HORTICULTURE

Frost Damage to Grapes in SW Michigan on April 23, 2002
Mark Longstroth

In 2002, over a week of summer like weather with highs climbing into the 80s and lows in the 60s caused rapid plant growth. We had cold wet rainy weather over the weekend with highs near 40 and lows near freezing. The weather cleared Monday evening resulting in wide spread frosts on Tuesday morning resulting in significant losses to all fruit crops. This was a classic radiation frost with clear skies and calm conditions.

In a radiation frost the ground cools by radiation to the open sky. The cold ground then cools the air above it. Since there is no mixing of the air due to wind the cold air layer becomes thicker and thicker and the bottom of the layer becomes colder and colder.  The cold dense air flows downhill like water, filling the hollows and valleys with cold air. This leaves hill sides and hilltops warmer than the surrounding area.  Wind machines are often used to mix the warm air above with the cool air below to keep fruit plantings warmer. On April 23, 2002 the difference between the cold air at the ground and the warm air above was only 3F. Growers reported little if any success using wind machines.

The Paw Paw - Lawton grape growing region recorded temperatures of 28 to 20 F. In the highest sites where temperatures of 28 F were recorded there was little damage.   For almost all other sites the damage was total. All the actively growing points on the vines were killed. I was stunned by the damage done to grapes. Southwest Michigan has about 375 grape growers with 12,800 acres of grapes, about 95% of Michigan total grape acreage. The grape crop in Michigan is worth over $20 million and suffered severe damage between sundown and sunrise.

To the left is what the most advanced buds in Lawton looked like on the evening of Monday, April 22. To the right is how they looked on Tuesday afternoon. After assessing grape damage on April 23 and 24, it appears we have lost 90% of the primary grape buds.   The unusually warm weather caused grapes to break dormancy earlier than normal. The average date for bud swell is April 27, but warm temperatures had caused buds to swell on April 16 and warm night temperatures (over 60F) caused the grapes to grow rapidly.

The inside of this dead grape bud reveals that the stem is killed as well as all the other parts of the bud. The grape plant will but out new shoots but they will have few if any flower buds so there won't be many grapes in Van Buren County this year except in the highest frost free sites.

These Pictures were taken April 24 at the base of the Lawton Hill were there were both live and dead grape buds in the vineyard. The bud on the left is alive, green and plump. You can even see the secondary bud growing at the base of the large primary bud. The buds on the right were killed by the freeze on April 23. It appeared that both the primary and secondary buds were growing. By Wednesday morning these buds were dry and shrunken and by the afternoon then crunched when you pinched them. Almost all the grape acreage in Van Buren and Berrien Counties look like the buds on the right.

By the end of April I was sure that the 2002 grape crop was lost. Other people were not so sure and events would prove them right. Dr. Stan Howell and Dr Tom Zabadal were unwilling to commit to any projections of the damage until after new growth began in grapes. Grape buds have 3 buds inside the bud we see. The first central bud is called the primary and has the potential to be the most fruitful. The secondary bud grows if conditions are very good or if the primary bud is killed or injured. The secondary bud has the potential to be about half as fruitful as the primary bud. The last bud is called the tertiary bud and there are no flower buds in this bud. Therefore if only the primary buds were killed there was still the potenital for a half crop. It would all depend on how fruitful the remaining buds were.

The Aftermath:

Cool weather followed this freeze and little growth was visible for almost a month. By late May, growers could see that the secondary buds were starting to push. Many growers believed that the 2002 April 23, freeze had killed both the primary and secondary buds and removed all crop potential for the 2002 crop year. Actually few secondary buds were killed. In 2002, there were many double primary buds due to a light crop in 2001 and many people thought these twin primaries were the primary and the secondary buds. The actual 2002 grape crop was about half of the average, due to fruit set on flowers that came from the secondary buds. In 2002, the secondary buds were very fruitful. This was probably due to the fact that we only had half a grape crop in 2001. Many of the shoots from these buds had 2 flower clusters, so the potential crop in 2002 was probably 2/3 of a normal crop, because of the fruitfulness of the crop.

There were other later frosts in May of 2003 which damaged many vineyards but the potential existed for a good crop and many growers did harvest good yields from their vineyards. The were other problems with the grape harvest in 2002. (See the regional report for Sept. 10, 2002). The main sources of loss to the crop after the April freeze event were the disease phomopsis cane and leaf spot and grape berry moth. Growers had saved on early sprays before bloom to determine what the crop potential was before appling costly pesticides. This allowed phomopsis rachis infection to become established. These infections were latent until the fruit began to ripen. As the fruit ripen rachis infections girdled the stems of the berries and caused heavy fruit drop.

Another source of loss was the heavy flight of grape berry moth in August and July. The warm summer season allowed a 3 generation to begin and this moth was fling in heavy numbers from early August all the way through harvest. This resulted in heavy losses also.

The take home message is even if you think that you have lost a crop do not neglect insect and disease control before bloom, and if grapes are scarce be aware that native grape pests will concentrate on the remaining grapes in the area.

The crop potential of grapes that have suffered an early spring freeze cannot be determined until the secondary buds have broken dormancy and grow out enough that the grower can determine the fruitfulness of the second flush of growth. This fruitfulness cannot be determined until a few weeks before bloom.

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Posted: April 27, 2002
Last modified: May 12, 2004