Does organic farming increase soil suppression against Fusarium wilt of melon?

Under Israeli organic farming conditions, compost (mostly made of animal manure) is annually applied as a source of plant nutrients, to enhance soil microbial activity and nutrient cycling and to improve soil structure. Composts are also known for their suppressive properties against soil-borne diseases. The objective of the present study was to assess the level of suppressiveness against Fusarium oxysporum f. sp. melonis (FOM) that may develop in soils with a history of organic farming as compared with conventional farming. Pairs of organic vs. conventional taxonomically identical soil samples were collected from adjacent plots, at various sites throughout the main agricultural areas in Israel. Out of 15 pairs, four of the organically managed soils were significantly more suppressive than their corresponding conventional plots. On the average, the area under the disease progress curve and the final disease incidence of the inoculated plants grown in conventionally managed soils were higher at 12% and 21%, respectively, than those of the plants grown in the organically managed soils. Both differences were significant when examined using two-factor ANOVA test (site and farming system). Microbial activity of the organic soils, as expressed by heat production rate, was positively correlated with soil suppressiveness and was significantly higher by a factor of 2.2 as compared with the conventionally managed paired soils. It is suggested that organic farming practices and especially compost application may lead, with time, to some reduction of the problems caused by FOM. This beneficial effect of organic farming seems non-specific to soil type or climatic region in Israel.