The purpose of the studies was to determine the occurrence of soilborne fungi pathogenic towards onion cultivated after spring rye and common vetch. The objects of the studies were the seedlings of onion ‘Wolska’ cv and the isolates of the following fungi: Fusarium culmorum, F. oxysporum, F. solani, Penicillium verrucosum var. cyclopium, P. verrucosum var. verrucosum and Pythium irregulare. As a result of the growth chamber studies it turned out that the fungi used in the experiment were pathogenic towards onion. The harmfulness of the studied fungi varied both between the species and between the isolates within the species. P. verrucosum var cyclopium and F. culmorum proved to be the most harmful since the smallest number of onion plants grew in those experimental combinations and they were the least infected. The most onion plants with the smallest proportion of the infected ones were found in the combination with F.  oxysporum.

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