HORTICULTURE
Assessing Frost Damage to
Fruit Buds of Fruit Trees.This file contains pictures of frost damaged flowers 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.
At or near the bloom stage where 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 those at lower sites. The percent of flowers killed in a frost may or may not relate directly to lost yield later in the season. With large fruited fruits (apples, peaches, plums and pears) The loss of 50% of the flower is not significant since we may only want 25% of the flowers to become fruit. For small-fruited fruits such as cherries, blueberries and grapes many small fruit are needed for good yields. Due to the small size fruit the number of fruit needs to be large to have a full crop. Crop losses due to freezing temperatures are almost always significant in cherries.
Stone Fruit
Early in the
season, flower buds are often cut open cross the bud to inspect the pistil. If this is
black the flower has been killed and the fruit will not form.Cherry buds contain several
individual flowers in each bud. The flowers in the cherry buds on the left look OK.
The
tart cherry flower buds on the right are from a frost a week later. The center
flower has 4 flowers and the one on the lower left was damaged or killed by a frost that
morning about 4 hours before this picture was taken. The bud to the left has 3
flowers and 2 are dead. Normally flowers killed in the bud are easily distinguished as the
bud continues to grow and the dead flowers stop growing.
A cut through this cluster of
sweet cherry buds show that they are all alive.
As the flower develops more you can tear it open to see if the pistil has been killed.
The entire pistil of the
sweet cherry on the left was killed. By the next day damage was easy to see. The flower on
the right shows partial damage to the flower. The style of this pistil was killed by a
frost earlier in the season. There is no place for the pollen to land and fertilize
this flower so this will never be a cherry. This kind of damage is very common in
cherries. People wonder why they have no cherries because the trees bloomed. If the pistil
is killed in the bud the flower will stop growing. If the style is killed and the pistil
remains alive the flower will continue to grow but since there is no way to pollinate the
flower and fertilize the ovule there will be no fruit and the flower will fall after
bloom.
Both
the pistil and the style are green in this sweet cherry.
As the flower opens you can look into the flower and see the pistil to see if it is discolored
Since peaches have large fruit and only a small number of flowers are needed
for a full crop. The loss of a large number of flowers does not mean the crop is severely
impacted. The peach blossom on the left is alive while the one on the right was killed. An
important factor in peach yields is the number of flowers on the tree i.e. is bloom heavy
or light.
After flowering, small stone fruit are in the shuck. The shuck is formed by the
floral cup. The shuck provides a little protection from the cold when the fruit is smaller
than the cup and not touching the walls. But when the fruit fills the shuck, the shuck
provides no protection at all.
The tart cherry flower cluster above shows little damage to the pistils inside the shuck.
In some cases the style was burned by frost but this will not hurt if the ovule
,which becomes the seed, has already been fertilized.
After shuck split, the fruit are exposed and very vulnerable to frost. Often
temperatures just below freezing can cause extensive damage.
The sweet cherry fruit above were frozen and have a "watersoaked" appearance
after they have thawed out.
The one green fruit on the left was not killed, but it is likely that it will have frost
marks, a russetting on the skin of the fruit where the surface cells were damaged.
Freeze Damage in Stone Fruit is similar to this Page.
Apples and Pears
Apples and Pears are different than stone fruit. The buds
of Stone fruit trees are either flower or leaf buds and all the flowers in a bud are about
the same age. 
In apples, the fruit buds are really small shoots
with flowers and leaves. 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. Another difference between apples and stone fruit is
that 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 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.
I have another file specifically for Freeze Damage in Apples.
