Effect of fungal, oomycete and nematode interactions on apple root development in replant soil
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CABI Agriculture and Bioscience Open Access
RESEARCH
Effect of fungal, oomycete and nematode interactions on apple root development in replant soil Emma L. Tilston, Gregory Deakin, Julie Bennett, Thomas Passey, Nicola Harrison, Felicidad Fernández and Xiangming Xu*
Abstract Background: Apple replant disease (ARD) is a phenomenon associated with poor tree establishment at sites where the same, or a closely-related species, has grown for at least 1–2 years. No single organism has been identified as the universal causal agent, but there is increasing evidence that multiple soil-borne plant pathogenic fungi and oomycetes form an ARD disease complex. Root damage caused by root lesion nematodes has also been implicated in facilitating the entry of pathogens into root tissues resulting in the development of severe ARD. Methods: We used a reductionist approach to determine effects of one or more members of the ARD complex on ARD in a number of selected rootstock genotypes with contrasting characteristics. Through a 15-month pot-based experiment in which semi-selective biocides were applied to soil from a replant orchard, we investigated (1) the nature of the interactions (i.e. antagonistic, additive or synergistic) between different groups of soil biota and ARD severity, and (2) whether rootstock characteristics modify ARD severity. Results: There might be competitive interactions between oomycetes and fungal pathogens in infecting apple roots and hence subsequent ARD development. Controlling all three ARD components (oomycetes, fungi, and nematodes) led to the best root development. However, these effects on root development were not manifested in the aboveground tree development 15 months after treatment. Specific soil biocide treatments against fungi and oomycetes led to large changes in soil microbial communities whereas the nematicide treatment led to least changes. In spite of the observed ARD, comparing rhizosphere microbial sequences among treatments failed to reveal candidate pathogens for ARD. Conclusions: Candidate ARD oomycetes and fungal pathogens are likely to engage in competitive interactions among themselves in infecting apple roots. Although soil amendments affected soil microbiota, such effects appear to be very unpredictable. Keywords: Competition, Oomycetes, Fungi, Root infection, Amplicon-sequencing Introduction Poor establishment of apple trees (Malus pumila Miller) at sites where the same, or a closely-related species, has grown for at least 1–2 years is a world-wide *Correspondence: [email protected] NIAB EMR, New Road, East Malling, West Malling, Kent ME19 6BJ, UK
problem affecting the production of both rootstocks and fruit trees (Mazzola and Manici 2012). Symptoms of apple replant disease (ARD) include uneven growth, stunting and shortened internodes aboveground, and discoloured roots, root tip necrosis and reduced root biomass belowground. The bearing of fruit is delayed by 2–3 years and yields are depressed by up to nearly 60%
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