MSU Extension

Introduction

The full extent of frost damage to the 2012 crop may not be known for a month, but the situation in some Michigan vineyards this spring has created a need for growers to reconsider their spray programs. Guidance on an adjusted insect and disease control program for frost-damaged vineyards is presented here. The comments below are intended to help growers reduce pest management costs while maintaining a program to address critical needs for vine protection.

We emphasize that there is no “prescription.” Growers need to assess their own sites, decide on whether vineyard blocks will be harvested, and then use regular scouting and knowledge of pest history to keep insect pests and diseases below levels that will cause economic injury. In a year like this, some vineyards will not need cluster protection because the crop is lost completely. These sites will then be better able to withstand leaf injury from insects so costs may be cut there, too. While the vines will also be able to tolerate more foliar disease, some level of disease control may still be needed to reduce inoculum production and ensure that the vines are healthy for 2013. Weekly scouting of vineyards, being aware of the crop load, and making decisions based on the facts will go a long way to ensure you are minimizing the cost of managing frost-damaged vineyards.

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 (see MSU’s Paolo Sabbatini’s article, “The effects early spring had on Michigan juice grapes”). 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, and increased canopy density which may impede thorough spray coverage.

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 pre-bloom, post-bloom, mid-July, and 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 crop and pest development and will cut down the cost of this service. This might take as little as one 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.

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 10 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 in frost-damaged sites, so only those vineyards where a high leafhopper infestation is discovered will need treatment.

Additionally, there are many highly effective insecticides for control of leafhoppers (seeMSU Extension publication E-154, “2012 Michigan Fruit Management Guide”), so these can be controlled quickly if discovered. If no crop will be harvested this year from a block, 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.

Cluster pests. A program for control of grape berry moth, which is the main pest of grape clusters in Michigan, 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. We do not recommend use of an immediate post-bloom insecticide to vineyards because this generation causes little injury, and this pest can be controlled well by use of well-timed sprays for generations two and three. Sampling vineyards in early 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, too, in case populations of grape berry moth continue to develop closer to harvest. This happened in 2010 when a fourth generation developed.

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 in your vineyards, cost savings can be achieved by applying border sprays to the outer 10 rows. Cluster sampling through vineyards 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. The other factor to consider is the weather; warm, dry conditions will be more conducive to powdery mildew, whereas cool, wet conditions and heavy dews (more common in late summer) promote downy mildew.

In Concord grapes, control of powdery mildew may not be needed at all, unless there is a concern about inoculum production for next year. In that case, one or two mid- to late season applications of an inexpensive sterol inhibitor fungicide (e.g., a generic tebuconazole) will suffice to reduce infection and production of cleistothecia. Alternatively, a single application of a contact fungicide, such as JMS Stylet Oil (or other oil), in late August to early September can knock out cleisthothecium production on existing colonies. In the latter case, thorough coverage is essential, for example, by using a higher spray volume and spraying every row.

Downy mildew is likely to be more harmful than powdery mildew, at least in ‘Niagara’ grapes, 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 a phosphorous acid product (i.e., Phostrol) is recommended to stop sporulation and further spread. A booster spray five days later will improve control. Scout again two to three weeks later to check if further control is needed.

Less costly protectant fungicides are copper products (for non-copper sensitive varieties), captan (not allowed on juice grapes after bloom), and Ziram. Some of the newer downy mildew fungicides, such as Reason and Forum, are also relatively inexpensive and are best applied as protectants. Obviously, the use of broader-spectrum materials will benefit control of other diseases as well and may give you more “bang for your buck” per spray application.

Fruit rot diseases. Black rot and Phomopsis are the main cluster diseases to be considered in juice grapes if there is sufficient fruit to harvest, especially if there is a lot of overwintering inoculum. If your vineyard had low disease pressure last year, 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 post-bloom spray, because the berries become naturally resistant to infection about four to five weeks after bloom. An inexpensive sterol inhibitor fungicide will suffice; ziram may be added to broaden the control spectrum to include Phomopsis and downy mildew.

Phomopsis control becomes important as soon as the flower clusters become visible, which will happen a little bit later this year and may be variable as we will rely more on the secondary buds. Phomopsis spores will be released during most rain events from bud break 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. During dry spells, fewer sprays will be necessary. The first post-bloom spray is also an important spray for Phomopsis and can be combined with the black rot spray. Mancozeb is a cost-effective material for use against Phomopsis prior to bloom, and Ziram or a phosphorous acid fungicide can be used after bloom. For growers who have already applied a dormant spray, this will help reduce disease pressure of Phomopsis and black rot through the season.

Coverage

Because cluster protection is the main focus of a reduced insect and disease control program for frost damaged sites, it is best to target sprays to the fruiting zone to maximize the effectiveness of sprays. Coverage is particularly important as increased canopy size due to a small crop may impede spray penetration. For effective grape berry moth and fruit rot 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.

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 diseases are not as bad as expected, and the cost of an application can be saved.

Drs. Isaacs and Schilder’s work is funded in part by MSU’s AgBioResearch.