A frost on May 3rd, 2004 damaged many of the fruit crops in Southwest Michigan. Grapes were especially hard hit in some areas. Damage was spotty and varied from light to severe.

For many growers there is still a potential for a good crop. MSU and National Grape Viticulturists have prepared this special newsletter to help guide Michigan grape growers as they determine what is their best course of action.

Questions regarding vine management after a spring freeze may be directed to the following MSU Extension and National Grape

Cooperative staff:

Cultural Practices, Insect & Disease Control

Al Gaus--269-944-4126
Mark Longstroth--269-657-7745
Terry Holloway--269-930-8291

Cultural Practices
Tom Zabadal--269-944-1477 x 206
Stan Howell--517-355-5191 x 1-311

Insect Control
Rufus Isaacs--517-355-6619

Disease Control
Annemiek Schilder--517-355-0483

By: Tom Zabadal, Stan Howell, Rufus Isaacs, Mark Longstroth, Terry Holloway and Al Gaus

Crop potential cannot be determined until the secondary buds grow and theirflower clusters are counted.

Assessing the Injury
It is not possible to immediately assess the extent of injury to vines after a spring freeze episode. It is possible to determine the mortality of emerged shoots. However, the buds in many vineyards were in various stages of bud swell and bud-break at the time of the May 3rd freeze. Therefore, it will be two weeks or more before the survival of all tissues, including the contribution of secondary buds, to the 2004 crop potential can be determined. The first step in response to spring freeze injury is patience. It will be the later part of May or even early June before a meaningful assessment can be made. At that time, specific procedures can be used to determine the extent of vine cropping potential  More information can be found at the MSU Extension web site www.msue.msu.edu/vanburen/disthort.htm.   However, even before this assessment is made, there are a number of cultural practices that need to be addressed by the grower as follows:

Weed Control - Under trellis weed control is fundamental to a healthy, productive vineyard. It is well documented that any competition by weed growth under the trellis will result in a decrease of vine size and productivity of a vineyard. Therefore, even under the most severe circumstances of spring freeze injury to a vineyard, it is important that weed control practices be maintained. Delays in the early season application of pre-emergent herbicides and later applications of systemic or contact herbicide sprays for established weeds will result in the need for even more effort later and at a greater cost to bring weeds under control. Many Southwestern Michigan vineyards do not need an additional stress from weeds to further depress vine size, particularly in 2004.

Disease and Insect Control - The importance of early-season applications of fungicides to vineyards has been increasingly appreciated in recent years, especially with regard to the control of several major grapevine diseases. Certainly, sprays at the early start of growth for control of Phomopsis Cane and Leaf Spot can be a major issue in many vineyards. The development of primary infections of both Downy and Powdery Mildews early in the growing season can also have a profound affect on disease pressure as the growing season progresses. Because the potential crop size will not be known for a number of weeks, growers should maintain early season sprays in order to insure that their crop, at whatever level, is harvestable.

As we have learned in recent years, a spring freeze is not likely to have much effect on insect pests. Growers should plan on an insect management program appropriate to the level of crop they have remaining in the vineyard. Grape berry moth is expected to be laying the first batch of eggs around bloom time again this year, and an immediate post bloom insecticide should be planned in vineyards with a history of problems with this pest.

Vine Nutrition - It’s well understood that when grapevines fail to crop due to a freeze or other causes, they typically exhibit strong vegetative growth in response to a lack of cropping stress. Therefore, a common tendency would be to reduce or eliminate nitrogen fertilization on grapevines that have been severely injured by a spring freeze. However, as we entered the 2004 growing season in Southwest Michigan, many juice grape vineyards had been crop stressed last year with a resulting dramatic decline in vine size. Therefore, 2004 presents an opportunity for growers to rebuild vine size in many vineyards. After this spring freeze, there will be a need to strike a balance between promoting vegetative growth of the vine and controlling excess growth, which could occur where the freeze has caused a major crop failure. As discussed in a previous newsletter, nitrogen fertilizers will be more efficiently utilized by vines when applied at several inches of shoot growth than at bud-break. Moreover, nitrogen applications after bloom may promote vine size development. Therefore, where vine size is considered adequate and there is the expectation of a moderate yield (4-6 tons/acre), a maintenance application of nitrogen fertilizer should be made between bud-break and bloom. In those vineyards where an increase in vine size is desirable, a supplemental nitrogen fertilizer application after bloom will be helpful. See the previous Michigan Grape Society newsletter (Vol. 5, Issue 2 in April, 2004) for details regarding these split applications.

The decision on vine nitrogen needs should be made for specific vineyards, and also for sections within a vineyard. For example, after this freeze the knoll areas in some vineyards are likely to crop more heavily than swale areas, which are inherently more fertile. Therefore, heavier nitrogen applications will often be justified in knoll areas than swale areas to maintain or increase vine size. This is where the individual grower vineyard assessment becomes the key factor to proper vineyard management.

The freeze of May 3rd will cause growers to apply more than the usual “individualized” management for specific vineyards or even portions of vineyards.

Economic Ramifications - Growers understandably may not want to continue a full set of cultural practices on severely frost-damaged vineyards. However, prior years’ experiences have demonstrated that there are other factors to consider. The federal crop disaster assistance program penalized growers who did not harvest their grapes in 2001 and 2002. The penalties resulted in lower disaster payments. In addition, crop insurance requires insured producers to harvest their crops or be subject to harvest appraisals. Harvest appraisals are subjective, and can overestimate the crop size resulting in reduced insurance payments.