I thought my dog had been vomiting all over my flower beds when I first saw these bright yellow patches. The dog was fine; what I saw was an aptly named fungus called Dog Vomit Slime Mold. It looks awful but is actually harmless to plants and animals. It grows on wood mulch, especially in damp weather, and works like earthworms, decomposing the wood. There are several things you can do to try to get rid of it.
- Patience. Do nothing and it will go away on its own in a week or so. It will grow larger and seem to move over several days. You will see it dry up, perhaps turn a crusty brown and then disappear. For how long? Maybe til next season or the next humid spell.
- Hit it with a spray from a garden hose. I think this just spreads it around, releases spores and creates problems in other areas. If it is growing up the trunk of a shrub or branch, and this disturbs you, you can use water and a soft bristled brush to remove it. I'd wear a mask around anything that can send spores into the air and you can breathe them in.
- Rake it apart. Same problems. You're just spreading it to a larger area.
- Gently pick it up with a straight edged shovel, bag it and put it out for your town's yard waste pickup. This worked for me. Since it is harmless, there's no need to spray toxic chemicals that might harm all the good organisms in your soil. This worked for me three years ago, and it hasn't reappeared since.