Northwest Michigan midseason vineyard update

We had a slow start to the season, with tepid temperatures and prolific precipitation.

July 26, 2019 - Author: ,

Riesling cluster
Photo 1. Riesling cluster at buckshot. Photo by Thomas Todaro, MSU Extension.

This season, wine grape producers in the Leelanau Peninsula and Old Mission Peninsula AVAs (American Viticultural Areas) are dealing with a combination of environmental factors that further limit the already narrow window for fruit ripening in Michigan’s short and highly variable growing season. In these AVAs, bud break typically occurs near May 15. However, the lower than normal heat unit accumulation (growing degree days, GDD) during the month of May likely played a major role in the delay of bud break by roughly seven to 10 days, up to 10-14 days on cooler sites (Table 1). In fact, May 2019 was the coolest in the past five years, and well below the five-year average (Table 1).

The “cool trend” does not stop there. This year, Leelanau Peninsula (1,052 GDD) and Old Mission Peninsula (957 GDD) have accrued the fewest recorded heat units than in any year during the past five years and are well below their respective five-year GDD average of 1,227 and 1,152 (Tables 2, 3 and Figs. 1, 2). This year is also different than the past five years with much greater amount and time of rainfall in Leelanau with 13.52 inches and 231 hours, and Old Mission with 14.02 inches and 237 hours (Tables 2, 3).

Graph 1
Figure 1. Minimum, average, and maximum temperatures and GDD accumulation in Leelanau Peninsula AVA from April 1 through July 25, 2019.
Graph 2
Figure 2. Minimum, average, and maximum temperatures and GDD accumulation in Old Mission Peninsula AVA from April 1 through July 25, 2019.

Phenology, fruit quality and crop control

While the tepid temperatures have generally slowed growth and development, prolific precipitation has induced rapid shoot growth, resulting in increased demand for canopy management practices (fruit zone leaf removal, tucking and shoot training) and has instigated increased concern and chemical applications for future disease pressure like downy mildew, powdery mildew, botrytis and sour rot. Grapes are currently between the phenological stage of large pea-sized on cooler sites and large buckshot at warmer sites (Photos 1-5).

Riesling fruitzone
Photo 2. Riesling fruit zone with adequate and uniform shoot spacing of four shoots per foot of canopy length. Photo by Thomas Todaro, MSU Extension.
Riesling fruizone
Photo 3. Riesling fruit zone after pruning to two-bud spurs, leaf removal, shoot tucking and clipping catch wires. Photo by Thomas Todaro, MSU Extension.
Close-up Riesling
Photo 4. Close up of Riesling fruit zone after pruning to two-bud spurs and canopy management. Photo by Thomas Todaro, MSU Extension.
Image 5 Merlot cluster
Photo 5. Merlot cluster grown on Old Mission Peninsula at buckshot stage of development. Photo by Thomas Todaro, MSU Extension.

It is critically important to apply fungicide cover directly to the fruit zone (after fruit zone leaf removal) before clusters enter the berry touch stage of development. After berry touch, the clusters are too compact to accomplish complete surface area coverage of the individual grape berries and the fruit will more likely sustain rot damage.

The first half of the growing season has been cooler than normal, and there is interest in crop reduction to enhance and speed ripening. If you are interested in crop reduction via cluster thinning to hasten fruit ripening, use the following rule of thumb:

  • Remove all fruit from shoots less than 12 inches long.
  • Retain a maximum of one cluster per shoot on shoots between 12 and 24 inches long.
  • Retain a maximum of two clusters per shoot on shoots longer than 24 inches long.
  • The sooner the crop is reduced, the better the impact on retained fruit. In other words, do not wait; the time to limit crop to see actual benefits is right now.

Many wine grape growers have indicated that the success of this year’s vintage is squarely dependent upon a warm second half of the growing season, suggesting that at this point, just like in major league baseball, September and October are the only months that matter.

Table 1. Five-year comparison (2014-2019) of heat unit accumulation (GDD) in Leelanau Peninsula and Old Mission Peninsula AVAs during May 1-31.

Year

Leelanau Peninsula

Old Mission Peninsula

2019

142

118

2018

362

327

2017

167

153

2016

277

260

2015

271

247

2014

214

185

5-year Avg. (’14-’18)

258.2

234.4

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Table 2. Five-year comparison (2014-2019) of heat unit accumulation (GDD) and rain in Leelanau Peninsula AVA from April 1 – July 25.

Year

(GDD)

Amount of rainfall (inches)

Time of rainfall (hours)

2019

1,052

13.52

231

2018

1,440

7.53

151

2017

1,202

9.7

193

2016

1,306

6.88

129

2015

1,115

7.09

159

2014

1,071

9.94

184

5-year Avg. (’14-’18)

1,226.9

8.23

163.2

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Table 3. Five-year comparison (2014-2019) of heat unit accumulation (GDD) and rain in Old Mission Peninsula AVA from April 1 – July 25, 2019.

Year

(GDD)

Amount of rainfall (inches)

Time of rainfall (hours)

2019

957

14.02

237

2018

1372

9.36

142

2017

1148

11.93

209

2016

1238

9.95

142

2015

1020

7.27

152

2014

982

9.31

193

5-year Avg. (’14-’18)

1152

9.56

167.6

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Tags: agriculture, fruit & nuts, grapes, msu extension


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