system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the whole world not covered in mildew?

Specifically in rain forests and things why is the whole thing not just covered in it?
gpt
Plants basically wage constant chemical warfare against each other. Look at pine trees for example: When the die they take a really long time to decompose because they are filled with resin. Insects can't chew through that and other plants and fungus (usually) has a hard time growing on it. 

Below living pine trees you won't find many other plants because the needles they constantly drop contain resin too. Eucalyptus trees do the same with eukalyptus oil. Species have to adapt (like Koalas) to these poisons before they can coexist or compete with these plants.

The same goes for mildew: It can't easily attach itself to plants that actively defend themselves, so it usually grows on dead plants, that cannot produce enough poison.