MSU Extension Grapes

Introduction

The recent cold weather in southern Michigan vineyards this spring has created a need for growers to consider adjusted insect and disease control programs 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 from key pests.

This article was published just a few weeks after this year’s freeze damage, and the actual remaining yield potential will not become apparent until after the secondary buds have pushed and fruit set is complete. So, these guidelines are dependent on managers making decisions about the level of crop remaining later in the season. 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 for next year.

Scouting

If a vineyard block has enough crop to be harvested, regular scouting can help avoid any more surprises. The Michigan State University Extension Mobile Guide for Grape IPM Scouting is available online as a PDF file that you can download to a phone, iPad, etc. and is free.

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 only an hour or two per week, but it can pay off with cost savings in pesticides and a better awareness of the crop and vine conditions. Consider this an investment in the long-term future of the vineyard. The Michigan State University (MSU) Vineyard IPM Scouting Form can help with keeping records of your scouting.

When cutting back on sprays, make every one count. Making sure that applications are made at the optimal stage to control 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 management

Foliage pests. Decisions about 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 no crop will be harvested this year, the cost of protecting vines from leaf-feeding pests like leafhoppers and beetles is unlikely to be economical in juice grape vineyards but may still be valuable in less tolerant hybrid and Vinifera vines. If a lower-than-normal crop is expected, managing foliage pests depends on population pressures and amount of damage seen during scouting.

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, rose chafers and leafhoppers than normal. If no immediate post bloom insecticide application is made, leafhopper infestation can be checked in mid-July to determine the need for controlling this pest.

The leafhopper threshold for juice grapes with a full crop during mid-summer is 10% 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 in frost-damaged sites, so only those vineyards where a high leafhopper infestation is discovered will need treatment.

Hybrid and vinifera wine grapes are less tolerant of foliage damage. If a lower-than-normal crop will be harvested, they can tolerate more leaf damage than during a full crop year, but leaf protection remains important for achieving fruit ripening and vine maturation. An immediate post bloom insecticide application is recommended. Leafhopper infestation can then be checked in mid-July to determine the need for any further actions to control this pest.

Regular scouting can be used to determine the need for and timing of interventions to control foliage pests. MSU's Vineyard IPM Scouting Form can help with keeping records of your scouting.

Cluster pests. A program for control of grape berry moth, which is the main insect 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 grape berry moth 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 also 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 10 rows. Cluster sampling in mid-July can help identify the vineyards where this strategy would be worthwhile.

We have also learned from past experience with frost damage that a short crop can potentially have higher berry moth infestation levels because the existing insect population is concentrated on fewer clusters. This highlights the need to focus efforts including scouting and crop protection into vineyards with a harvestable crop to keep berry moth under control.

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 still be needed to reduce inoculum for subsequent years. In that case, mid- to late-season applications of FRAC 3 fungicides might be sufficient to reduce further infections and produce chasmothecia. There are significant phytotoxicity issues in some grape varieties so beware of using sulfur or copper in sulfur sensitive varieties (i.e., juice grapes and some sulfur-sensitive hybrids).

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.

Scouting is recommended for vineyards throughout June and July. If downy mildew lesions are observed, an application of Captan (if it is allowed on your vines) can be made to eradicate the disease and stop further spread. Scout again two to three weeks later to check if further control is needed. For wine grapes, 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 curative activity and can stop disease development 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 moderate activity against Phomopsis and moderate activity against black rot. However, for these diseases, EBDCs (e.g., mancozeb) may be cost effective.

Fruit rot diseases. Black rot and Phomopsis are the main cluster diseases for juice grapes, especially if there is a lot of overwintering inoculum (fungi are not affected by a freeze). 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 post bloom spray because the berries become naturally resistant to infection about four to five weeks after bloom. FRAC 3 fungicides plus Ziram or even FRAC 3 fungicides alone will suffice. Other options are FRAC 11 fungicides, such as Abound.

Phomopsis control becomes important as soon as the flower clusters become visible. 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 periods, fewer sprays will be necessary. EBDCs (e.g., mancozeb) are a cost-effective material for use against Phomopsis prior to bloom, and Ziram can be used after bloom.

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. Applications of FRAC 9, 12 or 17 fungicides are most effective for control of this disease.

Coverage

Since cluster protection is the main focus of a reduced pest management 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 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, especially for products such as Intrepid that requires contact with eggs or young larvae. Although this will take more time, getting the maximum effect out of every spray is particularly important when yields are expected to be low.

Product selection

Under times of financial challenge, the temptation may be to choose the least expensive option to achieve control. It is good to keep in mind other factors, including if the product is effective under the current and predicted weather conditions; how long it lasts; and how well it controls 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, especially if you are targeting one pest. The MSU Fruit Management Guide contains recommendations on which products are most effective and their spectrums of activity. It is available for order from the MSU Bookstore in print or downloadable PDF format.

Our sincere thanks to Mike Reinke for reviewing an earlier draft of this article.