infected

I had a plant given to me by a friend about 3 months ago and at first all was good! Fast forward to the plant two weeks ago and i found this spot on the stem(picture)... It then continued to grow so I separated it from my tent and sanitized my tent before germinating and putting my seedlings...

I have read that clones from a plant that had mildew is a bad idea as it infects on a cellular level and will infect the clone some time later. but what about the seeds? if you germinated seeds that came from a previously infected plant, would the new plants get mildew?

One of the plants I have has just shown as a male after seven weeks. I removed it instantly but will it have infected the other plants? And what could have caused this?