Wis. Stat. § 891.29: Allegations of partnership.

Whenever in any action or proceeding a party shall allege in pleadings that named persons were partners at any particular time, or that as such partners they used any particular partnership name or style under which business was done, such averments shall be taken to be true unless expressly denied by the affidavit of the opposite party or someone in the opposite party’s behalf or by the opposite party’s pleading, within the usual time of pleading.