By Kathryn Carter, Tender Fruit and Grape Specialist, OMAFRA
Erika DeBrouwer, Tree Fruit Specialist, OMAFRA
April has been a month of wild weather with temperatures fluctuating widely. We are at bloom in tender fruit crops in some areas, and rapidly approaching bloom in other areas. Apples range across the province, but most staging is from tight cluster to pink. With temperatures Tuesday night approaching -1° C followed by considerable amounts of snow, and Wednesday reaching -4° C there are considerable concerns about frost.
Susceptibility to Frost
When buds and bloom are present, watch the weather conditions and forecast for frost. There is considerable variability in susceptibility to freezing between trees, cultivars, crops and stages of development. As flowers begin to swell and expand into blossoms, they become less resistant to freeze injury. Buds that develop slowly tend to be more resistant. As a result, some buds are killed at higher temperatures, while others are resistant at much lower temperatures. For a table of the critical temperatures for tree fruits refer to Reducing Frost Damage in Tree Fruit – ONfruit.
The duration of the cold event can also have a significant impact on the severity of the damage. Longer cold events tend to result in more significant injury.
Evaluating Frost Injury
After a frost event, it is good to evaluate fruit trees for frost/cold damage. It is important to evaluate cold damage in order to determine future crop protection efforts and thinning programs. It takes several hours for symptoms of frost damage to develop. As the frozen tissue warms up it will appear black or brown showing the extent of the damage.
It is best to evaluate damage by collecting shoots with flower buds and bring them indoors to warm up. Looking at buds in the field can give you a “feel” for the damage, but it is much more objective and accurate to determine exact levels of damage inside, with a good light and magnification if necessary.
Collecting 100 buds for evaluation will provide a good assessment. Collect 10 shoots (each with approximately 10 buds) from differing heights in the tree (high and low) and different locations within the orchard or orchard block. Separate cultivars (and locations/height within the tree) by bundling with flagging tape and a label.
Bring the samples back from the orchard and place the base of the shoots into a bucket (or a can) with water. Allow them to warm up at 21° C for a minimum of four hours, which will allow the damaged tissue to develop the characteristic brown-dark or brown-black color due to oxidation of phenolic compounds released by the injury.
Then cut the buds or blossoms on a vertically, using a very sharp blade. Stone fruit flowers (peaches, nectarines, cherries and plums) have a single pistil (the female part of the flower). If the pistil is brown or black after the breeze the fruit will not develop. Injury to peach pistils can be more difficult to assess due to the fuzz. Undamaged buds will be uniformly green throughout. For apples and pears frost assessment of the king bloom should occur, as it is the furthest advanced within the fruit cluster and is most likely to be killed in a frost event. Pistils of apples and pears are buried inside the base of the flower, meaning it is necessary to tear the flower apart (cross section vertically) to see if the centre of the flower is brown or black (Figure 1). Often times, king blooms may have damage, yet side blooms remain undamaged.
Assessing dead/injured flower pistils and ovaries in 100 buds (10 buds on 10 shoots) provides a good estimate of percentage crop loss. The following websites have some excellent pictures and resources for assessing frost injury for tree fruit:
- Assessing frost and freeze damage to flowers and buds of fruit trees – Fruit & Nuts (msu.edu)
- Evaluating Tree Fruit Bud & Fruit Damage from Cold – 7.426 – Extension (colostate.edu)
- Assessing fruit bud survival and crop potential – Fruit Growers News
- (74) Evaluating Freeze Damage to Peaches – YouTube
- (74) Assessing Spring Freeze Damage to Apples – YouTube
- (74) Assessing Spring Freeze Damage to Peaches – YouTube
The percentage of frost damaged blossoms may or may not relate directly to the amount of yields lost at the end of the season, depending on the amount of injury and the crop. With larger fruit (apple, pear, apricot, peach, nectarine, and plum) only about 10 percent of the blooms need to set for a full crop. However, smaller fruit like cherries need about 50 percent of the buds produced the previous summer in order to have a full crop. Uniform distribution throughout the tree is also important for full crop potential. Which is often an issue when frost occurs as generally the frost damage is more significant on the bottom of the tree then the top of the tree as cold air settles lower. Also often blossoms on the trees are in various states of development impacting their susceptibility to frost.
Peach Fruit Set After a Frost
Freeze-damaged flowers and fruitlets may drop or persist on trees. When flowers/fruitlets are completely frozen and damaged, the epidermis (skin), mesocarp (flesh), endocarp (stone or pit), and embryo tissue (seed in the pit) turn brown quickly and fruit drop off trees in approximately a week. Partly injured fruitlets may remain on the trees, appearing healthy and colouring normally, but stop enlarging. These fruit are often referred to as buttons, they have a normal epidermis, mesocarp, and endocarp tissue, allowing the fruit to persist on the tree. Often the seed (in the pit) is dead, the fruit will develop more slowly and be reduced in size. When growers are starting to thin green fruit (typically around 30 days after bloom), buttons resemble healthy fruitlets in appearance and size, making it extremely difficult to distinguish them from healthy fruitlets during the thinning process. As a result, useless buttons may be retained, reducing marketable yield.
The issue appears only under certain spring frost conditions, and some varieties are more susceptible then others. For nectarines frost injury can result in russeting or bumpy fruit.
Apple/Pear Fruit Set After a Frost
Apple and pear trees are similar to peach frost set in that, (1) damaged flowers and fruitlets can also persist on the trees, (2) when tissue is completely frozen and damaged it will turn brown and drop off trees in roughly a week, and (3) partly injured fruitlets can remain on the tree, appear healthy but will stop enlargement.
Apples and pears differ from peaches where clusters of flowers must be assessed to better understand the damage within the orchard. Bringing in branches to assess king blooms and side blooms is the best way to assess frost damage and make crop load decisions in the future. By measuring the base diameter of the branch, you can interpret crop load utilizing the Equilifruit Disk and by using your data from frost damage, you can compare the number of live buds to the full crop load.
Frost can cause organ development issues within the flower, meaning full pollination may not be possible, causing poor seed set. Frost mitigation applications can save these fruit that would have been dropped, but may result in parthenocarpy. These fruit are sterile, known to have small sizing and have a limited shelf life.
Frost damage in apple and pear can cause misshapen or russeting of fruit which affects their marketability. Frost also causes deformed bloom and fruit set of king blooms, but the most prominent indicator associated with frost damage is frost ring (Figure 2)
Currently many tree fruit are in various stages of bloom and are susceptible to frost. There is considerable variability in susceptibility to freezing between trees, cultivars, crops and stages of development. It takes some time for symptoms of frost damage to develop so keep that in mind when evaluating frost injury. Keep in mind that the percentage of flowers killed in a frost may or may not relate directly to the amount of yields lost at the end of the season depending on the extent of the damage and crop.