Introduction

Potato brown rot caused by the bacterium Ralstonia solanacearum race 3 biovar 2 is a quarantine disease with zero tolerance in the Netherlands.
The pathogen is soilborne and can persist in soil for a long time in infected host plant debris or by colonizing potato volunteer plants, alternative hosts or even non-host plants (Akiew & Trevorrow, 1994; Graham, Jones, & Lloyd, 1979; Granada & Sequeira, 1983). In the absence of potato, the pathogen was found to survive up to 3 years in soil (Graham et al., 1979). Cultural control through crop rotation is commonly used but requires long periods without host crops, and must be combined with strict control of volunteer host plants. Two seasons of non-host crops reduced wilt from 81% to only 22–49% (Lemaga, Kanzikwera, Kakuhenzire, Hakiza, & Maniz, 2001). Completely resistant potato cultivars are not yet commercially available (Zimnoch-Guzowska, Lebecka, & Flis, 2005). Most cultivars that seem to show (partial) resistance appear to be tolerant. The level of tolerance that is expressed can be negatively affected by environmental conditions (Boshou, 2004), such as high temperature (French & De Lindo, 1982), low light intensity and short photoperiods (Sequeira & Rowe, 1969). Chemical control of potato brown rot with currently available crop protectants is not effective (Hartman & Elphinstone, 1994; Lopez & Biosca, 2004). Development of more effective chemical control methods is not encouraged due to the general awareness about negative impacts of synthetic crop protectants on human health and the environment; this has led to the phasing out of an increasing number of crop protectants.
The latter is exemplified by the widely used and very effective soil fumigant methyl bromide (MB), which has been prohibited for agricultural use in developed countries since 2005 and will be prohibited in developing countries by 2015 (Duniway, 2002). Some countries, for example the Netherlands, started to restrict its use even earlier because of its toxicity (Braun & Supkoff, 1994).

In the Netherlands, potato brown rot is currently effectively managed by enforcing the statutory measures of the Brown Rot Control Directive 98/57/EC, which requires not growing potato or alternative host crops for a minimum period of four years in infested fields and applying strict control of volunteer plants in this period. However, a four-year period without host crops of R. solanacearum is a severe restriction for specialized potato farmers and costly and timeconsuming for inspection services. Therefore, there is a clear need to develop alternative practical, safe and effective management strategies that can shorten the time that no host plants can be grown.
Candidate methods include solarization (Gamliel & Stapleton, 1993; Katan, 1981), organic amendments (Conn, Tenuta, & Lazarovits, 2005; Gorissen, van Overbeek, & van Elsas, 2004; Lazarovits, Tenuta, & Conn, 2001) and Biological Soil Disinfestation (BSD). BSD is a new method, first described by Blok, Lamers, Termorshuizen, and Bollen (2000). With BSD, anaerobic soil conditions are induced by increasing microbial respiration through incorporation of fresh organic amendments (e.g. grass) and by reducing re-supply of oxygen by covering with airtight plastic sheets (Blok et al., 2000). Under these conditions the survival of a number of persistent soilborne pathogenic fungi and nematodes, including Fusarium oxysporum, Meloidogyne spp., Rhizoctonia solani, R. tuliparum, Sclerotinia sclerotiorum and Verticillium dahliae, was strongly reduced or completely eradicated in repeated field experiments over the last ten years (Blok et al., 2000; Blok et al., unpublished; Goud, Termorshuizen, Blok, & van Bruggen, 2004; Lamers, Wanten, & Blok, 2004). The effect of BSD on survival of R. solanacearum has not yet been tested. However, data obtained by 404 Eur J Plant Pathol (2007) 117:403–415 123 Termorshuizen et al. (2003) indicated that BSD could also be effective against R. solanacearum. They studied the effect of anaerobic mesophilic digestion of vegetable, fruit and garden waste on the survival of a number of pathogens including R. solanacearum and found that R. solanacearum was readily inactivated under these conditions.

The aim of the present study was to investigate the effect of BSD on survival of R. solanacearum in order to develop a practical, effective and sustainable management strategy for this important quarantine pathogen. Since many soil disinfestation methods have also a more or less drastic effect on non-target soil organisms, an attempt was made to also evaluate the effect of BSD on the soil bacterial community. Because R. solanacearum is a quarantine pathogen there are limitations to the experiments that can be carried out. The effect of BSD on the survival of R. solanacearum was first tested in a field experiment.
This was done in 1999 when potato brown rot had been recently detected in the Netherlands and a few heavily infested fields were available. In later years, the disease was under control and infested fields that could be used for further field experiments were no longer available. To further explore the potential of BSD two additional experiments were carried out in which field application of BSD was simulated, first using glass mesocosms under laboratory conditions and then using microplots situated in an outdoor quarantine area.

Materials and methods

General set-up

A field experiment was performed in a commercial field of starch potato at Dalerveen (province of Drenthe, the Netherlands) that had a high natural infestation with R. solanacearum. The effect of BSD was further studied using glass mesocosms under laboratory conditions and then in microplots consisting of concrete rings (1.2-m diam) filled with soil, which were positioned in the outdoor quarantine field of the Plant Protection Service and had a cage of metal wires around and above it. Anaerobic soil conditions were induced by increasing microbial respiration through incorporation of fresh organic amendments (grass or potato haulms) and by reducing re-supply of oxygen by covering with airtight plastic sheets (Blok et al., 2000). The soil was covered with airtight plastic tarps (Hermetix, a three-layered coextruded plastic ensilage film, 0.135 mm thick, produced by Klerks Plastic Industry, Noordwijkerhout, the Netherlands). Hermetix has a green and a white side, and the oxygen diffusion rate is 1400 ml O₂ m^–2 24 h^–1. The tarp was applied with the green side up.

Methods:

• Inoculation and isolation of the pathogen 
• Mesocosm experiment
• Microplot experiment
• Field experiment

Note: For more information about the experiments and their results, please download the full description by using the relevant link, at the bottom of this page.

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Writing Date : 2008-07-17
Latest update Date : 2008-07-21