Diverse effects of temperature on in vivo and in vitro germination of urediniospores of Neophysopella tropicalis

Tropical Asian grapevine leaf rust, caused by Neophysopella tropicalis , is an important grapevine disease in the tropics. The germination of N. tropicalis urediniospores in vitro is highly variable, even with standardized methodology. It has been demonstrated that at 30 °C, there is a low proportion of urediniospores germination on Petri dishes; however, at the same temperature, the severity of rust is remarkably high. The objective of this study was to estimate the proportion of urediniospores germination of N. tropicalis in vitro and in vivo at different temperatures. The in vitro assays were carried out with polystyrene dishes and the in vivo assays were performed on detached leaves of grapevines cv. Niagara Rosada and cv. Cabernet Sauvignon. A generalized beta model was fitted to the proportion of urediniospores germination as a function of temperature by non-linear regressions. The proportions of urediniospores germination of N. tropicalis were higher in vivo than in vitro, regardless the temperature. Urediniospore germination was inhibited at 35 °C. The optimal temperature for the germination of N. tropicalis urediniospores was 20 °C in vitro and 25 °C in vivo. The wide range of optimum temperature for the germination of N. tropicalis urediniospores on grapevine leaves may explain the importance of tropical Asian grapevine leaf rust in the tropics, as well as the increased disease severity observed at 30 °C.