Rational design of a survey protocol for avocado sunblotch viroid in commercial orchards to demonstrate pest freedom.

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Pretorius, L-S 
Jooste, AEC 
Geering, ADW 
Gilligan, CA 

Avocado sunblotch viroid (ASBVd) is a subcellular pathogen of avocado that reduces yield from a tree, diminishes the appearance of the fruit by causing unsightly scarring and impedes trade because of quarantine conditions that are imposed to prevent spread of the pathogen via seed-borne inoculum. For countries where ASBVd is officially reported, permission to export fruit to another country may only be granted if an orchard can be demonstrated to be a pest free production site. The survey requirements to demonstrate pest freedom are usually defined in export protocols that have been mutually agreed upon by the trading partners. In this paper, we introduce a flexible statistical protocol for use in optimizing sampling strategies to establish pest free status from ASBVd in avocado orchards. The protocol, which is supported by an interactive app, integrates statistical considerations of multistage sampling of trees in orchards with a RT-qPCR assay allowing for detection of infection in pooled samples of leaves taken from multiple trees. While this study was motivated by a need to design a survey protocol for ASBVd, the theoretical framework and the accompanying app have broader applicability to a range of plant pathogens in which hierarchical sampling of a target population is coupled with pooling of material prior to diagnosis.


Acknowledgements: We thank the anonymous avocado grower in South-east Queensland for practical support and engagement. We also thank the anonymous avocado grower in Mpumalanga, South Africa, for allowing access to orchards under their management and Lebogang Motaung and Ayanda Msweli for technical assistance in the laboratory. We thank Dr Alison Scott-Brown for assistance with literature retrieval and support with the final editing process and acknowledge the helpful comments given by the Editor and the two anonymous reviewers on an earlier version of this manuscript.

Viroids, RNA, Viral, Plant Viruses, Persea
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PLoS One
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Public Library of Science (PLoS)
Hort Innovation (AV18007)