Protective effect of organic substrates against soil-borne pathogens in soilless cucumber crops

• Organic substrates (wood and coir fibers) are good substitutes for peat.
• The microbial activity in substrates is not sufficient to predict protection.
• Growth conditions, substrate type and the microbiome impact on the plant protection.

The development of sustainable crop protection is expected by vegetable producers and highly encouraged by authorities. For crops grown in soilless systems, vegetable fibers are relevant for both agronomical and plant protection purposes. This work examines their potential against the soil-borne pathogen Fusarium oxysporum f. sp. radicis-cucumerinum.Wood fiber, coir fiber and peat were tested over two cucumber cropping periods. Fusarium blight symptoms were monitored on cucumber, and fungal community structure (PCR-TTGE) in substrates. Substrate sterilization and bio-augmentation with antagonistic strains were also studied; they did not modify protection. Compared to the other substrates, wood fiber increased protection at the end of the first assay, but did not during the second assay. Differences in crop season and plant density may have impacted on cucumber physiology and may have indirectly modified rhizosphere fungal community structure.The sole determination of microbial activity in substrates is not sufficient to predict protection. Growth conditions, substrate type and the microbiome altogether impacted on the protection of cucumber.