Background: The breakdown of resistance genes to stripe rust, often associated with the emergence of a new race, is a well-characterized pattern that strongly impacts both the composition of Puccinia striiformis f.sp. tritici (Pst) populations and cultivar deployment. We studied the case of a cultivar which recently experienced a consequent drop in resistance ratings in field trials. We tested 3 hypotheses to explain this observation: (i) emergence of a new race, (ii) increased aggressiveness on the considered cultivar within the same race, (iii) modifications in race frequency. Research findings: Cultivar A has experienced a major breakdown in 2019, with a drop in resistance ratings (assessed by Arvalis in field varietal trials) from 7 to 4 on the Arvalis scale (ranging from 1 most susceptible to 9 most resistant). This was associated with the emergence of a new race called Amboise, belonging to genetic group Pst10. In 2020, cultivar B, previously considered as resistant (7), experienced a large drop in resistance ratings (5). The same year, the rating of cultivar A also dropped again, from 4 to 3. The area cultivated with cultivar A (assessed by FranceAgriMer) was stable between 2019 and 2020, while the area of cultivar B increased dramatically during that period, reaching the same area as cultivar A in 2020. The Amboise race strongly increased in frequency (as assessed in the national French Pst survey) from 40% of Warrior (-) isolates in 2019 (first detection) to 65% of Warrior (-) isolates in 2020, and became in one year the most frequent race in the French population. To test the hypothese of a new race attacking specifically cultivar B, we pathotyped 13 samples from cultivar B and found that all belonged to the Amboise race. We tested 5 samples collected from cultivar A (in 2019 and 2020) on cultivar B at seedling stage and observed that all samples attacked cultivar B. We therefore concluded that no new virulence (at least at seedling stage) was involved. To test the hypothesis of an increase in aggressiveness, we assessed latency period (expressed in hours post inoculation or hpi) in optimal conditions for a set of 9 isolates, tested on cultivar A and B at seedling stage. The isolates included six samples from cultivar A (3 collected in 2019 and 3 in 2020) and 3 samples collected on cultivar B in 2020. We observed significant differences of latency period between isolates (quickest: 210.6 hpi, slowest: 230.9 hpi), but there was no difference between groups of isolates considering their cultivar of origin and sampling year. Latency period was generally shorter on cultivar B, but this was significant only for 3/9 isolates. Isolates from cultivar A (both years) were quicker on cultivar B (2019: 219.6 hpi, 2020: 218.3 hpi) than on cultivar A. For isolates from cultivar B (2019: 228.8 hpi, 2020: 226.4 hpi), we observed no significant difference depending on the tested cultivar. These results suggest that there was probably no major shift in aggressiveness (considering latency period at seedling stage) explaining the change of behaviour in cultivar B. Moreover, cultivar B is likely to have a weaker quantitative resistance at seedling stage (considering latency period) than cultivar A towards race Amboise. However, the limited drop of resistance of cultivar B in the field (compared to cultivar A) suggests that cultivar B might possess an adult resistance source that is not present in cultivar A. Conclusions: In this study, we gathered different sources of information and tested different hypotheses to explain a sudden drop in ratings for resistance to stripe rust of a French cultivar (cultivar B). We suggest that the breakdown of cultivar B was caused by the same race as cultivar A, but was observed with a delay. This delay could be explained by the increase in frequency of race Amboise in the mean time and possibly by the presence of an adult resistance source in cultivar B that protected it when the new race frequency was still relatively low. This case study could contribute to better understanding the shifts in cultivar resistance and improve resistance management strategies.