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The orchid Guarianthe skinneri (Bateman) Dressler & Higgins, with attractive mauve fowers, survives in small populations in forest fragments in Mexico and Central America. This orchid is vulnerable due to illegal extraction and habitat reduction. Attempts at sustainable cultivation are hindered by a disease, referred to in this study as the "Black Blotch", characterized in the feld by the development of black areas and necrosis on the surface of the pseudobulbs. Fungi were isolated from infected pseudobulbs of G. skinneri cultivated under semi-natural conditions. The fungal strains were identifed using morphological and molecular methods. Lasiodiplodia was isolated and described, with hyaline immature conidia and brown, septate mature conidia. The identity of fve of the isolates was confrmed as Lasiodiplodia theobromae and three isolates as Lasiodiplodia sp. Pathogenicity tests were carried out by inoculating the isolates on pseudobulbs of healthy, mature plants of G. skinneri. Symptoms appeared after 24 h, characterized by soft and slightly sunken light reddish-brown lesions, with a dark brown to black necrotic spot which increased in size, and, at 35 days, exudation, and detachment or rupture of the cuticle surface were observed. Pycnidia formed on the surface and between the cells of the spongy parenchyma of some of the pseudobulbs. The causal agents of the “Black Blotch” were identifed as L. theobromae and Lasiodiplodia sp., this being the frst report of Lasiodiplodia causing disease on Guarianthe skinneri. Inglés