I wounldn't dismiss your idea completely. It all depends on how one would define "heatlhy". If healthy means less blackspot, there may be an element of truth in it. There has been claims and observations that roses grown in cities are more resistant to black spot than those grown in "clean air". It is postulated that one of the airborne pollutants, sulphur oxide, when mixed with rain, has antifungal activity. And it has lead to claims from sites like these: [url]https://www.bbc.co.uk/gardening/advice/pests_and_diseases/nonflash_index.shtml?rose_black_spot[/url], that black spot is more common in city grown roses now that pollution is under tighter control and the use of coal has been significantly reduced.