Hello, this has been asked before but no answer was posted so I will ask again:
If I attack with a wither/infect creature and my opponent who controls a doubling season blocks with a creature, will there be twice as many -1/-1 counters being placed on the blocking creature?

119.3 (d) says wither/infect are just another way of dealing damage, so this would not be an effect.
119.4b says damage, that's been dealt is processed into its results, as modified by replacement effects that interact with those results (such as life loss or counters).
Does this apply here? Are wither/infect replacement effects?

This was the original question:
http://apps.magicjudges.org/forum/topic/23694/

Apologies for not replying to the first post. This question just fell through the cracks, but we'll answer it here and now.

No, they will not be doubled. Doubling Season only doubles counters that are added due to an effect. But combat damage is not an effect - it's a turn-based action. Wither and infect aren't replacement effects anyways - the rules tell you what to do if a source with wither or infect deal damage.

Replacement effects don't do anything themselves anyways - they modify what other things are doing. A replacement effect modifying what happens with combat damage doesn't make combat damage into an effect - it's still a turn-based action, not an effect, and Doubling Season still will not affect the counters added via wither or infect due to combat damage.

Nathan Long
Magic Rules NetRep