Zucchini (Cucurbita pepo L. var. giromontina) is a dependable vegetable in cultivation which is characterized by high fertility. Grown in the field and under cover, it is infected by a number of pathogens specific of the family Curbitaceae as well as those that are characteristic of other plantations. Actually, natural products such as plant extracts are more and more frequently used in plant protection from pathogens. The azoxystrobin (Amistar 250 SC, Syngenta) and extract of grapefruit (Biosept 33 SL, Cintamani Poland) were tested in vitro for their effectiveness to inhibit the linear growth of Alternaria alternata, Botrytis cinerea, Fusarium avenaceum, F. culmorum, F. equiseti, F. oxysporum, Rhizoctonia solani and Trichoderma hamatum. The azoxystrobin (0.05%, 0.1%) and extract of grapefruit (0.1%, 0.2%) were applied in different concentrations. The studies made use of a Petri dishes method recommended for testing fungicides in laboratory conditions. Extract of grapefruit was more effective than azoxystrobin. The extract of grapefruit inhibited the growth of A. alternata, B. cinerea, R. solani and T. hamatum already after 4 days and was the most effective after 12 days of experiment. Azoksystrobin inhibited the growth of B. cinerea and R. solani but did not inhibit the growth of F. equiseti. Microscopic analysis showed the morphological changes in fungal hyphae under the influence of tested substances.

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