See:
Greenhouse Plants, Ornamental - Impatiens Necrotic Spot
Symptoms Round, white, or tan leaf spots up to a quarter inch in diameter. Stem necrosis and wilting is frequent, which causes tissues to turn black. Chlorosis, leaf spots, and leaf drop may occur but are less frequent. Symptoms are variable and resemble those caused by other diseases and thus are not reliable for diagnosis.
Reference Wegulo, S.N., and Chase, A.R. 2017. Diseases of Snapdragon. In McGovern, R.J., and Elmer, W.H. (eds.) Handbook of Florists' Crops Diseases. Springer Int.