Well maybe it worked but there is no animation, so it is a bit non evenemental I had to look closely at the stats of the army you cast on you should see the difference.

And last but not least, to be sure it works, if you battle the army within 2 turns, you will see each unit in the army is correctly marked with 'wither' effect.

Actually, this has been one of the most used spells for me as it doesn't even require the enemy army to be in your territory.

The one I doubt seriously it works is infection (or is it contagion ? Do both exist ?). That one is supposed to propagate the bad effects on a unit to all others.