Sulfur is the oldest recorded fungicide and has been used for more than 2,000 years. Early in agricultural history, the Greeks recognized its efficacy against rust diseases on wheat. Although few homeowners grow their own wheat, sulfur can be a preventive fungicide against powdery mildew, rose black spot, rusts, and other diseases. Sulfur prevents fungal spores from germinating, so it must be applied before the disease develops for effective results. Sulfur can be purchased as a dust, wettable powder, or liquid. Do not use sulfur if you have applied an oil spray within the last month — the combination is phytotoxic (plant-killing). Likewise, do not use sulfur when temperatures are expected to exceed 80°F to reduce the risk of plant damage. Finally, there are certain “sulfur-shy” plants (including varieties of gooseberries, currants, apricots, raspberries, and cucurbits) that should never be treated with sulfur.