The fine milled sulfur stretches a protective net on the leaves. There are far more sulfur particles available to cover and protect the leave surface compared to dust formulations.
Sulfur particles, evenly distributed on the leave surface act immediately through contact and vapor action against target fungi. Sulfur vapor easily enters the fungal cell and is enzymatically converted to hydrogen sulfide being active against the pest. On the leaves Sulfur particles can be converted to sulfur dioxide under the influence of humidty, light and oxygen.
The efficacy of Sulfur as a plant protection agent depends on the concentration of elemental Sulfur in the vapor phase and as a consequence it depends on the temperature.