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There is increasing evidence that microbial volatiles (VOCs) play an important role in natural suppression of soil-borne diseases, but little is known on the factors that influence production of suppressing VOCs. In the current study we examined whether a stress-induced change in soil microbial community composition would affect the production by soils of VOCs suppressing the plant-pathogenic oomycete Pythium. Using pyrosequencing of 16S ribosomal gene fragments we compared the composition of bacterial communities in sandy soils that had been exposed to anaerobic disinfestation (AD), a treatment used to kill harmful soil organisms, with the composition in untreated soils. Three months after the AD treatment had been finished, there was still a clear legacy effect of the former anaerobic stress on bacterial community composition with a strong increase in relative abundance of the phylum Bacteroidetes and a significant decrease of the phyla Acidobacteria, Planctomycetes, Nitrospirae, Chloroflexi, and Chlorobi. This change in bacterial community composition coincided with loss of production of Pythium suppressing soil volatiles (VOCs) and of suppression of Pythium impacts on Hyacinth root development. One year later, the composition of the bacterial community in the AD soils was reflecting that of the untreated soils. In addition, both production of Pythium-suppressing VOCs and suppression of Pythium in Hyacinth bioassays had returned to the levels of the untreated soil. GC/MS analysis identified several VOCs, among which compounds known to be antifungal, that were produced in the untreated soils but not in the AD soils. These compounds were again produced 15 months after the AD treatment. Our data indicate that soils exposed to a drastic stress can temporarily lose pathogen suppressive characteristics and that both loss and return of these suppressive characteristics coincides with shifts in the soil bacterial community composition. Our data are supporting the suggested importance of microbial VOCs in the natural buffer of soils against diseases caused by soil-borne pathogens.

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Physical soil disinfestation is worldwide mainly applied in protected cropping systems or in small-scale intensive field crops. Continuous cropping of monocultures or different host plants for the same pest or pathogen often leads to heavily infested soils which forces growers to disinfest the soil thoroughly. The oldest method is steam sterilization which is applied for more than a century. In the 1970s hot water treatment of soils was developed in Japan. Hot water is applied to the soil surface to raise soil temperatures to lethal levels. Since the 1980s inundation became a new method in the Netherlands and is applied in bulb cultures. This so-called flooding of soils creates anaerobic soil conditions in which toxic compounds like greenhouse gases are produced. A new approach in soil disinfestation in the current century is hot air treatment which is developed in Israel. The method is based on blowing extremely hot air into soil particles which are thrown above soil surface into a heat chamber by a rotary spading device.In the Netherlands renewed interest has started in radiation of soil with micro waves. Radiation of soil or other substrates has been studied for decades. The mechanism is that the heat produced by radiation is lethal to pathogens and pests. Anaerobic soil disinfestation (ASD) with incorporation of fresh organic matter in soil is a method developed in the Netherlands. A new and promising Dutch development in ASD is the application of defined products. Advantages and disadvantages of all methods are presented in this paper.