Fungal endophytes are group of fungi that internally infect living plant tissues without causing any visible manifestation of disease. A major issue in endophyte biology is whether any endophytic microorganism is symbiotic in a host and pathogenic in another host. Here it is shown for the first time that endophytic Alternaria species which had been formerly isolated from the healthy members of the plant family Cupressaceae, and had shown biological activity against target fungi and bacteria in vitro, were capable of pathogenicity in potato (Solanum tuberosum). Indeed, six Alternaria alternata isolates i.e. POE43, POE72 and POE104 (isolated from Platycladus orientalis), CSE165 and CSE177 (isolated from Cupressus sempervirens) and CAE92 (isolated from Cupressus arizonica) were grown in Potato-Carrot-Agar medium, for 14 days, at 22±2 °C, under 16/8 light/darkness condition to obtain a conidial suspension of 105 CFU. Alternaria conidia were prepared and sprayed onto the foliar tissues of 6 weeks-old potato "Marfona". Results showed that under greenhouse condition, the symptoms of Alternaria's brown leaf spot were emerged on potato. This indicates that A. alternata which has been endophytic to cypress trees could be pathogenic on potato, and that cypress tissues may serve as a temporary niche for such fungi
