It’s not too late for dormant sprays in grapes
Dormant fungicide sprays in grapes can help reduce inoculum of Phomopsis, powdery mildew, black rot and anthracnose.
The cold spring has some advantages; for instance, allowing you to finish your pruning activities in grapes and even throw in some dormant sprays before the season starts. The goal of dormant sprays is to eliminate fungal pathogens that overwinter in or on the woody parts of the vine. While it is not possible to eradicate all inoculum, dormant sprays can kill or debilitate the fungus so it produces fewer spores, reducing disease pressure during the growing season.
Dormant sprays are useful for management of Phomopsis, powdery mildew, black rot and anthracnose. In some years, we have seen a reduction in downy mildew as well, but only with copper sprays. Since the downy mildew pathogen overwinters in leaf residue on the soil, it could be affected by copper residues that land on the soil surface.
In most years, we have seen a benefit from dormant sprays when rating diseases at harvest, but the degree has varied from none to 70 percent. As a rule of thumb, a 30 to 50 percent reduction in disease pressure can be expected on average from a single dormant spray. Results may not be as good in rainy springs, which probably lead to washing off of the material before it is able to do its job.
To cover your bases, two dormant sprays may be applied, in early and late spring or fall and spring. Dormant sprays should not be used as a stand-alone disease control measure, but can aid in reducing disease pressure during the season.
Products that can be used as dormant sprays are Lime Sulfur or Sulforix, Cuprofix or any other copper product, Sulfur (liquid form recommended), and JMS Stylet Oil or other dormant oil. Sticky formulations that don’t wash off readily are best. Application during a dry period and not right before a rainstorm can aid efficacy. Sulforix is calcium polysulfide (similar to Lime Sulfur) that has a lower rate of application.
I usually equate 1 gallon of Sulforix to 5 gallons of Lime Sulfur, and 2 gallons of Sulforix to 10 gallons of Lime Sulfur. Both products are corrosive to equipment and care must be taken to protect eyes and skin from exposure. Some growers spray PAM or oil on equipment prior to use of Lime Sulfur or Sulforix to protect against corrosion and facilitate washing off spray residues. An early-season spray of Manzate or Penncozeb (at 1 to 2 inches of shoot growth) will likely act similarly to a dormant spray by killing Phomopsis as it starts sporulating on old wood.
To get the maximum benefit out of dormant sprays, it is important to ensure thorough coverage of the trunk and canes and to spray every row. Airblast sprayers may not be the best means of application of dormant sprays; tower sprayers or boom sprayers spraying down onto the cordons from above may be better. In any case, closing off nozzles as needed and focusing nozzles on the cordons, lowering air intake, slowing down and spraying at a moderately low volume (e.g., 20 to 30 gpa) will allow better coverage of the canes while keeping the product fairly concentrated. Don’t use a high spray volume as it will dilute the product and result in run-off.
Dr. Schilder's work is funded in part by MSU's AgBioResearch.