Spring grape disease management: what to consider
Implementing these tips in your vineyard this spring will make your disease management a success.
Disease history of the vineyard
Most vineyards do not have a history of all grape diseases. Growers should focus their disease control efforts on the diseases they know are a problem for them; knowledge of the amount of disease the previous year(s) will be helpful in determining the amount of risk the vineyard is facing this year. Keep this in mind for next year and keep a record of disease occurrence in your vineyards, also, to gauge the efficacy of your disease control program.
Disease pressure will depend on the weather conditions, the cultivar grown, the age of the vineyard, the location and the training or canopy management system. For instance, mechanically pruned vineyards tend to have more Phomopsis than hand-pruned vineyards. If few sprays were applied last year due to a low crop caused by spring frost damage, more inoculum may have overwintered, so it would be beneficial to stay on top of the fungicide spray schedule this year, especially if you expect to have a large crop.
In addition, the wet spring so far may contribute to earlier powdery mildew, downy mildew and Phomopsis infections and potentially higher pressure than last year, but it all depends on if the wet trend continues.
If you are still in pruning mode, prune out dead canes and stubs as much as possible since they are the main sources of Phomopsis spores. Remove any fruit mummies still hanging on the vine, since these may release black rot spores. Also, remove large pieces of wood from the vineyard and burn them. This is especially important in Eutypa-infected vineyards, since dead wood remains a source of Eutypa inoculum for multiple years. While it is recommended to remove pruned canes from vineyards, most growers find it more practical to chop them up. This is fine, provided that the canes are well pulverized so that they can decompose quickly. Make two passes with a brush-chopping mower if necessary.
Timing of disease control measures
Timing of disease control measures is critical to success. Protectant fungicides have to be used before an infection period and are best applied before rain. While rainfall of more than 1 to 2 inches can substantially reduce fungicide residue, there is often sufficient active ingredients left to still provide moderate to good disease control. “Stickier” formulations are better and spreader-stickers may improve adherence of the fungicide to the plant surface. Applications should be made no more than two to three weeks apart, depending on weather conditions and how fast the shoots grow. If there are frequent rain events or the shoots grow rapidly resulting in dilution of fungicide residues, then the spray interval may need to be shortened to 7 to 10 days.
Between 1 and 10 inches of shoot growth, Phomopsis cane and leaf spot is the primary disease of concern. Clusters and shoots are vulnerable as soon as they become exposed. Young tissues are most susceptible. Spray timing trials have indicated that this stage is important for controlling cluster stem (rachis) and shoot infections, whether by actual protection of the tissue from infection or by inactivating the fungus as it is “waking up” from dormancy in the canes. Wet weather conditions during this period of rapid shoot elongation are ideal conditions for the infection and spread of Phomopsis.
Powdery mildew control should not be delayed in Vinifera and susceptible French hybrid vineyards past the 10-inch growth stage. However, in most Concord vineyards, powdery mildew control is not necessary and can wait until bloom. Black rot may be an issue in vineyards that had a problem the previous year, but control is usually not needed until immediate pre-bloom, as we generally focus on protecting the fruit only. Extended periods of wet weather are very favorable to most grape diseases. In general, if the leaves and shoots are wet for eight hours or longer, infection is possible if not protected by a fungicide (except for powdery mildew, which needs dry leaves for infection).
What fungicides to use early in the season
The fungicides that are most effective in controlling Phomopsis are also effective in controlling early-season (foliar) black rot. The broad-spectrum fungicide mancozeb (Manzate, Penncozeb, Maneb) is generally the most effective material, but Captan (captan) is also effective. Ziram is moderately effective and more effective at the 4-lb. than the 3-lb. rate. All of these fungicides can be tank-mixed with phosphorous acids to obtain systemic activity against Phomopsis, downy mildew and black rot as well (rates can be cut in half for both components in the tank mix, for example, 1.5 lb. Penncozeb + 2 pt. ProPhyt). It is recommended to save the strobilurins (e.g., Pristine, Sovran, Abound, Flint) until somewhat later in the season when they are needed for control of multiple diseases. They may also work better under higher temperatures.
Sterol inhibitors (Elite, Rally, etc.) can be used any time during the growing season, although they may not work well at temperatures below 40°F. Both of these groups of fungicides are prone to resistance development, so are best used at critical disease control periods – immediate pre-bloom until second post-bloom. Do not use these materials more than two to three times per season, and no more than twice consecutively.
Rotating with fungicides with a different mode of action can help delay the development of fungicide resistance and prolong fungicide efficacy. JMS Stylet Oil or sulfur may be used to control powdery mildew early in the season. However, powdery mildew generally is not a great concern at this time, except in susceptible cultivars and vineyards that had a problem with fruit infection the previous year. Sulfur can also be added to the tank mix as inexpensive “insurance” against powdery mildew. Do not apply sulfur to sulfur-sensitive varieties or tank-mix, or spray within two weeks of an oil spray.