Poison ivy / poison oak / sumac has a pretty intense chemical that requires a very little amount to cause problems — the average [accidental] dose is 100 nanograms. It contaminates surfaces invisibly, and will remain potent for 5 - 50 years. And it grows EVERYWHERE.

So it could be used for evil purposes.

To harvest and purify urushiol:

Urushiol rashes can get pretty severe [1] [2] [3] [4] so protect yourself when handling the stuff, and be aware of the harm you can cause.

Note that it can take 24 - 48 hours from exposure until a rash develops, so this isn't a fast-acting weapon.

If you were REALLY evil, you could burn or otherwise vaporize urushiol, which will give someone a rash inside their lungs. However, this is known to be fatal. So yeah, don't do that.