Amendment of soils with fresh and post-extraction lavender (Lavandula angustifolia) and lavandin (Lavandula x intermedia) reduce inoculum of Verticillium dahliae and inhibit wilt in strawberry

Soils that were naturally infested with high levels of Verticillium dahliae, the causal agent of wilt disease in strawberry, were amended with fresh and waste lavender, fresh and waste lavandin, hydrosols generated during essential oil production, BioFence (TM) (a mustard-based defatted seedmeal pellet), or water in microcosms to estimate their efficacy against microslecrotia of the pathogen. Single chemicals and mixtures of the chemicals detected from the substrata were also effective in microcosms. The mixtures were more effective than were the individual chemicals. Microplot evaluation of fresh and waste lavender and lavandin was also made in comparison to BioFence (TM) and water controls. Lavandin waste was compared to BioFence (TM) and untreated controls at three sites in field plots that were subsequently planted to strawberry. Disease incidence and severity were measured over time in the field. All of the Lavandula-based materials could be associated with large reductions in the numbers of viable of microsclerotia recovered in all but one experiment with greatest effect in microcosms and smaller effects in microplots and field plots, as could BioFence (TM) pellets. Due to the high levels of inoculum found at all field sites the reduction in pathogen inoculum density was not necessarily associated with a corresponding reduction in disease incidence, nor with severity as determined by yield. The monoterpenoids associated with the Lavandula spp. are of lower volatility than the isothiocyanates associated with crucifer decomposition and were detected for more than one week after materials were incorporated in soil. This suggests both differences in mode of action and the possibility of combining either the chemicals or the materials that produce them in order to further enhance efficacy. Several non-target effects were considered: numerosity and diversity of bacterial and fungal populations; infection by arbuscular mycorrhizal fungi; and functional diversity of soil microflora. No persistent non-salient effects were detected. (C) 2011 Elsevier B.V. All rights reserved.