The Vanishing Chocolate Trees

Cacao swollen-shoot virus (CSSV) is a plant virus that primarily infects cacao trees (Theobroma cacao) and has a major effect on crop yields. Within one year of infection yields decrease by 25%, and within two years by 50%. The trees are usually killed within 3 to 4 years. The virus is currently endemic in Togo, Ghana and Nigeria but it has claimed already over 200 million trees in this region that ranks among the top cacao producers in the world. Mealybugs (Pseudococcidae) are the key vectors transmitting the virus from tree to tree. 

The extend of viral transmission is mealybug species specific of mealybugs as virus vectors is species specific and varies depending to their favoured feeding sites on the cacao plant and with respect to the age of the plant. There are 61 species of mealybugs found on cacao, 19 of them have been reported in West Africa, where CSSV occurs.  Of those, 16 are thought to act as vectors.

Morphological keys for mealybugs require a high degree of expertise depending as they do upon characterisation of microscopic structures that are, in vivo, often obscured by filamentous wax exudates. The keys cannot be definitive as some species are known to exhibit misleading phenotypic plasticity and, with few exceptions, morphological keys for mealybugs describe only adult females leaving the peripatetic and therefore more pathogenically important juveniles largely anonymous. 

This calls for DNA-based identification methods and in a newly published study a group of researchers from the UK tested the utility of DNA Barcoding in this context. 

In this work we describe the use of a mitochondrial cytochrome c oxidase 1 (CO1)-based DNA barcoding approach to mealybug identification that circumvents many of these problems... In order to make a DNA barcoding approach a practical tool to support in situ breeding for improved pest resistance of tropical crops such as cacao we have also tested High Resolution Melt Analysis (HRMA) for CO1-based mealybug identification. 

Indeed HRMA allowed them to differentiate the mealybug species that affect cacao and which usually can not be identified by conventional morphological analysis. This find has important implications for research in the affected regions and the authors see its potential to facilitate breeding for resistance to CSSV and even other mealybug transmitted diseases.

HRMA allied to morphological characterisation of mealybug exemplars has immediate utility for the support of CSSV resistance screening in cacao. Once exemplar haplotypes have been established for all species found on West African cacao, reference DNA will be made available so that only HRMA will be necessary at remote sites for the identification of potential CSSV vectors. The approach makes dependence on access to DNA sequencing superfluous and its sensitivity means that samples can be characterised regardless of developmental stage thereby also benefitting quarantine applications.