MSU Extension Grapes

Weather

Last week began cool with highs in the low 70s and lows in the 50s. A high pressure system entered Michigan on Thursday, bringing temperatures in the mid-80s and several waves of scattered showers and storms that continued through the weekend. Total rainfall was variable. The northwest grape region had less than a half inch of rain. The southern regions had 2-3 inches with most coming on Sunday evening or early Monday morning. With the storms on Sunday and Monday came cooler temperatures. Highs stayed in the upper 60s for much of the state.

Severe weather was accompanied by rain in southwestern Michigan. Hail over 2 inches in diameter was reported in some areas as well as wind gusts over 60 miles per hour. Little damage has been reported from grape vineyards, but some apple and cherry orchards reported significant damage.

The forecast for all regions of the state is much like the past week. Thursday and Friday temperatures will be in the 80s, otherwise cooler 70s will be statewide for the week. There are significant chances of rain on Friday and Saturday, extending into Sunday for the southeastern corner of Michigan.

Even with the recent rains, soils are still dry, especially in the central and eastern lower portions of the state. At this time, most of lower Michigan is in a moderate or severe drought. Much of southwest grape growing areas and a portion of northwest growing areas are in a D2 (severe) drought condition. The significant rains in southern regions should alleviate some of this stress.

With the seasonal week, we picked up an average number of growing degree days (GDD) last week: 95-125 GDD base 50. The southwest region is 265 GDD base 50 ahead of the northwest region. Statewide, we are approximately one week ahead of the five-year average.

Vine growth

In southwest Michigan, vinifera grape bloom is ending. Juice and hybrid grape vineyards not impacted by the early May freezes are between buckshot and pea-size berries. Sites impacted by the freezes had varying levels of primary bud loss or foliar damage, with some areas as high as 90%. Many of these sites have just finished bloom. In these freeze damaged sites, a reduced pest and disease program may be warranted. See “Pest management approaches in a winter or freeze damaged grape vineyard” from Michigan State University Extension for recommendations.

In northern vineyards, many sites are in early to full bloom. The northwest had similar temperatures during early May, but the grapes were behind phenologically when compared to vines in the southwest, so they have survived the spring cold events without any significant damage.

See this chart for grape growth stages.

Horticulture

A quality soil test or a previous year’s petiole analysis is important in understanding what nutrients the vineyard needs. Early season nutrient management will most likely include nitrogen, zinc and boron. In addition, potassium and magnesium may also need to be managed at this time.

Grapevine nutrient status is determined by three methods: 1) Observing visual symptoms, 2) analyzing vine tissue samples and 3) performing soil tests. The pre-bloom and bloom time is a very important moment of the vine growing season to scout for visual symptoms of potential problems and deficiencies. Tissue analysis is the preferred tool for monitoring the nutrient status at the time of bloom, to identify potential nutrient deficiency observed in the vineyard.

This year, the extensive water stress is complicating how to monitor a vineyard for vine water stress or for nutrient deficiencies. During long periods of water stress vines will transport water and nutrients from the older, basal leaves of the growing shoots to newer leaves on the apical portion of the shoots as well as to developing clusters. Vines suffering severe water stress will develop symptoms of nutrient deficiency in basal leaves, potentially resulting in leaf loss.

Shoot thinning is finishing in northern vineyards. Shoot thinning is an important canopy management tool to improve air circulation, minimize disease pressure, reduce shading, and improve spray penetration. The right time for shoot thinning time is when the shoots are 5-12 inches long. For vinifera cultivars it is recommended to leave between three to five shoots per foot of canopy, with fewer shoots in red varieties and more in white varieties. Hybrid cultivars are more vigorous and ripen earlier, so it is suggested to leave four to six shoots per linear foot of canopy, for higher levels of yield per acre. For Concord and other native cultivars, the number of shoots per foot of canopy can reach as many as 15 shoots, especially in divided canopy trellis systems.

This is also the time to start to clean trunks from the extra shoots that can emerge from buds that were hidden beneath the bark. Suckering is an important early season operation as well, and one of the most challenging. It seems as if suckers have two weeks (for each variety) when they are easy to knock off, and this is the time of the season.

The removal of basal leaves from main shoots and lateral shoots developed from basal nodes is known as early leaf removal, which is an effective way to control yield and improve fruit quality.

Early leaf removal is usually performed prior to or during bloom, after shoot thinning has been done. This is an effective way to control yield and improve fruit quality. By removing the leaves at the bases of shoots at this time of year, airflow is improved and the vine’s physiology is modified. Read here for more detail: Early season vineyard management. In particular, early leaf removal is a viticulture management practice involving the removal of leaves from selected basal nodes along shoots around the time of bloom, and it has been extensively investigated as a means to simultaneously decrease 1) fruit set, 2) control yield per vine, 3) and reduce cluster rot.

Several studies have shown that the percentage of leaves removed as well as the specific timing of defoliation affect fruit quality and cluster rot incidence. When performed at pre-bloom, shoot photosynthesis and carbon allocation to cluster sinks are reduced, decreasing the flow of hexoses to inflorescences, which reduces flower fertility and amplifies the cultivar-specific sensitivity to flower abscission. As a result, fewer berries develop on the cluster, creating a looser cluster morphology. This effect, along with improved air flow around clusters, leads to a reduction in cluster rot for a number of tight-clustered cultivars, including Sauvignon blanc, Pinot noir, Pinot gris, Sangiovese, Riesling, and Vignoles.

Diseases

The rains in the last week produced disease infection events around the state. At this time of year, disease focus is on phomopsis, black rot, anthracnose, downy mildew and powdery mildew. During and for a few weeks after bloom, fungicides that include broad-spectrum/contact fungicides like the EBDCs (FRAC M3) and captan are effective and function similarly to dormant applications by sanitizing the vineyard. If a grower is organic, early season oils may also be used, which try to suffocate fungal spores and infected tissues.

With the exception of powdery mildew, these disease infections typically require rain events. It only takes 0.1 inches of rain above 50 F to trigger a possible infection. Viticultural practices that reduce canopy wetness such as good irrigation timing, leaf removal and good weed management can reduce many of these diseases in a vineyard. Typically, DMIs (FRAC 3), captan and EBDCs (FRAC M3) are effective for phomopsis, black rot, and anthracnose.

If powdery mildew is the only concern, there are a number of products that are effective (FRAC codes 3, 7, 11, 13, U8, 50, and U13 as well as sulfur). A combination of fungicides containing these FRAC classes should also be effective while helping with resistance management. Remember as you choose a fungicide check the guide for potential phytotoxicity of certain sprays on Concord grapes especially (this has been particularly noted for fungicides like Revus Top). Phytotoxicity risk is higher with high temperatures and quickly growing vines. Also there is a significant phytotoxicity risk with specific contact products such as copper and sulfur for Labrusca type grapes (Concord and Niagara).

Downy mildew can cause fruit infection and late season defoliation. During this time of year, grower focus should be on fruit infection which occurs around bloom. Flowers remain susceptible until two weeks after bloom. When making a chemical application, sprays should be timed prior to bloom and at bloom for optimal control. Downy mildew is caused by a fungal-like organism, so many site specific systemic fungicides that target other spring diseases do not work on downy mildew. Effective fungicides for downy mildew include products in FRAC codes 4, 11, 21, 40 and 45 as well as phosphorus acid salts and some biologically-based products.

As the season continues, it is important to remember to manage fungicide resistance and avoid applying similar products back-to-back. This is particularly important with site-specific systemic fungicides. To reduce the development of resistance with systemic fungicides:

• Do not make more than two applications per season of the same FRAC code.
• Do not make two consecutive applications of the same FRAC code.
• Rotate with unrelated fungicides in a different FRAC code that have efficacy on the target pathogen.
• Include a contact multisite fungicide into a program (e.g., sulfur, captan, oils or biological fungicides).

Insects

Catches of male grape berry moth from the first generation are continuing to fall in southwest Michigan and this flight is wrapping up. Based on the degree day model with late May dates for biofix, the second generation should begin in a week or two around July 4, depending on location.

Protection of clusters from larvae is focused on the second generation of this pest. Predicted egglaying starts at 810 GDD base 47 from wild grape bloom. Using the Berrien County bloom date of May 25, the Southwest Michigan Research and Extension Center Enviroweather station is currently at 588 GDD47. The Lawton Enviroweather station is at 539 GDD47 using May 29 as biofix. For a more accurate GDD number for your vineyard, your wild grape bloom date can be entered into the grape berry moth model in Enviroweather to predict when egglaying will start.

Check vineyards for rose chafer, potato leafhopper, and thrips damage, which has been observed in southwest Michigan. Thrips can feed on pollen and developing berries during bloom. As berries increase in size, corking on the skin can be seen. Unfortunately, no treatments can help at this time, as the damage was done a couple weeks ago.

Upcoming meetings

Our regular Southwest Michigan Monday Fruit IPM Updates are available online. You need to register to receive the Zoom link and password for these meetings. The webinars are free and one pesticide applicator credit is available for each meeting. We had over 70 growers attending our Monday meetings last year.

Viticulture Field Day will be returning to an in-person event this year. The event location is 12 Corners Winery in Benton Harbor, Michigan, on July 28, 2021. This will be an all-day event beginning at 9 a.m. The traditional steak dinner and wine tasting will return. There will be limited attendance. Pre-registration is highly recommended.

Related articles

• Southwest Michigan Fruit Update – June 22, 2021
• A Mobile Guide for Grape IPM Scouting in North Central and Eastern U.S.
• Early season vineyard disease management
• Early season vineyard management
• Michigan grape scouting report – June 16, 2021
• Periodical cicadas are emerging: Should Michigan fruit growers be concerned?
• Pest management approaches in a winter or freeze damaged grape vineyard