Systemic fungicides are an important tool for managing powdery mildew, the most common disease of cucurbit crops throughout the world. Unfortunately, most systemic fungicides are at risk for resistance development because they have single-site mode of action. Thus modification of one gene in the pathogen may be enough to enable the pathogen to resist the action of the fungicide. The cucurbit powdery mildew fungus has demonstrated a high potential for developing resistance. First detections of resistance to QoI (strobilurin) fungicides in North America occurred in 2002 and included New York. Although resistance has developed, QoI fungicides will continue to be valuable for managing powdery mildew and resistance to DMI fungicide until resistant pathogen strains become common. To use QoIs wisely, growers need to know the proportion of the pathogen population that is QoI resistant before the first application and how much the population changes with QoI use.