<i>Corynespora cassiicola</i>, the cause of Corynespora leaf spot disease, is one of the most important pathogens on cucumber in Japan. This fungus easily developed resistance to various fungicides such as benzimidazole, dicarboximide, QoI, and SDHI fungicides. Spray applications of SDHI fungicides were stopped and alternative fungicides have been used in the commercial greenhouse tested in this study as boscalid-resistant strains of <i>C. cassiicola</i> were found to be widely distributed in the area. In order to know the stability of boscalid resistance after withdrawal of SDHI fungicide applications, infected cucumber leaves were sampled every year between 2010 and 2013, single conidium isolates obtained, and mycelial growth tests conducted on fungicide-amended YBA agar medium to examine sensitivity. All of the 21 isolates collected from the greenhouse in 2013, three years after the withdrawal, still showed very high resistance to boscalid indicating that this type of resistant strains were stable in a greenhouse in the absence of selection pressure by SDHI fungicides. In contrast, these isolates all exhibited sensitivity to fluopyram which further confirmed the lack of cross resistance in very highly boscalid-resistant isolates to fluopyram as reported before.