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The Mexican buckeye Ungnadia speciosa, is one of the plants used in forestation programs in Mexico due its adaptive features and its shrub or small tree form; however production in the greenhouse is affected by pathogens that cause leaf blight, dwarf and fork stalks, making it impossible to use in plantations. The objective of this work was to determine the causal agent and evaluation of in vitro and in situ of benzalkonium chloride to control it. Plant tissue was collected in field and greenhouse and put in nutrient agar and potato dextrose agar. LOPAT and relevant biochemical test were conducted for identification. For the incidence and severity, the model proposed by French (1982), was used. The poisoned plate flock was used for the in vitro evaluation with 13 treatments and five repetitions, assessing the colony forming units (CFU); seven in situ treatments with three replications were used evaluating damaged leaf area. It was found that for Pseudomonas syringae, according to the Koch’s postulates involved in the symptoms of blight and deformities in the Mexican buckeye, field incidence was 12,9% and 36% in greenhouse; severity was 6,5% and 32%, respectively. Benzalkonium chloride satisfactorily controlled in vitro in dosages, and the best treatment in situ was 3 000 ppm (7 g/L), which showed a leaf damaged area of 17,33% and target treatment 73,09%.