MSU Extension Grapes

Weather

Last week began cool with highs in the low 60s and lows in the 40s. A storm system entered Michigan on Thursday, bringing rain and very humid conditions. Northern grape-growing areas also saw high temperatures in the mid-70s and lows in the lower 60s. Southern areas were 5-10 degrees warmer. The heaviest rains came on Friday and Saturday, but the unsettled conditions will continue through Wednesday for much of the state. Southern areas saw 3-6 inches of rain, northern areas an inch and a half.

The forecast for all regions of the state is for more settled conditions. High temperatures are expected to be in the mid- to upper 70s and lows near 60 for much of the week. A warm up is possible at the start of next week. After Wednesday, there are no significant chances of rain in the forecast.

The recent rains have helped alleviate the dry soil conditions, especially in the southern parts of Michigan. According to the drought monitor, most of lower Michigan is in a moderate or severe drought. The significant rains in southern regions from the past week will be reflected in the next update that comes out Thursday. Northwest Michigan drought conditions are not expected to change much. Even with the recent rains, all of Michigan is still in a deficit. Regular monitoring of soil moisture conditions is still recommended.

With the seasonal week, we picked up an average number of growing degree days (GDD) last week: 100-140 GDD base 50. The southwest region is 295 GDD base 50 ahead of the northwest region. Statewide, we are approximately one week ahead of the 5 year average.

Vine growth

In southwest Michigan, vinifera grape bloom has ended. Juice and hybrid grape vineyards not impacted by the early May freezes are at berry touch. 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 are between buckshot and pea-size berry on secondary shoots. 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, at many sites, the bloom is nearing an end and most varieties are at buckshot to pea size berry. The northwest experienced similar temperatures in early May, but the grapes were behind phenologically when compared to vines in the southwest, so most grapes survived the spring cold events without significant damage in the majority of the sites, spring frost damage has been reported in some vineyards in the Petosky area. See this chart for grape growth stages.

Horticulture

Shoot thinning should be finished in all areas of Michigan. Suckering and trunk cleaning is continuing. Trunk cleaning is the act of removing the extra shoots from the trunk 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 should be finished in southern Michigan, continuing in northern Michigan. It is usually performed around 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, it decreases fruit set, controls the yield per vine, and reduces cluster rot.

Diseases

The rains in the last two weeks produced disease infection events around the state. At this time of year, diseases that may begin to be visible are phomopsis, black rot, anthracnose, downy mildew and powdery mildew. That said management should be primarily targeting downy mildew and powdery mildew at this time. Fungicides that include broad-spectrum/contact fungicides like the EBDCs (FRAC M3) and captan are effective products and nice tools for resistance management. 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. As we get farther into the season it will be difficult to manage black rot infections.

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).

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.

Downy mildew can cause fruit infection and late season defoliation. 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 we approach berry touch, it is important to consider botrytis management. Several strategies contribute to good botrytis bunch rot management. These include opening up the canopy, properly applying fungicides and using resistant cultivars when possible. Good botrytis control depends on getting good coverage and this is the last chance to apply a fungicide to the inner part of the developing cluster. Fungicide resistance management is also important. The most effective products for botrytis are site specific and prone to resistance development. A new Michigan Grape Fact Sheet is now available for managing botrytis bunch rot.

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).

This is a good time to scout for suspicious issues related to other vine stressors. Many of these problems can be abiotic including nutrient related issues and graft incompatibility, but some of the biotic problems can be caused by grapevine trunk diseases or grapevine viruses. If you are interested in taking samples and testing, you can send them to MSU Plant & Pest Diagnostics. Please see these articles on grapevine trunk disease and sampling for grapevine viruses.

Insects

Catches of male grape berry moth have declined in southwest Michigan. Based on the degree day model with late May dates for biofix, egg laying of the second generation should begin this week.

Protection of clusters from larvae is focused on the second generation of this pest. Predicted egg laying starts at 810 Growing Degree Days base 47 from wild grape bloom. Using the Berrien County bloom date of May 25, the SWMREC weather station is currently at 782 GDD₄₇. The Lawton weather station is at 729 GDD₄₇ using May 29 as the average biofix in that county. The start of second generation egg laying is predicted to be July 1 for SWMREC and July 3 for Lawton. For a more accurate growing degree day number for your vineyard, your wild grape bloom date can be entered into the grape berry moth model in Enviroweather to predict when egg laying will start.

Once egg laying has begun, there are a number of insecticides that can be used to prevent grape berry moth infestation, but their timing is critical to give the best performance. Insecticides that affect eggs should be applied at the beginning of egg laying at 810 GDD₄₇ after wild grape bloom. The most commonly used examples are Intrepid and Altacor. Insecticides that are best used against berry moth larvae should be applied when eggs start hatching. This is 100 GDD₄₇ after the start of egg laying (910 GDD₄₇). At this time of year, this is approximately 5 days after the start of egg laying. There are a number of broad-spectrum insecticide options that have activity against young larvae such as Altacor, Delegate, and Verdepryn. Imidan is also effective against grape berry moth, but should be applied in water that is pH 5-5.5 for maximum activity. For vineyards that had significant berry moth damage last year or those that already have damage this year, a second spray targeting larvae should be applied two weeks after the upcoming spray. For more detailed information to help with choosing the right insecticide, see: Mid-season management of grape berry moth

Check vineyards for rose chafer, potato leafhopper, and thrips damage, which has been observed in southwest Michigan. Rose chafer activity has ended in southern Michigan, but they are still active in northwest grape growing areas. They feed on leaves and flowers. Populations are typically higher in vineyards with grassy areas and sandy soils.

 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.

Japanese beetles are now active in some vineyards, feeding on leaves. Labrusca grapes can tolerate damage from Japanese beetles, but vinifera and hybrid wine grapes with thinner leaves are more easily defoliated. Treatment is usually only necessary if significant defoliation is seen. A well timed broad spectrum insecticide treatment should help manage grape berry moths and Japanese beetles at the same time.

Upcoming meetings

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 29, 2021
• A Mobile Guide for Grape IPM Scouting in North Central and Eastern U.S.
• Early season vineyard disease management
• Mid-season management of grape berry moth
• Early season vineyard management
• Michigan grape scouting report – June 23, 2021
• Pest management approaches in a winter or freeze damaged grape vineyard