*Corresponding Author E-mail: awestphal@rsru2.tamu.edu © 2001 Association of Applied Biologists Ann. appl. Biol. (2001), 138:371-376 Printed in Great Britain 371 Impact of soil suppressiveness on various population densities of Heterodera schachtii By A WESTPHAL* and J O BECKER Department of Nematology, University of California, Riverside, CA 92521, USA (Accepted 5 January 2001; Received 27 September 2000) Summary Infectivity of second-stage juvenile (J2) populations of Heterodera schachtii was assayed with radish. The numbers of J2 in three-day-old seedlings were proportional to the numbers of J2 in two differently textured soils. In a microplot trial with a known H. schachtii-suppressive soil, half of the plots contained untreated suppressive soil, the other half contained the same soil, but methyl iodide-fumigated and therefore conducive. Both soils were infested with cysts introducing the equivalents of 0, 30, 60 or 120 H. schachtii eggs g -1 soil, kept moist for 2 months, and then planted to Swiss chard. The numbers of J2 in radish roots were proportional to the numbers of H. schachtii eggs introduced into the microplots, at a low level of detection in suppressive soil and at a high level in conducive soil. Growth of Swiss chard was not different at increasing infestation levels in suppressive soil, but growth was reduced in conducive soil proportionally to increasing nematode infestation level. Key words: Beta vulgaris, bioassay, biological control, density-dependence, Heterodera schachtii, sugar beet cyst nematode, suppressive soil Introduction Soils suppressive to plant-parasitic nematodes are of increasing interest because natural suppression has potential to contribute to the integrated management of plant-parasitic nematodes. One of the most studied examples of natural control of a plant-parasitic nematode is the biological suppression of the cereal cyst nematode Heterodera avenae Woll. in Great Britain (Kerry, Crump & Mullen, 1980). The cereal cyst nematode is reduced to below economic threshold levels, primarily by two fungi (Kerry et al., 1980). Even though ecological interactions of single antagonists and plant-parasitic nematodes, e.g. Hirsutella rhossiliensis Minter & Brady and Heterodera schachtii Schm., have been studied under laboratory conditions (Jaffee, Philips, Muldoon & Mangel, 1992; Jaffee, Tedford & Muldoon, 1993), little is known about the quantitative interactions of plant- parasitic nematodes with soil suppressiveness in a naturally suppressive soil. In studies of soil suppressiveness to soilborne plant-pathogenic fungi, the addition of a series of dosages of the pathogen of interest to soils of unknown status of suppressiveness has been useful to determine quantitative interactions of an unknown group of antagonists with the pathogen. Typically, the onset of disease symptoms in plants was observed at higher inoculum levels in soils that were suppressive than in conducive soils; relatively low inoculum levels caused disease in conducive soils (Burke, 1965; Louvet, Alabouvette & Rouxel, 1981; Smith & Snyder, 1971). For our study, we devised a bioassay with radish seedlings to measure the infectivity of second-stage juvenile (J2) populations of H. schachtii in two different soils. The assay was used to evaluate the effect of soil suppressiveness on various infestation levels of H. schachtii by comparing root invasion in suppressive and conducive soil in a microplot trial. The suppressiveness of the soil used in this study was reduced to non-detectable levels by biocidal soil treatments (Westphal & Becker, 1999). The assay has been used to monitor the onset of suppressiveness in soil transfer studies (Westphal & Becker, 2000). The objective of the current study was to examine the impact of soil suppressiveness on different infestation levels of the nematode. Materials and Methods Standardisation of bioassay Soils were collected from two different agricultural sites with known infestations of Heterodera schachtii. The first soil (RI) from field 9E at the Agricultural Operations field station of the University of California, Riverside was a Hanford sandy loam (60.9% sand, 29.6% silt, 9.5% clay, pH 8.2). This field was suppressive to H. schachtii (Westphal & Becker, 1999). Since 1975 it had been cropped almost exclusively to various host plants