For some grape growers, the first pesticide application of the season is lime sulfur shortly before bud swell. Others skip lime sulfur and start the season with a bud break fungicide application. So does skipping lime sulfur result in less disease control? Well, that depends on the disease you are trying to manage.
Dormant lime sulfur can be applied right before bud swell. As with any pesticide, be sure you have sound justification for spraying it, understand its limitations of disease control and effects on beneficial insects and the crop, and read the label to apply it safely and appropriately.
Sulfur vs. Lime Sulfur
Sulfur fungicides are composed mostly of elemental sulfur as the active ingredient. Sulfur can be applied throughout the growing season but is not as effective as conventional fungicides. Some grape varieties can be injured by sulfur (phytotoxicity).
Lime sulfur is just sulfur mixed with calcium hydroxide (calcium polysulfide). It is a contact fungicide and has demonstrated efficacy against powdery mildew. Several formulations are available. However, most formulations are not compatible to tank mix with other pesticide sprays.
Sulforix is also a calcium polysulfide product like Lime Sulfur but has a lower application rate.
Applications of sulfur should not occur when temperatures are above 85°F or within 14 weeks of an oil spray as these can result in burning of the foliage. Applications of lime sulfur products should not be used within 30 days of an oil spray outside of dormancy as this can result in phytotoxicity to the vine.
Setting Expectations: How Effective Are Dormant Lime Sulfur Sprays?
Dormant lime sulfur reduces the amount of overwintering fungal spores from anthracnose, phomopsis, black rot, and powdery mildew. However, it is most beneficial for the first three diseases listed and is not very helpful for powdery mildew control.
While it may reduce the amount of overwintering powdery mildew (PM) fungal spores, this does not necessarily translate into increased control of the disease. This is because PM continues producing secondary infections throughout the season, and this is primarily what drives the disease. In other words, killing off some of the overwintered PM spores won’t significantly reduce infection later in the season or have much impact on PM-related yield losses. In-season fungicide application is much better for fighting powdery mildew, by targeting secondary infections. So, pesticide costs would be better spent on in-season sprays for this disease (VEEN, Fall 2012, pp. 2-3).
However, phomopsis, black rot, and anthracnose infections are driven largely by primary (overwintered or early season) fungal inoculum rather than secondary infections. Therefore, dormant lime sulfur applications can meaningfully reduce infection by these pathogens.
- Dormant lime sulfur sprays are most beneficial in scenarios with heavy black rot, phomopsis, or anthracnose the previous season.
- The best approach to powdery mildew management is a solid in-season spray program that uses effective active ingredients at key points in the season.
Unintended Impacts on Beneficial Insect Species:
Dormant vineyards are home to beneficial insects including parasitic wasps and mites, which help keep insect pest populations in check. Additionally, bees are often active in late April or early May when these dormant lime sulfur is typically applied.
Lime sulfur is lethal to a plethora of insects, including beneficials. Some of these species occupy crevices in the grapevine’s bark or may already be flying around foraging for food. If the lime sulfur application is getting into the crevices to kill fungal spores, or contacts the vineyard floor, it is probably also killing some beneficial insects. This underscores why growers must weigh the value of dormant lime sulfur against the unintended consequences, before doing it.
For example: If your vineyard was largely disease-free last year, or you rarely have problems with black rot, phomopsis, or anthracnose, consider whether it is really worth it to spray lime sulfur, given the risks.
Timing of Dormant Sprays:
Always follow the label on your specific product to determine when and how to spray sulfur products. In general, “dormant” applications must be applied before bud swell.
For example, the Sulforix® label states: “During the DORMANT/DELAYED DORMANT period PRIOR to BUD SWELL apply a single application at 1 to 2 gallons of SULFORIX per 5 acre in sufficient water for thorough coverage.” It gives separate guidance for in-season applications.
Sulfur Sensitivity of Cold Climate Hybrid Grape Varieties:
According to research at University of Wisconsin, sulfur sensitivity varies among the cold climate hybrid varieties. Recommendations from this research include:
- Do not apply sulfur to Marechal Foch, Leon Millot, or Brianna
- Restrict sulfur to 1-2 applications per season for LaCrescent and St. Croix
- Hot temperatures (over 85F) increase the risk of sulfur injury
Water Volumes & Product Rates:
As with all pesticides, read the label for application instructions. There are several lime sulfur and sulfur formulations on the market, each with its own application rate. Therefore, it is not possible for us to recommend an application rate without knowing the exact product and formulation you have purchased.
When reading the label for application rate and tank volume, be sure to distinguish water volumes from product rates, and mix the tank accordingly. Dilute the lime sulfur in the recommended volume of water per acre. Every label should include a water volume range.
- Only use dormant lime sulfur if it is truly necessary for controlling black rot, phomopsis, or anthracnose based on previous years’ disease severity
- Dormant lime sulfur application is not a good strategy for controlling powdery mildew. Instead, rely on a good in-season fungicide program for powdery mildew.
- There are risks associated with sulfur and lime sulfur, including killing beneficial insects and causing phytotoxicity to certain grape varieties
- Follow the product labels for application timing, rates, water volumes, personal safety considerations, and other important information
Authors: Annie Klodd, University of Minnesota Extension Educator for Fruit Production; Leslie Holland, University of Wisconsin-Madison, Assistant Professor of Fruit Pathology