Pea–cereal intercropping may combine ecological disease regulation with improved land-use efficiency, but field evidence for pea powdery mildew responses on the pea component under temperate continental conditions remains limited. A two-year field experiment (2017/2018 and 2018/2019) was conducted in Novi Sad, Serbia, to evaluate the effects of intercropping on pea powdery mildew disease index (DI%), pea grain yield, seed physical quality traits, and land-use efficiency. Winter pea cv. Kosmaj was grown as a sole crop or in mixed intercropping (70% pea + 30% cereal seeding rates) with wheat, triticale, rye, or oat in a randomized complete block design with four replicates. Powdery mildew DI% was assessed at BBCH 71–75, while pea grain yield, thousand-seed weight (TSW), hectoliter weight (HLW), and yield-based land equivalent ratio (LER) were determined at harvest. Under the low natural disease pressure recorded in the study, intercropping was associated with lower DI% than sole cropping (approximately 2.8-fold lower on seasonal means; p < 0.001), but DI% did not show a significant independent effect on pea grain yield, TSW, or HLW after accounting for year and cultivation system. Pea grain yield was generally lower under intercropping, although the magnitude of reduction depended on the cereal companion; pea–triticale maintained pea yield closest to the sole crop, whereas pea–oat showed the lowest pea yield. TSW and HLW were generally higher under intercropping, but additional analyses indicated that these traits reflected different response patterns. All intercrops achieved LER > 1, with the highest values recorded for pea–triticale.