Histopathological evidences of early grapevine leaf senescence caused by Phakopsora euvitis colonisation

Grapevine rust, caused by Phakopsora euvitis, has recently been introduced in Brazil; however, this disease has spread nationwide, causing significant damage to vineyards. The pathogen reduces CO2 assimilation and photoassimilate availability in host plants. Grapevine rust causes early defoliation and yield reduction, in the subsequent crop, since storage of reserves is compromised. Infection, colonisation and defoliation processes caused by the pathogen are not completely understood. In order to shed light on monocyclic components of grapevine rust, we have carried out histopathological analyses of the interaction between Phakopsora euvitis and leaves of Vitis labrusca ‘Niagara Rosada’. Fully expanded leaves of grapevines were inoculated with a suspension of 105 urediniospores mL−1 of P. euvitis, and collected at 1, 2, 4, 8 and 22 days after inoculation. The samples were fixed and processed for analysis under light, scanning and transmission electron microscopy. Ultrastructural analyses indicated that the pathogen forms appressoria on the stomata and ordinary epidermal cells within 24 h of inoculation. Epidermal cells of the mature leaf did not present pre- or post-formed defense mechanisms to penetration of Phakopsora euvitis. However, mesophyll cells, when infected, produced callose and phenols to prevent fungal colonisation. Nevertheless, the deposition of these compounds, restricted to the haustorial neck, did not stop infection. Mesophyll cells from diseased leaves showed chloroplasts with plastoglobules and vacuoles filled with electron-opaque drops. Such structures are not present in healthy leaves but are present in healthy senescent grapevine leaves. Phakopsora euvitis seems to accelerate leaf senescence, which could be related to early defoliation observed in diseased plants.