Phomopsis diachenii belongs to the fungi recently detected on the herbs in the Polish climatic conditions. The purpose of this work was to study the fungus harmfulness to sowing material, seedlings, stems and leaves of caraway. The pathogenicity tests according to Koch’s postulates were carried out for 2 and 4 P. diachenii isolates using post-culture liquids, water suspension of conidia and discs of fungus sporulating mycelium to inoculate the plants’ parts. The studies were conducted both in the laboratory and a climatic chamber. P. diachenii caused disease symptoms in the form of necrosis on all plant organs tested. The ability for asymptomatic P. diachenii colonization of inoculated plants’ parts was determined. It was found that injuries of the plant epidermal tissue significantly increased the efficiency of inoculation. The performed pathogenicity tests and the results from electron microscopy SEM observation showed that the tested isolates of P. diachenii were occasional pathogens of caraway.

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