Applying fungicide treats black spot disease 0n apple trees, according to Garden Guides. Fungicide should be used during the spring to prevent the disease from spreading. Protectant and curative fungicides are two main types to use.
Garden Guides mentions that Scala, Chorus and Dodine are suitable protectant fungicides to use. Score, Systhane and Nustar are primary curative fungicides to apply.
SFGate mentions that lime sulfur fungicides eliminate black spot disease on apple trees, and it also treats fungal microbes and mildew. Lime sulfur fungicide also gets rid of pests like mites. It should be used during the late evening or early morning to avoid burning the foliage, and the fungicide shouldn't be applied in temperatures that are 80 degrees Fahrenheit or above.