Distribution of plant pathogenic Aphanomyces species in Sweden, Denmark and Lithuania and its relationship with soil factors

Abstract Three species of Aphanomyces, A. euteiches, A. cochlioides and A. cladogamus are devastating agricultural plant pathogens causing root rot disease on pea, sugar beet and spinach, respectively. Here we investigated the distribution of the three Aphanomyces species in parts of Sweden, Denmark and Lithuania using climate chamber soil bioassays, and tested for correlations with macronutrient levels in the soil. Aphanomyces euteiches was the most widespread pathogen found in 28% of the tested samples in Sweden and Denmark, while A. cochlioides had a more limited distribution, and was found in 22% of the samples. Aphanomyces cladogamus was only detected in Sweden and was present in 16% of the tested samples. Aphanomyces euteiches only infected pea plants, while A. cochlioides and A. cladogamus infected both sugar beet and spinach. There were significant ( P < 0.001) differences in disease severity index (DSI) between regions, pathogens and for the interaction between regions and crops. Soil calcium (Ca) content ranged from 82 to 3600 mg/100 g dry soil. No species of Aphanomyces was detected in soils with Ca levels above 250 mg/100 g soil, indicating a pathogen-suppressive effect from calcium. Similar results were observed from 1350 soil samples collected from commercial pea growing fields, as DSI values on pea remained very low (below 20 on a 0–100 scale) in soil samples with confirmed presence of A. euteiches and a Ca value above 210 mg/100 g soil. High Ca levels was a significant ( P < 0.05) predictor for low probability of presence of Aphanomyces spp. in soil.