MSU Extension Grapes

Introduction

The damaging weather conditions in southern Michigan vineyards this spring have created a need for growers to consider an adjusted insect and disease control program for frost-damaged vineyards. The comments below are intended to help growers reduce pest management costs while maintaining a program to address critical needs for vine protection.

Even though the current yield loss estimates are high, it should be kept in mind that the actual remaining yield potential will not become apparent until after the secondary buds have pushed and clusters have appeared. These guidelines are, therefore, dependent on managers making decisions about the level of crop remaining. If shoots were heavily damaged by frost but there are enough clusters to harvest some fruit, the focus should be on minimizing the cost of pest management inputs while maintaining quality and yield of the remaining fruit. In a year with a small crop load, the foliage will easily be able to produce sufficient sugars and mature the fruit as well as buds and wood for next year. Therefore, the need to protect the foliage from damage by insects and diseases is much lower. In fact, increased canopy size can become a problem due to increased shading, which leads to reduced formation of fruit buds.

Scouting

If a crop is to be harvested from a vineyard, regular scouting can help avoid any more surprises. At the very least, checking vineyards post bloom, in mid-July, and in early August can provide the minimum of information regarding development of key insect pests and diseases. If the cost of hiring a scout seems too much, try negotiating a lower price before canceling this service.

Alternatively, walking the rows once a week can help you keep up to date on vine and pest development and will cut down the cost of this service. This might take about 1 hour per week. It may not seem worth it to spend any time in some badly affected vineyards, but consider this an investment in the long-term future of the vineyard. Michigan State University’s Vineyard IPM Scouting Form can help with keeping records of your scouting.

Insect management

Foliage pests

Decisions for insect control will depend on the expected yield from each vineyard. If it is expected to be close to normal, a typical insect control program should be maintained to guarantee the expected yield and quality. If a lower than normal crop will be harvested, juice grapevines can tolerate leaf damage and still ripen the reduced crop. Because of this, it will be much less important to control Japanese beetles, rose chafers, and leafhoppers than normal. If no post-bloom insecticide application is made, leafhopper infestation can be checked in mid-July to determine the need for controlling this pest.

The threshold for juice grapes with a full crop at this time of the season is ten percent of leaves infested. Thresholds have not been developed for situations with a reduced crop, but they are likely to be much higher as the crop load decreases. Only those vineyards where a high leafhopper infestation is discovered will need treatment. If no crop will be harvested this year, the cost of protecting vines from leafhoppers and beetles is unlikely to be economical in juice grape vineyards.

Hybrid and Vinifera vines are less tolerant of insect feeding than juice grape varieties. A full program to protect the foliage in these varieties is important if any crop harvest is expected in order to achieve fruit ripening and vine maturation.

Cluster pests

Grape berry moth is the main pest of grape cluster. Therefore, it should be a priority to maintain a program for grape berry moth control if any grapes are to be harvested. This will help minimize crop loss this year and reduce the risk of high infestations next year. In a reduced crop situation, the remaining clusters will be under increased pressure from berry moth infestation. It is still important to identify wild grape bloom to determine grape berry moth biofix for the year. This is used to determine the timing for the second generation emergence when management usually starts for this insect. The second generation usually begins in late June to early July in southwest Michigan. If by this time an assessment indicates the crop will likely be harvested, a full grape berry moth program is warranted to protect from the increased pressure.

If harvest is unlikely, a reduced program can be implemented. Care should be exercised, however, because any areas not protected have the potential to become hotspots that could increase infestations in neighboring blocks. In these situations, a single application of Intrepid or Intrepid Edge at the typical 810 and 1,620 growing degree days (GDD) base 47 after biofix should suppress the berry moth population enough to reduce the pressure on neighboring blocks. The grape berry moth model in Enviroweather can be used to determine these timings.

It is worth keeping the sprayer on hand after veraison in case populations of grape berry moth continue to develop close to harvest. If this occurs and berries are at risk from infestation, a well-timed effective insecticide may be warranted prior to harvest to minimize risk of infestation in harvested berries. If grape berry moth infestation is restricted to wooded borders, cost savings may be achieved in some vineyards by applying border sprays to the outer ten rows. Cluster sampling in mid-July can help identify vineyards where this strategy would be worthwhile.

Disease management

Foliar diseases

The main foliar diseases that are important in Michigan juice grapes are powdery mildew in Concord and downy mildew in Niagara grapes. If no fruit will be harvested, foliar diseases are the only diseases that need to be considered. As with insects, vines with a small crop load will be able to tolerate more foliar disease.

In Concord grapes, control of powdery mildew may not be needed at all, unless there a concern about excess inoculum production for next year. In that case, one or two mid- to late-season applications of a sterol inhibitor fungicide will probably be sufficient to reduce further infections and production of chasmothecia. Oils can be a lower-cost alternative for control of powdery mildew. Oils have the added benefit of killing powdery mildew colonies on contact. Downy mildew can be more harmful than powdery mildew, as it can lead to severe defoliation and reduced winter hardiness of the vine. Even though vines with a small crop load can withstand more downy mildew than heavily cropped vines, it should not be allowed to go completely out of control. This is also important from the standpoint of overwintering inoculum for next year.

Scout the vineyards in mid-July. If downy mildew lesions are observed, an application of Ridomil will eradicate the disease and stop further spread. Scout again two to three weeks later to check if further control is needed.

Less costly alternatives are copper products (not to be used in Niagara or Concord), phosphorous acid fungicides (e.g., Phostrol, ProPhyt) and Ziram. Ziram is strictly a protectant, whereas phosphorous acid products have curative activity and can stop disease development for days after an infection has started (this is when the lesions are just starting to show). They don’t have much residual activity, however, so they may need to be tank-mixed with Ziram to get longer protection. The phosphorous acid products also have good activity against Phomopsis and moderate activity against black rot.

For wine grape growers that have already applied dormant sprays, you could expect a reduction in powdery mildew if you applied sulfur, and a reduction in downy mildew if you applied a copper fungicide. In small plot trials in Michigan, reductions of 40-60% were observed compared to untreated plots.

Fruit rot diseases

Black rot and phomopsis are the main cluster diseases to be considered if there is sufficient fruit to harvest, especially if there is a lot of overwintering inoculum (fungi are not affected by a freeze). Black rot control should be focused around bloom. The first and second post-bloom sprays being most important. There is generally no need to protect the fruit beyond the second post-bloom spray, because the berries become naturally resistant to infection about four to five weeks after bloom. Tebuconazole plus Ziram or even tebuconazole alone should suffice. FRAC 11 fungicides like Abound can also be effective.

Phomopsis control becomes important as soon as the flower clusters become visible. This will happen a little bit later in the year for secondary buds. Phomopsis spores will be released during most rain events from budbreak until about bunch closing. A peak in spore production usually occurs around the first and second week in May. This may be a good time to protect shoots from infection. The amount of overwintering inoculum can be estimated from the number of lesions on current season shoots and leaves. Fewer sprays will be necessary during dry spells. In many years, we have not seen a benefit from sprays beyond the first post-bloom spray.

EBDCs are a cost-effective material for use against Phomopsis prior to bloom, and Ziram can be used after bloom. For growers that have already applied dormant sprays, you can expect a substantial reduction in Phomopsis through the season. The only other sprays that may be needed are an Abound spray at bloom or first post-bloom, and in a wet spring, a EBDC pre-bloom. Pristine may be an effective option in Niagara, but the is some evidence of phytotoxicity on Concord and Niagara type grapes.

Botrytis bunch rot is usually only a concern in tight-clustered Vinifera and hybrid cultivars (e.g., Pinot Noir or Vignoles, respectively). Protection may be needed if conditions are wet between bunch closure and harvest, with veraison being a critical time. A bloom spray usually is not cost-effective. One or two applications of a fungicide in FRAC codes 9, 12 or 17 are most effective for control of this disease.

Coverage

Because cluster protection is the main focus of a reduced insect control program, it is best to target sprays to the fruiting zone to maximize the effectiveness of sprays. For effective grape berry moth control, spray deposits must reach the whole cluster. This becomes more challenging as the vine canopy grows. As the season progresses, spray volume should be increased and every row should be treated.

Field trials with an airblast sprayer have shown that a spray volume of 50 gallons per acre achieved substantially better disease control, particularly with protectant fungicides, than a spray volume of 20 gallons per acre. The same result was found for control of grape berry moth—increasing gallonage to 50 gallons provided better control than 20 gallons. Although this will take more time, getting the maximum effect out of every spray is particularly important when yield is expected to be low.

Product selection

Under times of financial challenge, the temptation may be to choose the least expensive option to achieve control. This may seem the best choice, but it is good to keep in mind other factors. For example, is the product effective under the current and predicted weather conditions; how long does it last; and how well will it control the target pest or disease?

In the long run, it may be more cost effective to use a slightly more expensive product that lasts longer than the cheapest option. Depending on existing pest and disease pressure, a lower labeled rate may be used, though.

Timing

When cutting back, make every spray count. Making sure that applications are made at the optimal stage for control of your target pest is another way to help cut costs. It may take a little more time to check vineyards closely every few days, but doing this can be a cost-effective way to improve the impact of your spray program. By doing this, you may also find that pests and diseases are not as bad as expected, and the cost of an application can be saved.