Soft rot of squash is caused by

Soft rot of squash is caused by Choanephora parasitica.
Choanephora is a genus of Zygomycota fungi. Choanephora species are known as plant pathogens.
Soft rot also known as Choanephora or wet rot is a disease most commonly found on summer squash under wet conditions.
This disease also infects other cucurbits including pumpkins and zucchini. The fungus can infect flowers or fruit during prolonged wet periods.