Effect of lignin-rich crop residues on the viability of Verticillium in organic greenhouse soils

Abstract

Healthy soils are of vital importance for organic greenhouse cultivation. Due to the intensity of the cropping system, soil-borne diseases are of great concern. To achieve a more resilient system, growers use multiple strategies, including resistant rootstocks, organic soil amendments, biological soil disinfestation and plant stimulants. Soil-borne pathogens that form (micro) sclerotia are particularly difficult to manage due to their persistence in soil. Previous studies have shown that incorporation of lignin-rich crop residues can decrease the viability of microsclerotia of Verticillium longisporum in cauliflower fields. The purpose of this study was to evaluate the effect of lignin-rich amendments on the microsclerotia of V. dahliae in soils from organic greenhouses with a history of Verticillium wilt in solanaceous crops. In the laboratory, two greenhouse soils (river and sea clay) were mixed with 10% (w/w) broccoli (stem), bristle oat (whole plant), reed (stalks or leaves), Ethiopian mustard (whole plant) and corn (stalks or leaves). In the river clay soil, corn stalks, broccoli stem, reed and bristle oat significantly decreased the amount of viable microsclerotia. In the sea clay soil, only reed leaves and stalks had a significant effect. Only in the sea clay soil, a significant negative correlation was found between the amount of lignin added and the amount of viable microsclerotia detected. These results indicate that in the river clay soil, mechanisms other than lignin may be involved in Verticillium control. Surprisingly, real-time PCR analysis showed that the amount of Verticillium tricorpus was much higher than that of V. dahliae in both soils. Sequencing of a Verticillium strain isolated from the sea clay soil showed that it should be re-classified as Verticillium isaacii (formerly V. tricorpus). It is necessary to investigate the role of these different Verticillium species, before new control measures against Verticillium are further developed.