Pest management in frost-damaged vineyards

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.  

Introduction

The damaging weather conditions in southern Michigan vineyards this spring has 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 for maturation of 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. A form to help with keeping records of your scouting is available at www.isaacslab.ent.msu.edu/grapescout/scout.pdf

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 beetle, rosechafers, 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. Although thresholds have not been developed for situations with a reduced crop, they are likely to be much higher as the crop load decreases. As mentioned above, the need for foliage protection will be low this year, so 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. If bearing vineyards of these varieties are infested by foliage pests, leaf protection remains important for achieving fruit ripening and vine maturation. Regular scouting can be used to determine the need for, and timing of, interventions to control foliage pests. See above for a link to a scouting form.

Cluster pests. A program for control of grape berry moth, which is the main pest of grape clusters, should remain a priority if any grapes are to be harvested. This will help minimize crop loss this year, and will reduce the risk of high infestations next year. Application of a post-bloom insecticide to vineyards that have a history of high GBM infestation is warranted if the vineyard will be harvested. Sampling again in the first half of July (same time as leafhopper samples above) can be used to determine whether the cost of further insecticide applications is warranted. 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 cleistothecia. Sulfur (for non-sulfur-sensitive varieties) and JMS Stylet Oil are lower-cost alternatives for control of powdery mildew. JMS Stylet Oil has 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.

I would recommend scouting of vineyards in mid-July. If downy mildew lesions are observed, an application of Ridomil can be made to eradicate the disease and stop further spread. Scout again 2 to 3 weeks later to check if further control is needed. Less costly alternatives are copper products (for non-copper sensitive varieties), phosphorous acid fungicides (e.g., Phostrol, ProPhyt) and Ziram. Coppers and Ziram are strictly protectants, whereas phosphorous acid products have strong curative activity and will stop disease development for up to 6 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 growers that have already applied dormant sprays, you can 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). Luckily, most vineyards experienced low disease pressure in 2005, so fungicide applications may not be as critical this year. Black rot control should be focused around bloom, with the first and second post-bloom sprays being most important. There is generally no need to protect the fruit beyond the second postbloom spray, because the berries become naturally resistant to infection about 4 to 5 weeks after bloom. Elite + Ziram or even Elite alone will suffice. Other options are strobilurins, such as Abound. Phomopsis control becomes important as soon as the flower clusters become visible, which will happen a little bit later this year as we will rely more on the 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, which 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. During dry spells, fewer sprays will be necessary. In many years, we have not seen a benefit from sprays beyond the first post-bloom spray. Mancozeb is 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 postbloom, and if a wet spring, a mancozeb pre-bloom. Pristine may be a cost-effective option in Niagara, but the label claims a risk of phytotoxicity on Labrusca-type grapes. Botrytis bunch rot is primarily a concern in tight-clustered Vinifera and hybrid grapes. Protection may be needed if conditions are wet in the period 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 like Vangard are most effective for control of this disease. Scala may be a lower-cost alternative for Botrytis control.

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 and so 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 gpa achieved substantially better disease control, particularly with protectant fungicides, than a spray volume of 20 gpa. 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 on sprays, make every one 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/or diseases are not as bad as expected, and the cost of an application can be saved.

Insect and disease control approaches in frost-damaged Concord or Niagara vineyards.

Timing
No harvest
Partial harvest
Budswell/
1-2 inches of shoot growth
Sprays of sulfur or copper at this time may be an inexpensive means to reduce powdery and downy mildew during the season and inoculum production for next year.
Sprays of sulfur or copper at this time can provide a substantial reduction in Phomopsis and black rot at harvest; powdery mildew will also be reduced by sulfur, and downy mildew by copper.
Pre-bloom
No insect or disease control needed
Control of Phomopsis needed only if it was a problem last year.
Bloom/
Post-bloom
No insect or disease control needed.
Controls only needed if history of GBM pressure in that vineyard
If field has history of black rot and/or Phomopsis, this is the best time to apply at least one spray for control. First post-bloom most important.
Mid-season
Foliage protection from insect pests is unlikely to be needed.
Scout for downy mildew and treat if infections are common.
Check clusters for GBM infestation. Treat only if infestation is detected.
If controlling black rot and Phomopsis, stop after 2nd post-bloom spray. Scout for downy mildew and powdery mildew and treat if infections are common.
Late-season
Foliage protection from insect pests is unlikely to be needed.
Scout for downy mildew and powdery mildew and treat if infections are common.
Check clusters for GBM infestation. Treat only if infestation is detected.
Scout for downy mildew and treat if infections are common. At this time, it is probably too late for powdery mildew to have a negative impact.

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