HORTICULTURE
Assessing Frost Damage to
Apple Buds, Flowers, Fruit and Trees.
Mark Longstroth
As apple trees begin growth in the spring, buds begin to swell
and lose the ability to withstand cold temperatures. As the buds, develop warmer and
warmer temperatures (still below freezing) can damage them. The killing temperature is
often called the critical temperature and is defined as the temperature that buds can
withstand for a half-hour. I have posted a table of critical bud
temperatures. In general there is a range of temperatures over which damage occurs
with more buds damaged at lower temperatures until all the fruit buds are killed. Often
the freeze will only damage some of the flowers such as the most developed ones or flowers
in the bottom of the tree.
This file contains pictures of frost damaged buds, flowers and fruit for
growers and home fruit growers who wish to determine frost damage after a freeze. People
want to know if a frost has damaged fruit immediately after the freeze. It is best to wait
several hours (until the afternoon) to let frozen tissues thaw. Dead and damaged tissues
will turn black or brown.
Near bloom, freezing temperatures of 28 F will result in 10% loss
and 24 F in 90 % loss. In that temperature range we can expect to see 50% or more loss in
tree fruit plantings. Fruit on higher sites or in the tops of trees will be less damaged
than lower buds. With large fruited fruits like apples and pears, the loss of 50% of the
flower is not disastrous since we may only need 25% of the flowers to become fruit.
Unfortunately, the flowers lost are the king bloom in the center of the flower cluster.
An apple flower cluster is shown on the right. In apples the flower in the center
of the flower cluster is the oldest and has the potential to be the largest fruit. This
central flower is called the king bloom and is the most desirable of the flowers in the
cluster. The king bloom is also the most advanced and therefore the most likely to
be killed in a frost. Several weeks after a killing frost it is not unusual to see the
side blooms larger than the king bloom this means that the king bloom was killed earlier
in the Spring. In apples, the pistil is buried in the floral cup at the base of the flower
and not exposed above it as in stone fruit. 
This means that it is often necessary to tear the flower or the bud apart
to see if the center of the flower is brown or black. The flower on the left is a king
bloom killed by frost. When checking apples from frost damage check the king and side
blooms separately.
PreBloom
When apples buds are in tight cluster, it is easy to cut across the bud to see if
any of the flowers are damaged. You can start slicing at the top of the bud and slice down
through the king and side blooms. With a steady hand you can slice down through the buds
lengthwise.
All the flower buds above were not damaged by the freeze.
When freeze damage occurs early in bud development you may not be aware of damage.
If you examine the first leaves out, the spur leaves, they are often crinkled as are the
leaves in the photo on the right. These leaves were crinkled by a frost that damaged the
leaves that were exposed but did not damage the flower buds inside them. At other times
some of the flower buds will be damaged. If the pistil has been damaged in apples, usually
that flower will stop growing. Below is an example of a king bloom that was killed in the
bud and stopped growing. On this spur the side blooms continued to grow and are now
noticeably larger than the dead King bloom.
Bloom
The pictures below were taken after the May 2004 frost.
Frost/freezes can cause browning of the petals especially in Golden
Delicious. The picture above shows damaged petals and some of the styles in the middle of
the flower are black indicting that the flower was killed.
The picture above shows a dead king bloom and live side bloom. The picture below shows
live king bloom. The king bloom has the potential to be the largest apple fruit in
the cluster. Apple growers would prefer to set the king bloom and thin off the side bloom.
But having some live side blooms is better than losing your crop to a freeze.
Sometimes you need to tear apart the flower to check and see if the inside is brown.
This flower was killed by frost and you can see that the inside of the flower is black.
Even though there were many apples that survived the
May frost, some of the flowers that did survive were damaged and the fruit showed frost rings.
These Frost Rings were still visible at harvest. Often a frost
ring will constrict the growth of the fruit.
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Posted: April, 26 2005
