This study was conducted to evaluate the interactive effects of twelve culture filtrates of Fusarium culmorum isolates (G14, K8, YLVC16, SK20, CAV6, T21, GOL2, GOL18, YOVA21, YOVA22, YOVA27, K17) and Pratylencus thornei (SK24 isolate) on nematode disease severity and nematode reproduction in four barley varities i.e. Tosunpaşa, Tarm92, Burakbey, Aydanhanım in this study. The F. culmorum isolates and P. thornei were taken from Lake regions in Türkiye and cultured under controlled conditions. Disease severity and nematode reproduction were assessed eight weeks post-inoculation. The experiment included three treatments: (1) only F. culmorum culture filtrate isolates (FCUL), (2) only P. thornei (N) and (3) co-inoculation of P. thornei and F. culmorum (N+FCUL). The experiment was laid out following comletely randomized design under factorial arrangement with 5 replications for each barley variety and each isolate treatment. All barley cultivars were found to be susceptible to F. culmorum. Culture filtrates of F. culmorum K17 and CAV6 isolates resulted in the highest disease severity. Simultaneous inoculation of P. thornei and culture filtrates of G14, YLVC16, and SK20 significantly increased disease severity across allbarley cultivar. The highest mean nematode reproduction rate occurtsred in Burakbey (7.10), and the lowest mean nematode reproduction was in Tosunpaşa (4.60). The nematode reproduction in the culture filtrates of YOVA27 and K17 isolates increased in all varieties in the co-inoculation (N+FCUL) treatments. However, it was determinated that the nematode reproduction rate was lower in nine culture filtrates of F.culmorum (G14, K8, SK20, CAV6, T21, GOL2, GOL18, YOVA22, YOVA21) treatments than only nematode treatment. Lesion nematode reproduction had different in only nematode and the co-inoculation with culture filtrate of F. culmorum isolates treatment in barley varieties. However, the culture filtrates of F. culmorum negatively affected P. thornei; and no direct relationship with fungal aggressiveness was established. These findings enhance our understanding of nematode-fungus interactions and may aid in developing new integrated disease management strategies.