Identification of resistance to Phytophthora ×alni could provide the basis for a management strategy against alder decline in riparian ecosystems in Europe. Our objectives were to test methods to evaluate the resistance of riparian alders to the disease, and to screen alder genotypes for resistance. P. ×alni isolates were compared for their aggressiveness (lesion length on stem) and sporulation capacity (sporangia). While no difference of lesion lengths was found between isolates, sporangia production was dependent on isolate, highlighting the need to select carefully isolates for inoculation methods dealing with zoospores suspension. Inoculation tests carried out at different periods of the year revealed a seasonal change in susceptibility to the disease, with the period from June to September being the most efficient for inoculation tests. Stem-wounded inoculations tests carried out on excised shoots were unreliable for evaluating the level of resistance of alder genotypes to P. ×alni infection, with divergent results between two successive years or between two inoculation periods during the same year. In contrast, a method which mimics the natural conditions of infection based on flooding of rooted cuttings in artificially infected river water was found promising. Another method based on the inoculation of foliated terminal shoots with zoospore suspensions was found to be repeatable and could be used for high throughput analyses. Altogether, the results show a continuous resistance response from highly susceptible to moderately resistant genotypes. This suggests that breeding might be a useful strategy to manage alder decline caused by P. ×alni.