Asian grapevine rust (Phakopsora euvitis) is a serious disease, which causes severe leaf necrosis and early plant defoliation. These symptoms are unusual for a strict biotrophic pathogen. This work was performed to quantify the effects of P. euvitis on photosynthesis, carbohydrates, and biomass accumulation of grapevine. The reduction in photosynthetic efficiency of the green leaf tissue surrounding the lesions was quantified using the virtual lesion concept (β parameter). Gas exchange and responses of CO2 assimilation to increasing intercellular CO2 concentration were analyzed. Histopathological analyses and quantification of starch were also performed on diseased leaves. Biomass and carbohydrate accumulation were quantified in different organs of diseased and healthy plants. Rust reduced the photosynthetic rate, and β was estimated at 5.78, indicating a large virtual lesion. Mesophyll conductance, maximum rubisco carboxylation rate, and regeneration of ribulose-1,5-bisphosphate dependent on electron transport rate were reduced, causing diffusive and biochemical limitations to photosynthesis. Hypertrophy, chloroplast degeneration of mesophyll cells, and starch accumulation in cells close to lesions were observed. Root carbohydrate concentration was reduced, even at low rust severity. Asian grapevine rust dramatically reduced photosynthesis and altered the dynamics of production and accumulation of carbohydrates, unlike strict biotrophic pathogens. The reduction in carbohydrate reserves in roots would support polyetic damage on grapevine, caused by a polycyclic disease.

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Asian grapevine leaf rust (AGLR), caused by Neophysopella tropicalis, is a problem for viticulture, especially in latitudes lower than 25° S, which include the most significant production regions in Brazil. Climate change has raised new concerns in agriculture as temperature can affect the resistance of plants to pathogens. With the aim of understanding how air temperature rise affects the AGLR pathosystem, measurements of leaf gas exchange and epidemiological and histopathological analyses were carried out on control and inoculated leaves of Vitis labrusca ‘Niagara Rosada’ grown at 25°C and 30°C. The lesion density and rust severity were higher at 25°C than 30°C, and the ratio between adaxial surface necrosis and the abaxial surface area occupied by pustules was >1 only at 30°C, presenting a necrosis not associated to the pathogen lesion. In fact, leaf necrosis was identified on control plants kept at 30°C and associated with gerontoplasts, representing accelerated leaf senescence. The AGLR pathogen reduced gas exchange and photosystem II activities at 25°C, with no difference between control and inoculated plants at 30°C. Our results indicate that AGLR is sensitive to increasing air temperature. However, the accelerated leaf senescence caused by the combination of N. tropicalis infection and temperature on Niagara Rosada can lead to high leaf damage.