I have a white powder on part of my lawn. What is it?

You have a harmless fungal growth commonly referred to as “powdery mildew”. Here in Manitoba it rarely becomes severe enough to damage the lawn. Powdery mildew is usually most prevalent during periods of high humidity and cool nights. It is also indicative of a lawn that has high nitrogen content. While a fungicide is available to treat severe cases, I have never seen a case severe enough to warrant a fungicide treatment. Given a few days, weather conditions usually change. The grass blades will continue to grow and the infected blades will simply get cut off by your lawn mower. Powdery mildew is also a common problem in many tree species.