Aims: We investigated the ultrastructural and molecular interactions between 'Candidatus Phytoplasma mali' and the apple endophyte Epicoccum nigrum in the experimental host Catharanthus roseus to determine whether inoculation of endophyte could trigger defence reactions in the host. Methods and Results: Apple proliferation (AP) symptom severity was evaluated in AP-grafted plants that were treated by E. nigrum and compared with untreated controls. Phytoplasma concentration was quantified by real-time PCR in treated and untreated plants. Ultrastructural observations revealed that in endophyte-treated periwinkles, modifications to phytoplasmas, such as irregular shape and cytoplasm confined to the periphery of the cell, and plant cytological changes, such as abundant callose depositions and P-protein aggregations in the sieve elements, occurred. AP-grafted plants that were treated by the endophyte (E. nigrum) showed a reduction in symptom severity; in particular, flowers appeared normal in shape and size, when compared with uninfected controls. Real-time PCR indicated that phytoplasma concentration in AP-grafted plants treated with E. nigrum was about 2 center dot 8 times lower than that in untreated ones. Conclusions: These results demonstrated that the inoculation with E. nigrum influenced phytoplasma infection in C. roseus plants; plant ultrastructural modifications allowed us to hypothesize an enhancing host defence response. Significance and Impact of the Study: At present, curative protocols against this phytoplasma are not available. Alternative approaches are thus required to reduce disease spread. Our study might represent a first step in the clarification of plant-phytoplasma-endophyte relationships to find possible strategies for the control of phytoplasma diseases.