Can I use fungus gun to control black spot on mango trees?

Unfortunately, the product Yates Fungus Gun is only registered for ornamental plants not for edibles. To control black spots on Mango, try spray the plant every 4 weeks from flowering to fruit setting with 'Yates Liquid Copper Fungicide'. A broad-spectrum fungicide for control of a wide range of diseases on fruit, vegetables and ornamentals.

Hope this helps and happy gardening :)