Assessment of sulfur phytotoxicity on Vitis vinifera grapevines in northern Georgia

Grapevine powdery mildew (GPM), caused by the fungal pathogen Erysiphe necator, is an economically important disease wherever grapes are grown. Elemental sulfur is a contact fungicide used as a management tool in powdery mildew control programs. However, sulfur has been reported to induce phytotoxicity when hot temperatures and high relative humidities (RH) occur simultaneously. Therefore, grape producers in northern Georgia (U.S.) often avoid sulfur applications during mid-to late summer when higher temperatures and humid conditions are more persistent. In this study, we assessed the phytotoxicity response when Microthiol Disperss (sulfur) was applied at high temperatures to Vitis vinifera and interspecific hybrid cultivars with American grape heritage in northern Georgia. On days that were selected for their predicted high temperatures (≥30 °C), the highest label rate of sulfur was applied at 3:00 p.m. or 7:00 p.m. to plots of vines in four V. vinifera cultivars located across four commercial vineyards (2023 and 2024) and plots of three hybrid cultivars located at a University of Georgia Durham Horticulture Research Farm (2024). One week after application, treated leaves were assessed for phytotoxicity symptoms and compared to untreated controls. Significant foliar phytotoxicity symptoms, scorch or discoloration, were not observed on V. vinifera grapes when sulfur applications were conducted at times that coincided with moderate to relatively higher temperatures (21.7–33.1 °C) and concurrent relative humidity measurements of 40–80 %. Though high temperatures (>30 °C) often occurred at the time of sulfur applications, RH >70 % did not occur simultaneously at these warmer temperatures, both of which are purportedly required to induce phytotoxicity. Based on a review of historical data over the last 21 years, we can further predict that simultaneous conditions of high temperatures and humidities are unlikely to occur in northern Georgia, and therefore, sulfur-induced phytotoxicity is highly unlikely to occur on V. vinifera vines grown there. The known combination of high temperatures and high RH that reportedly causes phytotoxicity appeared to be exceedingly rare and likely short-lived. However, we observed a phytotoxic response on the Vitis hybrid ‘Crimson Cabernet’ when sulfur was applied at 3:00 p.m. (31.7 °C, 45 % RH), displaying greater scorch damage when compared with untreated controls and sulfur application at 7:00 p.m. (27.8 °C, 56 % RH) (P ≤ 0.05). These studies support season-long sulfur use on V. vinifera grapes in Georgia, with very limited danger from phytotoxicity. However, this is not necessarily true when sulfur is applied to all Vitis hybrids, and these should each be reviewed independently for the potential of sulfur phytotoxicity.