Unseen population build-up of Pratylenchus in organically grown clover grass leys

Abstract. Legumes constitute up to 50 % of all crops in organic rotations surrendering these susceptible towards a range of soil-borne pests and diseases, such as plant-parasitic nematodes of the genera Meloidogyne and Pratylenchus. Here, we ask how plant-parasitic and other free-living trophic nematode guilds are affected by four diverse organic farming strategies, i.e. three stockless organic farm types “Cash Crop”, “Soil Fertility”, “Vegan” Farm type and one “Mixed” Farm type which includes cattle, each with individually designed legume cropping frequencies.

These farm types have been set up with individual fertilization strategies in four replicates in 2017 in an organically managed long-term experiment. Soils were always sampled in spring between 2022 and 2024 (not all Farm types were sampled each year). Nematodes were extracted from soils using Oostenbrink elutriators, enumerated and eventually identified to the genus and/or family level. Morphology and molecular analysis of the 18S region helped to identify Pratylenchus to the species level. Average over all years, the Mixed Farm type harbored 3356 nematodes 100 ml soil^-1, between 37 and 52 % more than the other Farm types. Plant-parasitic nematodes were dominant in all farm types constituting 40 % (Soil Fertility Farm type) to 80 % (Mixed Farm type) of the total nematode community. Pratylenchus was the most abundant genus with up to 1,300 specimen 100 ml soil^-1 within the treatments of the Mixed Farm type. Organic fertilizer had inconsistent impacts on nematode trophic groups but negatively affected the herbivorous family of Tylenchidae. A strong positive correlation (R² = 0.78, p = 0.004) of clover grass biomass production and numbers of Pratylenchus in the Cash Crop Farm type was observed in the last sampling year. Such a hidden population build up emphasizes further monitoring of the population dynamic of involved Pratylenchus spp. over the course of the rotation.