Freeze Alert for April 20/21

Please note that the full phenology report will be provided tomorrow, along with damage estimates from the previous freeze event and other relevant data. This alert is issued in advance of the freeze expected Monday night into Tuesday morning.

These temperatures pose a serious risk to fruit crops. Apple, peach, and pear crops may experience varying levels of damage depending on developmental stage and site-specific conditions. Apples are at petal fall in Adams County and at full bloom in Center County.  Although fertilized flowers and young fruitlets are less sensitive to freezing temperatures than blossoms at full bloom, exposure to critical temperatures can still result in tissue injury, leading to embryo damage, fruitlet abortion, or subsequent quality defects. There is also potential for frost ring formation under the forecasted conditions.

Frost rings are a type of freeze injury observed on apples when young fruitlets are exposed to subfreezing temperatures shortly after bloom, typically between petal fall and the 3 to 10 mm fruit stage. They develop under mild to moderate frost conditions, often around 28 to 30°F, where the fruit survives but the outer epidermal cells are damaged. As the fruit continues to grow, the injured tissue fails to expand normally, resulting in a thin, circular, often russeted band encircling the fruit. The position of the ring reflects the size of the fruitlet at the time of the freeze, effectively marking when the injury occurred. While frost rings do not usually cause fruit drop, they can reduce market quality, particularly for fresh fruit, due to their cosmetic impact.

Immediate Action Recommended

Growers should activate all available frost protection measures immediately, including wind machines, sprinkler systems, or orchard heaters where feasible. Pay close attention to low-lying areas and frost pockets, which may experience temperatures 2-4°F colder than weather station forecasts. Monitor site-specific conditions throughout the night, as microclimatic variation can significantly influence the extent of injury. Please see this Penn State Extension article on critical frost temperatures and Michigan State University article on what growers can do if a freeze is coming.

Assessing Freeze Damage in Fertilized Flowers and Fruitlets

Evaluating freeze damage becomes more challenging as fruit development progresses beyond bloom. During flowering, damage assessment is relatively straightforward. Cutting flowers longitudinally reveals brown or black discoloration of the pistil, style, and ovary, while healthy tissues remain white to light green, allowing for a clear distinction between viable and injured flowers.

However, once fertilized fruitlets reach approximately 3/8 to 1/2 inch in diameter, assessment becomes more complex and time-dependent. Freeze-damaged fruitlets may show a slightly shrunken appearance and contain brown or darkened seeds when cut open, whereas healthy fruitlets exhibit green centers and normal embryonic development. External symptoms are often delayed, and injury may not become visible for 7 to 10 days after the freeze. For accurate evaluation, cut fruitlets both longitudinally and cross-sectionally to examine the flesh and developing seed area.

Severely damaged fruitlets typically abort and drop within about a week. Partially injured fruitlets may remain on the tree and appear normal externally, but often fail to enlarge due to nonviable seeds that cannot produce the hormones required for proper fruit growth. For reliable assessment, wait at least 48 hours after the freeze before sampling, and evaluate a minimum of 50 fruitlets per variety across different canopy positions and orchard locations. Documenting damage by variety and position will help guide subsequent management decisions.

In Pennsylvania, most locations have already lost the king bloom in apple, and in some cases, even the first lateral, particularly in areas like Rock Springs. As a result, in much of the state, the largest remaining lateral flower in the cluster will determine final fruit set, provided it is not injured by the upcoming frost event.

Using PGRs to Improve Fruit Set at Bloom

Promalin® and Perlan® (GA4+7 and 6-BA) function as a form of "hormone replacement" during the bloom period, supplying the plant growth regulators that are normally produced after successful fertilization. Under natural conditions, fertilized ovules develop into seeds that produce cytokinins and gibberellins, which signal the tree to retain and support fruit development. When fertilization fails due to frost injury or poor pollination conditions such as low bee activity, these signals are not produced, and the tree typically aborts and sheds the flowers. Promalin and Perlan provide these missing hormonal signals externally, allowing otherwise sterile flowers to be retained and develop into normal-sized fruit, though fruit may have reduced seed set or be seedless.

Applications must be closely synchronized with active flowering and frost recovery. If frost occurs during bloom, Promalin or Perlan should be applied as soon as possible after tissues have thawed, ideally within 24 hours of the event. Delayed applications may still provide some benefit, but efficacy declines as flowers move beyond the effective pollination window.

Recommended rates are typically one pint per acre under normal conditions and up to one quart per acre under severe frost conditions below 26°F. Apply in approximately 100 gallons of water per acre to ensure thorough coverage, and do not include surfactants. Applications are most effective under slow-drying conditions, provided that frost-damaged tissues have fully thawed prior to spraying. It is important to note that these PGRs can sometimes enhance fruit set on marginally injured or frost-ringed flowers/fruit, which may not always be desirable depending on the extent of damage and final crop load objectives. Promalin and Perlan are not preventative treatments, cannot protect blossoms from freezing injury, and cannot repair or recover tissues that have already been killed by frost—their role is strictly to support fruit set from surviving, viable flowers.

Weather Outlook and Recommendations

Following Tuesday morning's freeze event, temperatures will gradually moderate through the remainder of the week. Highs are expected to climb back into the upper 50s to mid-60s by the weekend, with overnight lows settling into the upper 30s to mid-40s. The recovery will bring a return to more seasonable conditions for late April, though growers should remain vigilant as additional scattered frost events remain possible in low-lying areas during clear, calm nights. Rainfall chances appear limited through the weekend, providing good conditions for conducting freeze damage assessments and making informed crop management decisions.

A petal fall application of calcium chloride can be included to support cell wall integrity in developing fruitlets, particularly following stress events such as frost or fluctuating temperatures. Avoid calcium chloride applications when temperatures exceed 85°F; if a softer option is preferred, liquid calcium formulations can be used safely at this stage. Apply at the rate of 3-5 lbs of CaCl2 per 100 gallons of water.

Thinning Recommendations

Due to the back-to-back freeze events, many growers have appropriately held off on blossom thinning applications. Given the uncertainty about final fruit set and the extent of freeze damage still becoming apparent, it would be prudent to delay chemical thinning until fruitlets reach 6-8 mm in diameter. At this stage, damaged fruitlets will be more readily identifiable, allowing for better assessment of actual crop load before committing to thinning applications. This delay will help avoid over-thinning in blocks where freeze damage has already reduced fruit set, while ensuring adequate thinning in areas that escaped significant injury. Growers should conduct careful variety-specific damage assessments before making final thinning decisions and adjust application rates accordingly based on observed survival percentages.

If growers plan petal fall applications later this week or over the weekend, NAA or 6-BA products can be used for mild thinning, with rates adjusted based on observed fruit set and overall tree stress. Carbaryl can also be included at 1 pint per acre as part of the petal fall thinning program, where appropriate and consistent with label guidelines.

For southern Pennsylvania, the carbohydrate balance model on the NEWA website currently indicates that approximately 60-75 degree hours above 4°C have accumulated since bloom. Based on the forecast for this week, only mild thinning activity from chemical thinners is expected at present. However, there is no strong indication that conditions next week will be more favorable for thinning. Therefore, after assessing frost damage on Wednesday or Thursday, thinning sprays can be considered from Friday onwards to ensure that available windows for thinning are fully utilized.