He started from 10,000 seeds, selected 3,000 plants, then 30 cultivars. It took him 20-30 years.
Gary Zill's Mango Variety Development Project - from Truly Tropical channel: "How did Gary Zill develop so many new mango varieties??!
Gary Zill has been a major player in the Florida Mango Industry for years. In the late 1990s, he undertook a very ambitious project to develop new superior mango varieties. In this video, Gary tells all about his incredible project that has resulted in 20-30 wonderful new varieties."
See more here: http://www.livebettergarden.com/2019/05/most-new-mango-varieties-come-from-this.html