Applied Soil Ecology 15 2000 243–251
The role of low molecular weight organic acids from decomposing rye in inhibiting root-knot nematode populations in soil
Robert G. McBride
a,∗
, Robert L. Mikkelsen
a
, Kenneth R. Barker
b
a
Department of Soil Science, North Carolina State University, Raleigh, NC 27695-7619, USA
b
Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695-7616, USA Received 26 August 1999; accepted 17 January 2000
Abstract
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 mmoll. 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 mmoll 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.
Keywords: Low molecular weight organic acids; Meloidogyne incognita root-knot nematode; Secale cereale rye; Soil amendment
1. Introduction
Plant-parasitic nematodes are invertebrate worm-like animals that require a susceptible host plant on which
to feed in order to complete their life cycles. More than 1200 species of nematodes have been found to attack
plants with virtually every crop being susceptible to a
∗
Corresponding author. Tel.: +1-919-515-2655; fax: +1-919-515-2167.
E-mail address: rgmcbridunity.ncsu.edu R.G. McBride.
particular assortment. Nematode control includes the use of resistant plant cultivars, application of chemi-
cal nematicides, and cultural practices, such as tillage and crop rotation. Alternative methods of control
include the addition of organic materials to the soil andor microbial antagonists Jairajpuri et al., 1990.
It has long been observed that the addition of or- ganic materials to the soil can benefit plant growth.
Organic amendments can also limit the severity of plant-parasitic nematode damage Akhtar and Alam,
1992, 1993; Akhtar and Mahmood, 1994. Nematode
0929-139300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 9 - 1 3 9 3 0 0 0 0 0 6 2 - 7
244 R.G. McBride et al. Applied Soil Ecology 15 2000 243–251
control resulting from the use of organic amendments has been attributed to several factors, including toxic
effects of nitrogen, predatory fungi, nematodes, in- sects, and mites, as well as organic acids and their
interactions. The contribution of organic acids from decomposing organic matter has been assumed to be
the causative factor for many years Stephenson, 1945; Johnston, 1959; Sayre et al., 1964; Badra et al., 1979.
However, no research has been done to measure organic acids produced in situ and their subsequent
influence on nematode infection.
Low molecular weight organic acids have been re- ported to be present in anaerobic soils following the
addition of plant material Gotoh and Onikura, 1971; Chandrasekaran and Yoshida, 1973. In well-aerated
soils, these acids generally exist in low concentrations and only for relatively short periods of time, being
rapidly broken down and utilized by bacteria and fungi Schwartz et al., 1954; Hollis and Rodriguez-Kabana,
1966; Lynch, 1991. These soluble organic products are generated as metabolic by-products from soil or-
ganisms, such as bacteria and fungi, and are also ex- creted from plant roots McLaren and Peterson, 1967.
The limited quantities and transient nature of low molecular weight organic acids in aerobic soils ap-
pear contradictory to the nematicidal merit these acids have received. Our objective was to quantify the per-
sistence and quantity of the five most cited low molec- ular weight organic acids in the soil solution after the
addition of fresh rye. We also sought to determine any associated change in the root-knot nematode pop-
ulation, a worldwide pest responsible for damage to nearly every food crop.
In the first experiment, the effects of three applica- tion rates of rye on the root-knot nematode infection
and the concentration of five low molecular weight or- ganic acids in the soil solution were evaluated. A sec-
ond experiment quantified the rate at which five low molecular weight organic acids were removed from
the soil solution over a 10 h period.
2. Materials and methods
