Parasitism is a harmful relationship between microorganisms and their host. Biotrophic fungi, such as powdery mildew and rusts, are obligate parasites that extract nutrients from living plant cells using specialized structures like haustoria and appressoria. These pathogens weaken plant defenses by inhibiting enzyme secretion, making them particularly dangerous. Blumeria graminis, causing powdery mildew, affects all cereals and forms a white coating. Pucciniales, or rusts, represent the largest group of biotrophs, with complex life cycles involving up to five spore stages, complicating their identification. Resistance to chemical pesticides and environmental concerns have driven interest in alternative, safer, plant protection solutions. One promising group is coumarin and its derivatives, known for antifungal properties and biodegradability. Our study aimed to test the effectiveness of selected coumarin compounds in inhibiting the growth of powdery mildew and rust under laboratory conditions. These findings may support the development of modern, eco-friendly fungicides. Our studies have shown that the tested coumarin derivatives were characterized by varying effectiveness in inhibiting the growth of biotrophic pathogenic fungi. Histological studies showed that two compounds led to a reduction in the number of pathogen structures, suggesting their potential effectiveness in inhibiting the spread of infection.