Abiotic characteristics of soils suppressive to Aphanomyces root rot

Soils showing suppressiveness to Aphanomyces root rot of pea in bioassays and field experiments were surveyed in an area intensively cultivated with vining pea in southern Sweden. By examining the relationships between disease suppression, soil mineralogy, and selected physicochemical properties of 24 soils with different degrees of suppressiveness, the suppressive soils could be divided into two groups, mainly on the basis of their textural characteristics. Both soil groups are developed in unsorted sediments. The first group, S1, consisted of soils with a low content of clay (9–12%), and a high content of sand (56–73%). The second group, S2, consisted of soils with a clay content of 19–21%, high pH (>6.7), and a high content of calcium (>17 cmol kg−1). The ratio of the peak intensity of vermiculite–smectite to the peak intensity of illite–kaolinite in the X-ray diffractograms was high in these soils, and an increase in disease suppression was closely related to an increase in this ratio. There were also significant correlations between disease suppression on one hand and content of clay, calcium and pH on the other. The results suggest that soils disease suppressive to Aphanomyces root rot can be found by searching for soils with specific abiotic characteristics.