BACKGROUND: Silicon (Si) application to miniature potted roses can decrease severity of powdery mildew (Podosphaera pannosa) and this is associated with increased accumulation of callose and hydrogen peroxide (H₂O₂) as well as hypersensitive (HR) cells. We used microscopy, gene expression and specific inhibitors of callose and H₂O₂ to determine how effective these plant responses are in stopping infection. RESULTS: Pathogen arrest in Si-treated (Si+) plants was accompanied by increased accumulation of callose and H₂O₂ in papillae and HR cells, respectively. These responses were reduced by application of specific inhibitors (2-deoxy-d-glucose for callose and catalase for H₂O₂), which increased disease severity in Si+, but not in Si− plants. As markers for HR and callose, expression of the HR-specific gene hsr203J and the wound-related callose synthase GSL5, respectively, was studied. An up-regulation of expression was only seen after isolation of HR cells with laser capture microdissection. The up-regulation was higher in Si+ than in Si− plants and occurred concomitantly with more efficient photosynthesis in Si+ plants at high disease severity as compared to Si− plants. CONCLUSION: Silicon-mediated activation of callose and H₂O₂ are decisive factors in the defence of rose against P. pannosa and these responses were accompanied with more efficient photosynthesis to strengthen the plant. Only by isolation of HR cells using laser capture microdissection as compared to analysis of whole leaf tissues allowed detection of elevated transcript levels of hsr203J and GSL5 at infection sites as markers for HR.