 They're also studying trees at the Arnold Arboretum of Harvard University to see what kinds of fungi are already present there. This 281-acre arboretum has over 16,000 trees, including species from all over the world. Some of these species that grow in the arboretum were planted in central gardens around the world. So they can compare stuff and see what stuff is going on. We're trying to exclude the things that are always present from the ones that are causing disease, the plant researchers said.