Bacterial leaf spot of Ilex paraguariensis caused by Pseudomonas syringae

Yerba mate (Ilex paraguariensis) leaves and twigs are dried and used to make mate, a popular herbal tea in Southern Brazil, Argentina, Paraguay and Uruguay. In February 2021, yerba mate leaves with water-soaked spots, surrounded by chlorotic halos were observed in four nurseries at the Ilopolis and Arvorezinha counties, Rio Grande do Sul (RS) State, Brazil. Bacterial colonies were recovered from the symptomatic leaves. Based on morphological characteristics, LOPAT tests and the DNA sequence of four house-keeping genes, the causal agent was identified as Pseudomonas syringae. To fulfil Koch's postulates, pathogenicity was confirmed by inoculating leaves of yerba mate seedlings with a bacterial suspension obtained from one of the isolates, under laboratory conditions. Inoculated leaves developed small dark spots after 4 days post-inoculation only in plants that were wounded prior to the inoculation, indicating that mechanical disruption of the plant tissue favoured bacterial penetration. To the best of our knowledge, this is the first worldwide report of P. syringae causing bacterial leaf spots on yerba mate.