Powdery mildew on sunflower type plant

I purchased a plant a while ago and left it in the pot. After about 10 days, it got powdery mildew on the leaves. I sprayed it with milk/ water solution which helped, and cut off all the affected leaves. There is still the odd leaf that is coming up with some mildew showing. I will spray it again. Is the plant safe to plant in the ground, or will it affect the area? The plant is a brown-eyed Susan
type plant ( sunflower ?) Thank you.