The sensitivity of Aspergillus niger and Fusarium oxysporum f. sp. cepae to fungistasis in onion-growing soils

Twenty-seven soil samples were collected from the onion (Allium cepa L.) fields of Tekirdag province, Turkey. These samples were investigated for the sensi-tivity of Aspergillus niger V. Tieghem (AN) and Fusarium oxysporum Schlechtend.: Fr. f. sp. cepae (H. N. Hans.) W. C. Snyder H. N. Hans (FOC), known as the causal agents of black mould and basal rot of onion, respectively, to soil fungistasis. Fungistasis was evaluated using two methods: inhibition of pathogen spore germination by volatile compounds from the soil and determination of the antagonistic fungal population of soil samples. Volatile compounds in twelve of the soil samples strongly (>70%) inhibited spore germination of only AN. Inhibition rates of volatile compounds were not correlated with physical and chemical characters of the soils. Fungi isolated from soil samples were evaluated for their antagonism to both pathogens using dual cultures. The population of species causing over 70% inhibition of radial growth on pathogens was calculated in soil samples. The presence of both volatile compounds inhibiting spore germination of AN and populations of fungi antagonistic for AN and FOC were observed in six of the soil samples. The possible effects of two fungistatic mechanisms in soils on disease development by these pathogens are discussed