Weather

Last week weather was variable. The week started with highs in the 80s statewide. A front passed through Michigan on Tuesday and Wednesday bringing a tenth to a half inch of rain to grape growing regions as well as a 10 degree drop in temperatures. Another front on Thursday and Friday dropped temperatures even further and added another tenth to three quarters of an inch of rain around the state. High temperatures on the coldest days were in the upper 40s for much of the state, and mid-50s in the southwest. Frost watches and warnings were widespread around Michigan Starting Thursday morning continuing through Sunday morning.

The next week is expected to be more consistent. The week will start with high temperatures in the low 70s climbing to the mid-80s by the weekend. Lows will climb from the low 50s to the mid-60s. There is a chance of scattered showers on Wednesday in southern Michigan and on Friday in northern grape growing areas. No significant amounts are expected.

Total rain for the week was a third to one inch. Soils are still very dry, especially in southwestern portions of the state. At this time, most of lower Michigan is in a D1 level drought. Portions of Van Buren and all of Allegan counties and areas just to the north and east are in a D2 (Severe) drought condition. This is extremely unusual at this time of year.

With the variable week, we picked up an average number of growing degree days GDD last week: 50-75 GDD base 50. The southwest region is 195 GDD base 50 ahead of the northwest region. Statewide, we are 100-140 GDD base 50 ahead of the five-year average.

Vine growth

In the southwest, a lot of variability exists between vineyards. A series of classic radiation frost/freeze events starting the morning of May 5 and continuing through May 14 caused damage to grapes in many parts of southwest Michigan. Those sites impacted by the freezes had varying levels of primary bud loss or foliar damage, with some areas as high as 90%. In many of these sites, secondary shoots are 3-6 inches long and clusters are visible. 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 southwestern sites not damaged by the freezes, shoot growth continues. Shoots are 10-18 inches long in many areas. Clusters are elongating and separating. Bloom has been seen on some early hybrid wine grapes.

In northern vineyards, many sites have shoots that are 6-12 inches long. Clusters are beginning to elongate. 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. Be careful with the amount of nitrogen applied in your vineyard. Too much can cause flowering to be delayed or aborted and increase vegetative growth with detrimental effects on fruit quality (canopy shading), cold hardiness and sensitivity to foliar and cluster diseases. Too little nitrogen will result in reduced vigor, stunted growth and poor fruit quality.

Grapevine nutrient status is determined by three methods: 1) Observing visual symptoms, 2) analyzing vine tissue samples and 3) performing soil tests. The prebloom and bloom time are very important moments 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. Tissue analysis may be performed on different types of vine tissues (petiole and leaf), however, petiole tissue analysis is recommended in our region because of the lack of historical data for nutrient range recommendations of leaf blade tissue.

If you observe leaf symptoms and a potential nutrient deficiency, collect petiole samples from vines reporting symptoms and from asymptomatic vines. The two samples should be sent and analyzed separately for comparison.

Shoot thinning is underway in southern vineyards now and may begin in northern vineyards in the next week or two. 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.

Diseases

At this time of year, disease focus is on phomopsis, black rot, anthracnose and powdery mildew. Before bloom, fungicides that include broad-spectrum/contact fungicides like the EBDCs (FRAC M3) and captan are effective and function similar to dormant applications by sanitizing the vineyard. For organic growers, early season oils may also be used, which try to suffocate overwintering fungal spores and infected tissues. Early season management should target phomopsis, black rot, anthracnose and powdery mildew, and as we approach bloom, downy mildew.

With the exception of powdery mildew, these spring disease infections typically require rain events. It only takes 0.1 inches of rain above 50 degrees 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.

If wet weather is in the forecast, make sure plants are protected as we head into bloom, particularly from downy mildew cluster infections which are typically susceptible up to 12 days prebloom.

Insects

For southern Michigan grape growers, the insects to be on the lookout for are rose chafer and potato leafhopper. Potato leafhopper have been reported on other crops in Michigan already, and so both should be expected in vineyards soon. These are only a challenge in certain sensitive winegrape cultivars. Rose chafers can be found feeding on leaves and clusters. Potato leafhoppers feed on the undersides of leaves. Most cultivars are very tolerant of potato leafhopper damage and control isn’t often warranted.

Male grape berry moth have been trapped for a few weeks in southwest Michigan. Wild grape bloom is used as biofix for grape berry moth models. Record the date when 50% of the clusters on wild grape are at 50% bloom. This date can be entered into the grape berry moth model in Enviroweather to predict when egglaying will start. Wild grape bloom was widespread in areas of Berrien County on May 25 and was observed in Van Buren County on May 29. With the variability in bloom date that can occur from site to site, it is important to scout for wild grape bloom on your farm to determine your local biofix.

In southwest Michigan, protection of clusters from larvae is focused in late June or early July when egglaying by the second generation of this pest starts. In high pressure sites where this pest has been a problem in recent seasons, early treatment for protection of the clusters at bloom in mid-June may also be warranted. Product selection can be guided by what other pests may be present, such as potato leafhopper, rose chafer, etc. Treatment recommendations for all of these insects can be found in the MSU Fruit Pest Management Guide (E-154).

Tumid gallmaker has been seen in hybrid wine grapes in southwest Michigan. This insect was an issue for some growers in susceptible vineyards last year during early cluster development. Scouting for early detection is key for this insect. For small vineyards, hand picking and removing infestations can be an effective management technique.

If there is a high infestation or if clusters are affected, Movento is an effective treatment. It works by moving into the plant and metabolizing to create the toxin. This should be applied at the first sign of infestation to provide enough time for it to get into the vine and be converted into the toxin. Movento needs to penetrate the waxy leaf surface, so application with a penetrating adjuvant is essential to get control.

Grape cane girdler damage has also been reported in southwest Michigan. This weevil can produce damage that looks similar to some tumid gallmaker damage. It produces red galls that usually have a slit or opening on one side on shoots just above the nodes. While the damage is noticeable, it rarely impacts yield.

Regional and national news outlets have been reporting on Brood X of the periodical cicada lately. This insect emerges once every 17 years. It is only expected in south central and southeastern portions of Michigan, but is not expected to be an issue for growers. We do not expect periodical cicadas to be a significant concern for grape growers in Michigan, but if you are interested, check out this article on the periodical cicada.

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.

Related articles

• Southwest Michigan Fruit Update – June 1, 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 – May 26, 2021
• Periodical cicadas are emerging: Should Michigan fruit growers be concerned?
• Pest management approaches in a winter or freeze damaged grape vineyard