All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
There is no doubt that Zina (fornication & adultery) is a great sin. It is, in fact, one of the gravest sins. Whoever commits it, is unqualified to be a spouse to some righteous one unless he/she repents and returns to Allah in word and in deed.
Allah Says (what means): {The fornicator does not marry except a [female] fornicator or polytheist, and none marries her except a fornicator or a polytheist, and that has been made unlawful to the believers.} [Quran 24:3]
As for your question, it is forbidden to spread such a rumor about this woman since Muslim women are primarily considered chaste unless there is evidence that proves otherwise. At the same time, if it is proved that she has committed Zina, you are asked to conceal that.
But, if she admits committing Zina or if it becomes confirmed by some definite evidence, and she has not informed you before marriage - you are free to undertake any of the following choices:
1 -To retain her in all decency, urge her to repent and conceal her past. This choice is better, especially if you wish and expect to have a good life with her in the future, forgetting all about this matter and if you think she will become righteous.
2 -To part from her decently. This second choice is better if you think you will always be obsessed by her state or if you do not expect her to return to the right path.
Allah knows best.