Abstract
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 Tagetes as a control option. In a pot experiment, Tagetes patula or Tagetes tenuifolia 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 Tagetes treated ARD soil did not reduce root growth compared to uninoculated plants, irrespective of Tagetes species and incorporation. In plots of five apple tree nurseries or orchards, either Tagetes 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 Tagetes plots for all five farms. Apple rootstocks showed overall a significantly higher increase in shoot base diameter when grown on Tagetes-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, Tagetes alleviated ARD by changing the nematode community in soil.
Highlights
Apple replant disease (ARD) has been recognized throughout pome fruit production regions of the world and has been studied extensively [1,2,3,4]
We investigated whether nematode communities recovered from ARD soil affect apple plant growth and whether pre-culture of Tagetes in ARD soil reduces the negative effect of the nematode community on apple plant growth
Since different Tagetes species produce different biocidal compounds, which may be exuded or not from the root, we tested whether T. patula and T. tenuifolia differ in their effect on nematode-mediated ARD, and whether incorporation of Tagetes into the soil is necessary for mitigation of nematode-mediated ARD
Summary
Apple replant disease (ARD) has been recognized throughout pome fruit production regions of the world and has been studied extensively [1,2,3,4]. Fruit yield and quality are significantly reduced [6]. Consensus regarding the causality of replant disease has not been fully realized, mitigating measures such as soil pasteurization or fumigation significantly improved the growth of apple plants, which gives evidence that the disease is caused by biotic factors [7]. Replant disease, known as soil fatigue, was described as the phenomenon that soil gradually loses its capacity to support growth of a specific plant after replanting without any obvious reasons, especially if plant damage cannot be attributed to known pathogens or plant-parasitic nematodes [8,9].
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