Effect of chemical fumigation on soil fungal communities in Spanish strawberry nurseries
Introduction
Application of methyl bromide (MB) to agricultural soils before planting has been the basis to control nematodes, soil-borne pathogens (such as Phythophthora cactorum, Verticillium spp., Rhizoctonia spp., Pythium spp., and Fusarium spp.) and weeds for many years in strawberry nurseries in Spain (Duhart et al., 2000, De Cal et al., 2004). The chemistry and air pollution potential of this fumigant (Ibekwe et al., 2001a) as well as its ability to destroy stratospheric ozone have been extensively documented (Yung et al., 1980, Prather et al., 1984). MB utilization is being phased out in the European Union (Anon., 2000), and indeed in 2005, MB will be forbidden in EU countries (Batchelor, 2002). 1,3-Dichloropropene (1,3-D), chloropicrin, dazomet, metam sodium (both of them methyl isothiocyanate precursors), metam potassium and dimethyl disulphide in combination with different films have been tested as chemical alternatives to MB in Spanish strawberry nurseries (De Cal et al., 2004). Most of these fumigants show a wide activity range (Elliott and Des Jardin, 2001, Ibekwe et al., 2001a) but their effects on strawberry soil fungal communities are largely unknown (Mark and Cassells, 1999, Martín and Bull, 2002). Soil fumigants usually have damaging effects on beneficial microorganisms; therefore, identification of new alternatives to methyl bromide should be based on the efficiency against pathogens but minor side effects on beneficial microorganisms.
Soil fungi, such as other soil microorganisms, are critical to soil environment (Dalal, 1998). They can act as both sources and sinks for many elements and as agents of nutrient transformation and pesticide degradation (West, 1986, Elliott and Des Jardin, 2001, Imberger and Chiu, 2002). They also associate to strawberry plant roots in beneficial or harmful modes (Menge, 1982, Robertson et al., 1988, Mark and Cassells, 1999, Elliott and Des Jardin, 2001). Even in absence of known pathogens, strawberry crops have exhibited an improved growth response when planted into soil fumigated with MB. One of the possible reasons for this evidence is that fumigation may alter the microbial composition of the soil, either enhancing beneficial colonizers or reducing populations of damaging rhizosphere colonizers (Martín and Bull, 2002). For that reason, it is critical to know the effects of biocides such as MB and its chemical alternatives have on fungal communities, both beneficial and detrimental, of strawberry nursery soils.
In this paper, we have evaluated the effect of MB and some alternatives to MB, together with different films, on beneficial and detrimental soil fungal communities in Spanish strawberry nurseries.
Section snippets
Treatments and experimental design
Experiments were carried out during 5 years (1998–2002). Two different experimental fields located in Navalmanzano (Segovia, Spain) (A) and Arévalo (Avila, Spain) (B) were used for the experiments. In years 2000, 2001 and 2002, the fields had not been fumigated with MB for at least 6 years prior to the experiment. Strawberry or horticultural crops, and strawberry or cereal were cultivated at Navalmanzano and Arévalo fields, respectively, in former years. A randomized complete block design with
Results
No significant differences (P < 0.05) among fungal populations isolated on PDAs, FUS, PY, and VER at the first and second sample date from control plots (without soil fumigation) were obtained except for population estimated on PDAs, FUS, and VER in 2000 in both experimental fields and on PDAs and FUS in 2002 in Vinaderos (Fig. 1). Phytophthora spp. and Rhizoctonia spp. were only sporadically isolated in strawberry nursery soils.
A reduction in soil fungal populations isolated in PDAs were
Discussion
Soil fungal populations in Spanish strawberry nurseries were clearly reduced in number and composition after pre-plant soil fumigation. MB:Pic, 1,3-D:Pic, Pic alone, dazomet, metam sodium, and metam sodium fumigations had been reported to show a large effect on soil fungi, reducing populations in different soil crops, reaching 100% in some cases (Locascio et al., 1997, Pelagatti et al., 1998). In 2002, reduction of soil populations was not observed in Navalmanzano. This year a compost was
Acknowledgements
This research was supported by Project INIA SC 97-130 (Plan Sectorial, Ministerio de Agricultura, Pesca y Alimentación). We wish to thank Viveros Rio Eresma SA (Navalmanzamo, Segovia, Spain), Viveros California SAT (Tordesillas, Valladolid, Spain), A. Millán, Y. Herranz, and C. Simón for technical support.
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