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Q: Big white mushrooms sprout overnight in our lawn after a heavy rain. Is there anything we can put down on the grass to kill them? Fannie Fungi
A: No. Mushrooms, no matter their size or color, are but a very small and visible part of an underground fungus that survives by breaking down organic matter in the soil. In many cases, this organic matter is dead roots or buried wood. In a sense, mushrooms are the fruit of the fungus. They rise from the ground to spread spores so that the fungus can reproduce. The body of the fungus, called the mycelium, sometimes covers the ground with a whitish film, but usually remains invisible underground. It can be huge, too. The mycelium of a single Armillaria fungus discovered in Oregon covers about 2-1/2 square-miles, making it the largest living thing on Earth. Godzilla is a distant second.
Fungi thrive in moist conditions, so that's why mushrooms appear after a heavy rain. There is no fungicide you can put on the lawn that will kill the mycelium below. When the mycelium has completely consumed the organic matter in the soil, there will be no more mushrooms.