide estimates of flower bud mortality and survival at representative locations.