In this Issue:

IPM as a Risk Management Tool? Recent Thoughts about Taking Time to Plan


Vegetable Insect Pest Update

Snap Bean Seeding Rate Considerations for 2005

New Registration for Headline Fungicide


Weekly Pest Sampling Data/Critical Spring Temperatures

Upcoming Field Days


Questions and Answers Regarding Apple Freeze Damage and Thinning

Assessing Frost Damage to Apple Buds, Flowers, Fruit and Trees

Current and Coming Events

Weekly Trap Counts

Apple Scab Infections

Insect, Pest Fact Sheets

Vol 2 No.2   May 16, 2005

Assessing Frost Damage to Apple Buds, Flowers, Fruit and Trees

By Mark Longstroth, MI State University Extension, Van Buren County, District Horticulture Agent, April 26, 2005
(Reprinted with the Author’s Permission)

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. 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 article 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 28F will result in 10% loss and 24F 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 upper left. 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 lower left is a king bloom killed by frost. When checking apples for frost damage, check the king and side blooms separately.

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.

These flowers were undamaged by the freezes that crinkled the leaves. All these flower buds are undamaged
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 above 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.

The center bud was killed earlier and stopped growing


The pictures below were taken after the May 2004 and 2005 frosts.

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.

Healthy Side Bloom in apple

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 frost.

Healthy king bloom

Even though there were many apples that survived the May frost, many 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.
Frost ring on young apple (click for larger image)Frost Ring at Harvest (click for a larger image)



(From: Washington State University Extension Bulletin 0913)

Bud Development Stage **










Old Standard Temp. 1










Ave. Temp for 10% Kill 2










Ave. Temp for 90% Kill 2










* For Red Delicious, Golden Delicious and Winesap approximately 1 degree hardier; Rome Beauty, 2 degrees hardier; After petal fall all varieties are equally tender.
** Bud development stages: 1 - Silver Tip; 2 - Green Tip; 3 - Half-inch green; 4 - Tight Cluster; 5 - First pink; 6 - Full pink; 7 - First bloom; 8 - Full bloom; 9 - Post bloom
1 Critical temperatures as previously published.
2 Average temperatures found by research at the WSU Research and Extension Center, Prosser, to result in 10% and 90% bud kill.


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Co-Editors: Bill Hutchison (, Department of Entomology, University of Minnesota, Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Management and Development Division, and Suzanne Wold-Burkness (, Department of Entomology, University of Minnesota

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Wednesday to Jeanne Ciborowski, 651-297-3217, , MDA, 90 W. Plato Blvd., St. Paul, MN 55107-2094. You can access the Newsletter at the U of MN web site in htm format at: and at the MDA web site in pdf format at:

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