Replant disease of apple is common to all major apple growing regions of the world. Difficulties in defining disease etiology, which can be exacerbated by abiotic factors, have limited progress toward developing alternatives to soil fumigation for disease control. However, the preponderance of data derived from studies of orchard soil biology employing multidisciplinary approaches has defined a complex of pathogens/parasites as causal agents of the disease. Approaches to manipulate microbial resources endemic to the orchard soil system have been proposed to induce a state of general soil suppressiveness to replant disease. Such a long-term strategy may benefit the existing orchard through extending the period of economic viability and reduce overall disease pressure to which young trees are exposed during establishment of successive plantings on the site. Alternatively, more near-term methods have been devised to achieve specific quantitative and qualitative changes in soil biology during the period of orchard renovation that may lead to effective disease suppression.

The enzymatic hydrolysis of glucosinolates (GLs), typical compounds of Cruciferae, produces molecules with fungitoxic activity. Eleven GLs and their enzymatic hydrolysis products obtained by myrosinase were tested in vitro against Fusarium culmorum. Toxicity of hydrolysis products from glucoiberin, glucotropaeolin, sinigrin, and epiprogoitrin were assayed on eight plant pathogenic fungi. The results showed (i) the native GLs showed no fungitoxic activity, whereas their hydrolysis products inhibited fungal growth depending on their chemical structure; (ii) the hydrolysis products from glucoiberin, glucoerucin, glucocheirolin, and glucotropaeolin were the most effective, with 50% inhibition of fungal growth at 0.1 mg/mL; (iii) the fungitoxic activity of hydrolysis products obtained from glucoiberin, glucotropaeolin, sinigrin, and epiprogoitrin was confirmed on eight pathogenic fungi, with different responses depending on their chemical structure; (iv) the most effective hydrolysis products were those from glucoiberin, showing EC50 values of 0.05 mg/mL on Rhizoctonia solani, Sclerotinia sclerotiorum, Diaporthe phaseolorum, and Pythium irregulare and a minimum inhibitory concentration varying from 0.1 to 1.2 mg/mL. Keywords: Cruciferae; isothiocyanates; Fusarium culmorum; myrosinase

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Background and aims High-throughput 454 pyrosequencing was applied to investigate differences in bacterial and fungal communities between replant and closely situated control non-replant (fallow) soils. Methods The V1-V3 region of the bacterial 16S rRNA gene and the ITS1 region of fungi from the different soils were sequenced using 454 pyrosequencing (Titanium chemistry), and data were analysed using the MOTHUR pipeline. Results The bacterial phyla Proteobacteria, Actinobacteria and Acidobacteria dominated in both fallow and replant apple orchard soils, and community composition at both phylum and genus level did not significantly differ according to NP-MANOVA. The fungal phyla Ascomycota, Zygomycota and Basidiomycota were dominant, and communities also did not differ in composition at either phylum or genus level. High positive Pearson correlations with plant growth in a plant growth assay performed with apple rootstocks plantlets were detected for the bacterial genera Gp16 and Solirubrobacter (r: >0.82) and fungal genera Scutellinia, Penicillium, Lecythophora and Paecilomyces (r: >0.65). Strong negative correlations with plant growth were detected for the bacterial genera Chitinophaga and Hyphomicrobium (r: <−0.78) and the f u n ga l g en er a Ac remo ni um, F us ar iu m an d Cylindrocarpon (r: <−0.81). Conclusions Study findings are in part consistent with those of previous research, but also highlight associations between apple plants and certain microbial genera. The functional role of these genera in affecting soil health and fertility should be further investigated.