White blister rust, caused by the oomycete Albugo candida, is a widespread disease of Brassica crops. The Brassica relative Arabidopsis thaliana uses the paired immune receptor complex CSA1-CHS3/DAR4 to resist Albugo infection. The CHS3/DAR4 sensor NLR, which functions together with its partner, the helper NLR CSA1, carries an integrated domain (ID) with homology to DA1 peptidases. Using domain swaps with several DA1 homologs, we show that the LIM-peptidase domain of the family member CHS3/DAR4 functions as an integrated decoy for the family member DAR3, which interacts with and inhibits the peptidase activities of the three closely related peptidases DA1, DAR1, and DAR2. Albugo infection rapidly lowers DAR3 levels and activates DA1 peptidase activity, thereby promoting endoreduplication of host tissues to support pathogen growth. We propose that the paired immune receptor CSA1-CHS3/DAR4 detects the actions of a putative Albugo effector that reduces DAR3 levels, resulting in defense activation.