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| Pest management in frost-damaged vineyards |
| Rufus Isaacs, Entomology Annemiek Schilder, Plant Pathology |
| Introduction The unique situation in Michigan grapes 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 flower clusters have appeared. 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. ScoutingIf 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. 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. |
| Insect management
Foliage 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 third week 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 (as they did in 2005). 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 diseasesThe main foliar diseases that Michigan juice grape growers need to worry about 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. Control of powdery mildew in Concord grapes may not be needed at all, unless there is a lot of disease leading to 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 inoculum production. Highly susceptible Vinifera varieties and hybrids may need closer monitoring and control. Sulfur (for non-sulfur sensitive varieties) and JMS Stylet Oil are lower-cost alternatives for control of powdery mildew. However, they may need more frequent application. 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 common, an application of Ridomil can be made to eradicate the disease and stop further spread. Scout again 2-3 weeks later to check if further control is needed. Protectants such as copper (for non-copper sensitive varieties), Bordeaux mixture, and ziram are less costly alternatives, but these will not eradicate the disease. They will only serve to prevent further infections and may need to be reapplied if weather conditions are conducive to the disease. Fruit rot diseases Black rot and Phomopsis are the main cluster diseases to control if there is sufficient fruit to harvest, especially if there is a lot of overwintering inoculum (fungi are typically not affected by a freeze). If very little disease was present last year, fungicide applications are not as critical. Black rot control should be focused around bloom, with the first and second post-bloom sprays being most important. If there is a lot of overwintering inoculum and conditions are conducive, an immediate prebloom spray may also be warranted. However, there is generally no need to protect the fruit beyond the second postbloom spray, because the berries become naturally resistant to infection. Phomopsis control becomes important as soon as the flower clusters become visible, which will happen a little bit later this year as the secondary buds are only starting to push now. If there is a lot of overwintering inoculum, Phomopsis spores will be released during most rain events until about bunch closing. If there was not much disease the previous year, Phomopsis sprays may only be necessary until pea-size berry. If you don’t know how much disease there was last year, the amount of overwintering inoculum can be estimated from the number of lesions on current-season young shoots and leaves. During a dry spell, fewer sprays are necessary as well. Last year, there was no benefit from sprays beyond the first post-bloom spray. The EBDCs are cost-effective materials for use prior to bloom, and Ziram can be used after bloom. Botrytis bunch rot is primarily a concern in Vinifera and hybrid grapes. Protection may be needed if conditions are cool and wet in the period between bunch closing and harvest, with veraison being a critical time. A bloom spray typically is not cost-effective. One or two applications of a fungicide like Vangard 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 and so as the season progresses, spray volume should be increased and every row should be treated. 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. |
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05/24/11 Contact: E. Haney