1Plant Pathology and Biological Control Unit, Swedish University of Agricultural Sciences, PO Box 7035, SE-750 07 Uppsala, Sweden; 2Sweden Nestlé R&D Centre Bjuv AB, PO Box 520, SE-267 25 Bjuv, Sweden

Background and objectives
Pea root rot caused by Aphanomyces euteiches is the most destructive root disease of vining pea in Sweden [1]. The main objective of this study was to evaluate the occurrence of soil suppressiveness to Aphanomyces root rot. In a suppressive soil, the disease incidence remains low despite the presence of the pathogen, a susceptible host plant and suitable climatic conditions [2]. Disease suppression in soils in southern Sweden was assessed by using a greenhouse bioassay and the field relevance of the bioassay was evaluated by growing vining pea in monoculture in field experiments on soils ranging from highly conducive to disease suppressive.

Materials and methods
Soil suppressiveness to A. ;euteiches was assessed in a bioassay where soil samples collected from the fields were mixed with an inoculum consisting of dried oospores in talcum powder (about 800 oospores/ml of soil). The soils were left for 7 ;days in the greenhouse and thereafter sown with pea seeds. After 4 ;weeks, the plants were assessed for pea root rot using a DSI (disease severity index) ranging from 0 (=healthy plants) to 100 (=dead plants) [1]. Pea monoculture field trials were carried out on six soils ranging from conducive to suppressive, but with similar climatic conditions and history of crop rotations (pea grown once in every 6 ;years). The yield was measured every year and root pathogens were isolated on selective agar media. Roots collected from these trials were comminuted with a homogenizer and the number of oospores/g of root was counted in a haemocytometer.

Results and conclusions
By growing vining pea in monoculture on a soil that reached a high DSI in the greenhouse bioassay, we obtained a severe infestation of A. ;euteiches by the second year. The yield decreased each year of monoculture and the increase of oospores in the root tissue was very rapid with a high amount of oospores the fourth year (91,000 oospores/g of root). On a soil with a low DSI in the greenhouse bioassay, the build-up of disease problems was markedly slower and in the fourth year of monoculture the yield remained equal to the average yield in the area (5300 ;kg/ha) and the number of oospores was still low (2900/g of root). The occurrence of A. ;euteiches in the roots could be verified by isolations done on selective agar medium. Since A. ;euteiches was established in all field experiments, but the disease (assessed as number of oospores in the roots) remained low in a soil assessed as suppressive in the bioassay, we conclude (1) that some soils have a high degree of disease suppressiveness and (2) that our bioassay have relevance for field conditions. These conclusions are further supported by the high and stable yield during 4 ;years of monoculture on a suppressive soil. The soils in the area tested have different geological origins owing to the movement of continental glaciers, and it appears that the mechanisms for disease suppression, biotic or abiotic, are associated with soil type.

1. Persson L, Bødker L, Larsson-Wikström M, 1997. Plant Disease 81, 171-174.
2. Alabouvette C, 1986. Agronomie 6, 273-284.