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Duration: From Nov 1, 2015 until Mar 31, 2025

About the Project:

What causes replant problems in tree nurseries and fruit farms that repeatedly grow apples in the same soil? The interdisciplinary team at ORDIAmur has studied this complex topic and developed and tested sustainable countermeasures based on the latest findings.
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Apple harvest - Research into overcoming replant disease is essential to secure yields and keep soil health ( photo: Lukas Lühs).

Goal and approach

In fruit tree nurseries and apple orchards worldwide, there is a risk of high crop losses if apples are planted in soils where apples have previously been grown. The causes of these replant problems are not fully understood and sustainable management measures are missing. The BonaRes project ORDIAmur aimed to develop alternatives to chemical soil disinfection that are based on a better understanding of apple replant disease and are economically and ecologically viable. Therefore, researchers from 14 German universities and research institutions worked on the topic.

At three locations, reference plots with apple disease were created through repeated apple cultivation and control plots were created on which no apples were cultivated. Humus, nutrients and structural properties were analysed in the soil from these plots and the organisms, primarily nematodes, fungi and bacteria, were characterized using classical isolation approaches and modern sequencing techniques. Plants grown in regrowth and control soils were analyzed for differences in gene expression and the content of secondary constituents in roots. Important partners were also farms that were interviewed for risk analyses and where counteractive measures were tested.

Most important findeings

Depending on the availability of new sites, the disease is a problem in apple and even more so in fruit tree production, for which commercially available diagnostics and economical, reliably effective countermeasures are still missing. In most cases, companies take a pragmatic approach to the risk of disease. Apple roots react to the replant disease and produce phenolic substances, including defense substances (phytoalexins), which are partly released into the soil. However, this defense mechanism of young plants becomes a problem when these substances lead to an accumulation of microorganisms at the soil-root interface that are able to tolerate these substances. As a result, the diversity of the rhizosphere microbiome, which provides important functions for plant growth, is reduced. Interestingly, microbial genes that encode enzymes for the degradation of phenolic substances are less strongly expressed in replanted soils. Strikingly, replanting disease is a very localized phenomenon. Both the imbalance of soil organisms and the reaction of the plant are highly localized. Microorganisms were isolated that could compensate for the observed deficits if the soil or plants were inoculated with them: an inoculum consisting of a bacterial strain that can break down phenolic compounds, a plant growth- stimulating bacterial strain and arbuscular mycorrhizal fungi was tested in greenhouse experiments. This inoculum shows positive effects in the treatment of replant disease, as does a second inoculum consisting of a mixture of already known plant growth-promoting bacteria and fungi.

However, trials under field conditions on the reference areas and with practice partners have shown only partially positive effects of the inoculations so far. Another option currently being tested to restore a resilient, diverse soil microbiome is the cultivation of catch crops, in particular catch crop mixtures in cooperation with the BonaRes project CATCHY. There are also clear differences in the communities of nematodes and springtails in the larger soil organisms in replicated and control soils.

The involvement of phytopathogenic nematodes postulated in the literature could not be confirmed. Rather, certain free-living nematodes and the microorganisms associated with them are important. Tolerant plants would be a sustainable solution. For this reason, apple rootstocks and rootstock varieties were tested in apple replant diseased soils. None of the genotypes tested were tolerant on all soils. However, individual genotypes were superior in certain soils. The consortium was able to show that apple plants in soils with replant disease exhibit strongly altered gene expression patterns, which made it possible to identify indicator genes that can be used to assess the severity of post-conversion disease as well as the effectiveness of countermeasures.

Outlook

The ORDIAmur project will run until the end of March 2025. A large central experiment is currently being evaluated to identify factors that help to predict the risk of replant disease. This should make it easier for producers to plan their cultivation. In addition, the derived management strategies are being tested with the aim of making soil life more diverse and resilient.

People and Partners

Project Leaders

  • Traud Winkelmann

    Leibniz University Hannover

Partner Organizations

  • Landwirtschaftskammer Schleswig Holstein

  • Julius Kühn Institute (JKI)

  • Helmholz Zentrum München

  • Helmholtz Centre for Environmental Research – UFZ

  • University Bayreuth

  • TU Braunschweig

  • Hochschule für Wirtschaft und Technik Dresden

  • Zentrum für Betriebswirtschaft im Gartenbau e.V.

  • Rheinische Friedrich- Wilhelms-Universität Bonn

  • Hochschule Bonn-Rhein-Sieg

  • TU Dortmund

  • Leibniz Centre for Agricultural Landscape Research (ZALF)

  • HU Berlin

Contacts

  • Traut Winkelmann

    Gottfried Wilhelm Leibniz University Hanover