Coconut Lethal Yellowing Diseases: A Phytoplasma Threat to Palms of Global Economic and Social Significance.

Geoff M Gurr,Minsheng S You,Mark M Ero,Charles F Dewhurst,Anne C Johnson,Carmel A Pilotti,Bree A L Wilson,Gavin J Ash

doi:10.3389/fpls.2016.01521

Abstract

The recent discovery of Bogia coconut syndrome in Papua New Guinea (PNG) is the first report of a lethal yellowing disease (LYD) in Oceania. Numerous outbreaks of LYDs of coconut have been recorded in the Caribbean and Africa since the late Nineteenth century and have caused the death of millions of palms across several continents during the Twentieth century. Despite the severity of economic losses, it was only in the 1970s that the causes of LYDs were identified as phytoplasmas, a group of insect-transmitted bacteria associated with diseases in many other economically important crop species. Since the development of polymerase chain reaction (PCR) technology, knowledge of LYDs epidemiology, ecology and vectors has grown rapidly. There is no economically viable treatment for LYDs and vector-based management is hampered by the fact that vectors have been positively identified in very few cases despite many attempted transmission trials. Some varieties and hybrids of coconut palm are known to be less susceptible to LYD but none are completely resistant. Optimal and current management of LYD is through strict quarantine, prompt detection and destruction of symptomatic palms, and replanting with less susceptible varieties or crop species. Advances in technology such as loop mediated isothermal amplification (LAMP) for detection and tracking of phytoplasma DNA in plants and insects, remote sensing for identifying symptomatic palms, and the advent of clustered regularly interspaced short palindromic repeats (CRISPR)-based tools for gene editing and plant breeding are likely to allow rapid progress in taxonomy as well as understanding and managing LYD phytoplasma pathosystems.

Highlights

The review is timely because a lethal yellowing disease (LYD)—Bogia coconut syndrome (BCS) in Papua New Guinea (PNG)—has recently been reported from Oceania for the first time (Kelly et al, 2011), and LYDs remain a serious issue in other regions with, for example, a current outbreak in Mozambique threatening the livelihood of over three million people (Bila et al, 2015b)
Since the advent of molecular approaches to study phytoplasmas, there have been frequent changes in taxonomy this review provides an update on the taxonomic status of the phytoplasma taxa associated with different LYDs
Phytoplasmas affect a large number of economically important crops worldwide, and this has spawned an extensive research literature (Bertaccini et al, 2014) in which it is clear that phytoplasmas have proven challenging to manage

Summary

INTRODUCTION

Phytoplasma-associated diseases occur in many plant species and, reflecting their great economic importance, there is a large body of literature available focusing on phytoplasmas in agriculture (Bertaccini et al, 2014), vectors (Weintraub and Beanland, 2006; Weintraub, 2007), biology (Christensen et al, 2005; Namba, 2011; Maejima et al, 2014; Bertaccini, 2015), diagnosis and classification (Lee et al, 1998b, 2000; Bertaccini, 2007; Firrao et al, 2007; Duduk and Bertaccini, 2011; Harrison et al, 2014) and genomics (Kube, 2011). Among the more serious phytoplasma diseases are the lethal yellowing-like diseases (LYDs) of palms that have caused major outbreaks leading to the losses of millions of coconut and other palm species (Jones, 2002; Eziashi and Omamor, 2010). All reviews published since 2010 have a narrow focus on particular geographical areas and diseases such as: Ghana and Mozambique (Danyo, 2011; Harrison et al, 2014); the Carribean (Ntushelo et al, 2013c); Southern Asia (Ramjegathesh et al, 2012); or the distribution of phytoplasmas within palms (Oropeza et al, 2011). Coconut provides a staple food and serves as a cash crop in many developing countries; copra (coconut “meat”) being one of the few sources of cash income for many households (Bourke and FIGURE 1 | World map of annual coconut production and current published occurrences of lethal yellowing-type diseases of palms (see Table 1 and text for full detail and caveats) production data from FAO (2016)

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Findings

CONCLUSIONS AND OUTLOOK