Apr 7, 2021

Cold, overnight temperatures in Grand Rapids region could impact apple crop

Over the past two weeks, the Grand Rapids, Michigan, region has seen an acceleration of growing degree day accumulation and associated bud development due to unusually warm temperatures.

This was followed by cold temperatures and snow in some locations overnight from April 1 to 2, 2021, with lowest temperatures between the hours of 6 and 8 a.m. It may feel like an April fool’s joke by Mother Nature, but it is too early to make any assumptions about crop damage at this time. Fortunately, we expect the actual frost damage is minimal.

Were temperatures low enough for frost damage to occur?

In most areas in the Grand Rapids region, apple buds were at approximately green tip Friday morning (April 2), with early varieties at slightly more advanced stages. The critical temperature for frost injury at this stage is approximately 18 degrees Fahrenheit for 10% kill or 10 F for 90% kill, according to the Tree Fruit Critical Temperatures table and Picture Table of Fruit Freeze Damage Thresholds.

However, there is some variation in these temperatures. Evaluations were conducted on Red Delicious, a variety very sensitive to frost damage. In the experiments, buds were held at 30 mins before evaluation. Actual damage in the orchards will depend on cultivar, duration of the cold temperatures, and other site-specific conditions.

The table below shows the actual low temperatures for weather stations in the Grand Rapids region over April 1-2, as well as the difference between the critical temperature and recorded temperatures for damage at green tip. In only a few locations, indicated with an asterisk (*), recorded temperatures dropped below the critical temperature for 10% damage (18 F).

Bud assessments to evaluate injury

Bud samples can be collected and evaluated to assess frost damage. It takes several hours for any damage to become evident, so always wait until the afternoon or the next day following periods of cold temperatures. Begin by collecting approximately 25-50 floral buds per block. Where there is variation within the block, collect separate samples to assess these areas independently. Pay special attention to differences in elevation, variety and growth stage.

For each sample, cut each bud in half down the middle with a sharp razor the long-way (from top to bottom). For cherries, buds can be cut in half the opposite way (across). Using a hand lens or a dissecting scope, look for the floral organs (immature flowers) within the bud. At this growth stage, it should be fairly easy to see the king flower and at least two lateral flowers. Evaluate king and lateral flowers separately. King flowers mature earlier than the lateral flowers, so are at a more advanced growth stage and more sensitive to cold temperatures than laterals.

Keep track of damage. Brown tissue indicates damage and bright green tissue indicates healthy uninjured tissue. Record the number of king flowers and laterals that are damaged out of the total number of buds collected. Make sure to evaluate the damage right away—if you wait, the cut tissue will oxidize as it is exposed to the air, and the tissue will turn brown on its own (not frost damage).

What can we expect going forward?

Several bud assessments were conducted in the Sparta, Michigan, area after the cold event Friday morning. A minimal amount of damage was observed in king flowers in more advanced buds. In particular, early cultivars such as Ginger Gold and Zestar. Some damage was also found in more advanced buds on other cultivars (0.25-inch green). Laterals were by and large not impacted at this time. Slightly more bud damage was reported in other areas, with more advanced development that experienced low temperatures of 17 F Friday morning.

Before getting too worried, remember that apple trees can withstand a considerable amount of damage and stress before the crop will be significantly impacted. Trees produce many more flowers than the number of apples that will be harvested. Any frost at this time is more likely to amount to a little “natural thinning.” Other damage that may occur could include russeting or frost rings, but it is much too early to know if this was the case.

As the spring weather and tree growth continues, buds are becoming more sensitive to cold temperatures. Scientists at MSU (Todd Einhorn, Greg Lang, Amy Irish-Brown, Nikki Rothwell, Bill Shane and Anna Wallis) have been collecting data on bud hardiness across the state since the end of November 2020. In this work, buds are collected at least every other week and evaluated for their tolerance of extremely low temperatures. Watch this video to learn more about the project and see what cold temperature damage looks like. In our work, we are observing rapid changes in bud hardiness in our sampling.

It is still early in the spring, and we are likely to get more cold nights and early mornings over the next month. If frost damage is a concern, bud damage should be evaluated prior to petal fall so that adjustments can be made to thinning programs to compensate for any damage that may have occurred.

– Anna Wallis, Michigan State University Extension

Photo at top: Cold temperature injury to apple flower buds, with differing levels of severity. Left: king flower and one lateral are dead. Center: king and both laterals are dead. Right: healthy bud with no cold temperature injury. Photos: Anna Wallis/MSU Extension