My wifes has a lady friend who's mother is a master gardener.
Its my understanding there are courses one takes for this.
Little bit of info
Master Gardeners are active in all 50 states in the United States and four Canadien provinces.According to the 2009 Extension Master Gardener Survey, there are nearly 95,000 active Extension Master Gardeners, who provide approximately 5,000,000 volunteer service hours of per year to their communities. Once volunteers are accepted into a master gardener program, they are trained by cooperative extension, university, and local industry staff in subjects such as, Taxonomy, Plant Pathology, Cultural Growing requirements, Nuisance wildlife management , and Integrated Pest Management.
I think there required to participate community servive as well but not exactly sure as it may be different from state to state.