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Early season frost damage in apples

On the evening of April 30-May 1 several apple growing regions in Ontario experienced frosts which will likely result in crop loss. The extent of the damage varies throughout the province with some areas receiving little or no damage, and other areas reporting high levels of injury. The highest level of damage has been reported in the Norfolk and London areas with reports of temperatures reaching as low as -4° or -5° C. The effect of temperature on fruit damage is dependent on the stage of development the crop is at.

Critical Spring Temperatures for Temperate Tree Fruits

When frost damage has occurred in your area it can be useful to try to assess the amount of frost damage to apple buds and flowers. Often the extent of frost injury doesn't show up on the blossoms or buds for 2-3 days. The entire extent of the damage to the crop is not usually apparent until after June drop, when you have a better indication of the impact on fruit russetting, malformed fruit, and fruit drop.

Prior to June drop frost damage can be assessed by looking at the pistils of the flowers. If the pistils are black or brown (Fig. 1) the blossom is likely killed.

Figure 1. Browning of floral parts is evident shortly after frost damage has occurred [Photo J. Cline].

It is often necessary to tear the flower or the bud apart to see if the center of the flower (ovary) is brown or black (Fig. 2).

Figure 2. Using your fingers or a small razor blade, cut open the small fruitlet and look for evidence of browning [Photo J. Cline].

When checking apples for frost damage, check the king and side blooms separately. The king bloom is the most advanced and therefore it is the most likely to be affected by frost near bloom.

Frosts and freezes can cause browning and damaged petals especially in varieties like golden delicious. Some apples can make it through frosts, but later in the season can show signs of frost rings or malformation (Fig. 3).

Figure 3. Frost rings and misshapen/malformed fruit are often evidence of earlier frost damage to flowers. Some reliance on hand thinning to selectively remove these unmarketable fruit may be necessary [Photo J. Cline].

Frost damage is important to keep in mind when determining a thinning program and a pest management program for the year. For more information on thinning after a frost refer to Dr. John Cline's article in this issue of Hort Matters.

Learn more:

• Chemical thinning of apples in the response to frost damage to flowers