Mo. Rev. Stat. § 509.140

It shall be sufficient to aver the ultimate fact of the capacity of a party to sue or be sued or the authority of a party to sue or be sued in a representative capacity or the legal existence of a corporation or of an organized association of persons that is made a party. When a person desires to raise an issue as to the legal existence of any party or the capacity of any party to sue or be sued or the authority of a party to sue or be sued in a representative capacity, he shall do so by specific negative averment, which shall include such supporting particulars as are peculiarly within the pleader's knowledge. When a party so raises such issue, the burden of proof thereon shall be placed upon the opposite party.