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You can use a fungicide to help control powdery mildew, but you should apply it judiciously as it can damage soil health and persist in the soil. Most of the time, proper watering, surface drying, and limiting shade can control powdery mildew without needing harsher interventions. If you have a repeated problem with powdery mildew, you can also try planting a shade-resistant type of grass that will be less susceptible to the fungus.

If you use a starter fertilizer designed for this purpose, you can overseed and fertilize at the same time. If you opt to use a fertilizer for established grass or one with a weed control component, it's best to wait at least four to eight weeks after seed application to allow the new grass to take hold and establish itself. 

Technically, yes—and you may have some successful growth—but this isn’t the ideal way to fill in sparse areas. By clearing leaves and debris, amending the soil, and aerating it properly before you lay the seeds, you’re providing the most ideal conditions for them to germinate.

It’s not required, but it is helpful if your goal is to produce new growth. Any fertilization should occur after the lawn is aerated and before the compost is applied. If you also plan to spread seed, you’ll want to do so after the fertilizer is applied and once the ground is damp, but not wet.