Improving the Utility of Sulfur for Grapevine Powdery Mildew Management

Walt Sanders and Phil Brannen
Department of Plant Pathology, University of Georgia

The Role of Sulfur in Grape Disease Management

Sulfur is one of the most commonly used fungicides in vineyards – accounting for nearly 30% of all vineyard fungicide applications in the U.S. Applications typically begin early in the season and continue throughout, with some growers even applying sulfur after harvest to maintain leaf health and to reduce overwintering fungal inoculum. Sulfur is also a critical resistance management tool, as the E. necator pathogen does not develop resistance to sulfur. Despite its effectiveness, sulfur has limitations. The residue it leaves behind can provide disease protection for up to 21 days under ideal conditions, but it is easily washed away by rain, necessitating frequent reapplication. Moreover, sulfur is less effective than systemic fungicides, requiring growers to supplement its use with other fungicides that carry a risk of resistance development. Increasing the frequency and efficacy of sulfur applications on Vitis vinifera grapes would be of value for numerous reasons, and the below research results, derived in part through a grant from the Southern Region Small Fruit Consortium, have provided valuable information to this end.

Surfactants: A Potential Solution for Improved Sulfur Efficacy

To improve sulfur’s effectiveness for disease control, many producers will tank mix sulfur with adjuvants, commercial products that alter the properties of the applied solution. Surfactants are a class of soap-like adjuvants used to enhance the performance of pesticides by altering the physical properties of water. Surfactants reduce surface tension, allowing fungicides like sulfur to spread more evenly across leaf surfaces. This increased coverage can lead to improved disease control. Some surfactants also exhibit antimicrobial properties which can have a direct impact on disease control. Surfactants have been extensively studied in herbicide applications, but their role in fungicide performance is less well understood. Our recent study in 2023 and 2024 examined whether surfactants can enhance sulfur’s effectiveness against GPM. We found that certain non-ionic surfactants, such as Hi-Wett and Silwet L-77 in particular, can improve sulfur’s ability to control GPM, likely due to increased spray coverage. However, the precise mechanism—whether it is purely due to better spreading, an added antimicrobial effect, or both—remains unclear. Of the three surfactants reviewed in these trials, the Silwet L-77 surfactant, when combined with sulfur, provided the best and most consistently positive response in reducing GPM (Tables 1 and 2).

Understanding Sulfur Phytotoxicity

Sulfur-induced phytotoxicity, referred to as “sulfur burn,” commonly manifests as necrotic tissue mostly on the margins of leaves, but leaf discoloration can also occur. Research suggests that sulfur phytotoxicity occurs under high temperatures and humidity levels, though the exact thresholds are debated. Some studies report that damage occurs on V. vinifera varieties when temperatures exceed 30°C (86°F) and humidity surpasses 75%, while others suggest a slightly lower humidity threshold of 70%. These thresholds are different for many if not most interspecific hybrid grape varieties (e.g., Chambourcin) and those with genetics from American grape species (e.g., Norton/Cynthiana), which are particularly susceptible to sulfur damage, sometimes experiencing phytotoxic effects at temperatures as low as 24°C (75°F).

One should never use sulfur on highly susceptible varieties (Fig. 1). Though V. vinifera grapes are known to be less prone to sulfur injury, due to concerns over phytotoxicity, many growers still generally avoid sulfur applications during hot weather. Withholding sulfur could leave vines vulnerable to GPM, particularly if alternative fungicides used in place of sulfur no longer work due to resistance development in the E. necator pathogen population. Our recent study examining this topic in the northern Georgia region indicated that V. vinifera grape varieties may not be as sensitive to sulfur-induced damage even as previously thought. After applying sulfur with the highest labeled rate during hot and humid conditions, we found that sulfur did not cause phytotoxicity, even at temperatures as high as 32°C (90°F).

Environmental Factors and Practical Considerations for Potential Sulfur Injury

After failing to induce sulfur burn at such high temperatures, we took a closer look at the weather conditions in northern Georgia to see how often the temperature and humidity reached the levels necessary to cause burn in the first place. Historical weather data from Dahlonega, Georgia, suggests that these conditions—temperatures above 30°C with humidity above 75%—are relatively rare. Within 21 years of data collection, only 12 days during the summer months met the thresholds to cause sulfur burn. Furthermore, the conditions rarely lasted longer than 45 minutes, meaning even when they occurred, they likely did not last long enough to cause damage. Still, the impact of sulfur applications varies by region, grape variety, and specific weather conditions at the time of spraying. Growers should consider local climate data when deciding whether to apply sulfur and should conduct small-scale trials before widespread use on sensitive grape varieties.

Conclusions

Sulfur remains a cornerstone of GPM management, particularly in regions where fungicide resistance is a growing concern. The addition of non-ionic surfactants can improve sulfur’s efficacy, likely by enhancing spray coverage, but efficacy improvement can vary from year to year and from product to product. However, our research does generally support the use of non-ionic surfactants with sulfur. Concerns not addressed by our research are whether added surfactants can improve disease management with other fungicides or whether phytotoxicity could occur from mixing surfactants and other chemicals in tank mixes. We recommend that producers always apply tank-mixes to a limited vineyard area first to determine whether phytotoxicity occurs. In addition, jar tests are recommended to determine in-tank compatibility when mixing chemicals.  

For V. vinifera grapes, sulfur applications—even at high temperatures—appear to pose minimal risk of phytotoxicity in northern Georgia. However, interspecific hybrids with American grape ancestry are more susceptible to sulfur phytotoxicity and should be monitored closely for sulfur damage – if sulfur is utilized at all on these. With so many new hybrids being grown, great care should be taken to avoid sulfur damage, and we do not have sufficient information at this time to address the potential for sulfur damage on most hybrids. In practice, vineyard managers should weigh the benefits of improved disease control against the potential risks of increased phytotoxicity. By carefully monitoring weather conditions, growers can optimize sulfur applications to maximize disease control while minimizing plant stress, and the use of sulfur on V. vinifera grapes should carry limited risk of damage. This information will likely apply to the whole of the southeastern wine grape industry.