Fred Rogers Productions says it's deeply saddened by Joanne's passing, adding ... "The loving partner of Fred Rogers for more than 50 years, she continued their shared commitment to supporting children and families after his death as chair of the board of Fred Rogers Productions."

The company says ... "Joanne was a brilliant and accomplished musician, a wonderful advocate for the arts, and a dear friend to everyone in our organization."

Joanne was married to Fred for 50 years, as he became famous for "Mister Rogers' Neighborhood," and helped carry on his legacy of kindness and caring after he died of stomach cancer in 2003.

During election season, Joanne made headlines for publicly supporting Joe Biden, describing herself as a "very big Biden fan" and saying ... "I think we all need somebody like Biden who can give us little pats on the back."

Like her husband, she's hailed as a hero in their hometown of Pittsburgh, and the city paid tribute to her Thursday, calling her one of its "favorite neighbors" and thanking her for her dedication to its community and its children.

Joanne is survived by her 2 sons, James and John. She was 92.