| Summary: | Apple replant disease (ARD) is a severe problem in orchards and tree nurseries caused by yet unknown soil biota that accumulate over replanting cycles. This study tested the contribution of nematodes to ARD, and cultivation of <i>Tagetes</i> as a control option. In a pot experiment, <i>Tagetes patula</i> or <i>Tagetes tenuifolia</i> were grown in ARD soil, incorporated or removed. Nematodes extracted from untreated ARD soil and washed on 20-µm sieves induced ARD symptoms when inoculated to apple plantlets growing in a sterile substrate. In contrast, nematodes from <i>Tagetes</i> treated ARD soil did not reduce root growth compared to uninoculated plants, irrespective of <i>Tagetes</i> species and incorporation. In plots of five apple tree nurseries or orchards, either <i>Tagetes</i> or grass was grown on ARD soil. Nematodes extracted from the grass plots and inoculated to apple plantlets significantly reduced plant growth compared to nematodes from <i>Tagetes</i> plots for all five farms. Apple rootstocks showed overall a significantly higher increase in shoot base diameter when grown on <i>Tagetes</i>-treated plots compared to grass plots, while this effect differed among farms. Plant-parasitic nematodes were too low in abundance to explain plant damage. In conclusion, <i>Tagetes</i> alleviated ARD by changing the nematode community in soil.
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