The role of low molecular weight organic acids from decomposing rye in inhibiting root-knot nematode populations in soil

Organic soil amendments have been employed as an alternative to or in combination with, chemical nematicides and cultural practices to control plant-parasitic nematodes. Rye (Secale cereale L.) has been shown to be effective in minimizing the damage caused by root-knot nematodes (Meloidogyne incognita (Kofoid and White) Chitwood) when grown as a cover crop and then incorporated into the soil prior to planting. It has been suggested that the release of low molecular weight organic acids during the decomposition of rye is the cause of the nematicidal effects. This study was conducted to quantify the concentration and persistence of formic, acetic, propionic, butyric, and valeric acids in soil solution following the incorporation of fresh rye foliage. Formic and acetic acids were detected by means of ion exclusion chromatography, primarily in the first 24 h following addition of rye, and at concentrations <450 mmol/l. The effect of the rye treatment on the root-knot nematode population was determined by growing tomato plants (Lycopersicon esculentum Mill.) in the rye-amended soil and assessing the nematode damage to the root systems. Despite the low concentrations of organic acids detected, the rye treatment resulted in a significant suppression of root-knot nematode activity. To determine the fate of these acids in soil, an addition of each acid was made to a field soil resulting in a soil water concentration of 1500 mmol/l for each acid. Soil solution samples were collected every 2 h for 10 h and analyzed for the five added organic acids by means of ion exclusion chromatography. The concentration of all acids declined by 54–97% over the 10 h incubation. Although low molecular weight organic acids may be one of many factors that contribute to restriction in root-knot nematode damage, these acids do not appear to be solely responsible for the nematicidal effect of the rye. © 2000 Elsevier Science B.V. All rights reserved.