What are these black patches in
my lawn?

This is brown patch. It is a fungus that gets on the grass. The best way to treat it without using chemicals is to make sure there is good drainage around the grass. Also, do not overfertilize. Aeration may also help.
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Related Videos:
Lawn Aeration
What caused these patches on my lawn? Is it army worms or herbicide or fertilizer?
Spring Dead Spot

Related Resources:
UMass Extension Turf Program
Turfgrass Diseases: Brown Patch (Causal Fungus: Rhizoctonia solani)