Calcium concentrations of soil affect suppressiveness against Aphanomyces root rot of pea

The potential for field soils to cause Aphanomyces root rot of pea (Pisum sativum) was estimated for a large number of samples from commercial pea fields over a period of 5 years, using a greenhouse bioassay. The aim of the research project was to gain a mechanistic understanding of soil suppressiveness to the disease. Regression analysis showed that of the measured soil variables (Ca, Mg, K, P, pH), soil Ca concentrations had the strongest (negative) correlation with disease prevalence, and also a significant negative correlation with disease severity in samples with confirmed presence of the disease. Greenhouse bioassays using a set of non-infested soils inoculated with artificially produced oospore inoculum of the casual organism Aphanomyces euteiches, showed a similar negative correlation between soil Ca content and disease severity. Disease severity was not consistently affected by soil sterilisation, but was lowered by the addition of two different Ca salts. In contrast, addition of sodium bicarbonate to two soils lowered the content of water-soluble Ca in the soils and increased disease severity. Studies of cultures of A. euteiches exposed to varying Ca concentrations in vitro showed that zoospore production was inhibited at submillimolar concentrations, while mycelial growth was stimulated or unaffected. We conclude that free Ca is a major variable controlling the degree of soil suppressiveness against A. euteiches, and that inhibition of zoospore production from oospores is a possible mechanism.