Potato brown spot caused by Alternaria alternata (Fr.) Keissler is one of the most destructive diseases of potato worldwide. The present study was conducted to identify the causal agent, and to test the efficacy of several fungicides with different modes of actions against A. alternata. Five different chemical fungicides including; Copper oxychloride (Sufer Copper Oxychloride® 50% WP), Carbendazim (Carbendazim Aria® 60% WP), Penconazole (Penconazole®  20% WE) , Mancozeb (Qadri Mancozeb®  80% WP) and Flutriafol 6.94% + Tebuconazole 20.8% (Topgaurd® 30% SC) were tested at four different concentrations of; 20, 100, 300 and 500 mg\ l, using the poisoned food technique. Among the tested fungicides, the highest in vitro inhibition (%) of mycelial growth of the pathogen was recorded on using Flutriafol 6.94% + Tebuconazole 20.8% causing complete inhibition (100 %) at 100, 300 and 500 mg\ l, followed by Penconazole causing 100 % inhibition at 300 and 500 mg\ l; moreover, it caused strong inhibition of 90.17 % at 100 mg\ l. Both fungicides proved to be the most effective expressing strong inhibition potency of 93.75 % even at the lowest concentration of 20 mg\ l after 5 days of fungal incubation, although the inhibitory efficacy decreased slightly over time after 10 days of incubation. Mancozeb was the third effective fungicide causing strong mycelial inhibition of (41.91, 75.24 and 84.21 %) at 100, 300 and 500 mg\ l, respectively. The least mycelial growth inhibition was observed with Copper oxychloride (8.77 %) followed by Carbendazim (21.05 %) at maximum concentration of 500 mg\ l. For the best of our knowledge, this is the first report of A. alternata as the fungal causal agent of potato brown spot in Afghanistan.