Abstract

Aphanomyces root rot, caused by the oomycete Aphanomyces euteiches, is the most important and destructive disease of pea. Since the inoculum is persistent in the soil, and no resistant cultivars or means of chemical control are available, the disease is difficult to manage. The occurrence of soil suppressiveness against the disease was previously reported, and the main aim of this study was to understand the mechanism. By analysing chemical data for naturally infested soil samples from a large number of fields, we found that disease suppression was correlated to the Ca content of the soil. This was also true for artificially inoculated soil samples. Suppressiveness was only inconsistently affected by soil sterilisation, and was concluded to be abiotic in nature. Amendments of non-suppressive soils with both neutral and alkaline Ca salts strongly reduced disease severity. In contrast, amendment of suppressive soils with NaHCO3 solutions lowered the content of soluble Ca, increased pH and made the soil less suppressive. It was concluded that high concentrations of Ca in the soil solution was a major factor causing suppressiveness. The effect of Ca in the field was evaluated by adding a neutral Ca salt, CaSO4 in a naturally infested field trial. A clear delay in the onset of disease symptoms was seen at all doses evaluated. A quantitative molecular detection method based on quantitative PCR was developed and compared with the greenhouse biotest traditionally used to detect the pathogen in soil samples. Using the qPCR assay, it was possible to detect and quantify the pathogen at concentrations under 10 oospores g soil-1. The Swedish population of A. euteiches was analysed using co-dominant DNA sequence markers. It was found that the population was dominated by a globally distributed genotype, but that distinctly different genotypes also exist. These results were interpreted as evidence of a predominantly clonal mode of reproduction, but also of occasional sexual recombination.