Pathogen Profile #4

Hello Robigalia reader! Welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inboxes.

As a PhD student, I rely on a steady supply of chocolate to get me through each week. If you’re anything like me, you’ve probably noticed the price of chocolate skyrocket recently. A perfect storm of factors have contributed to this, including climate change and plant diseases.

Cocoa swollen shoot disease (CSSD), caused by several Badnavirus species, including Cacao swollen shoot Togo B virus (CSSTBV), is one of the most significant diseases affecting West African cocoa production. First identified on Theobroma cacao in Ghana in 1936, CSSD is now estimated to account for up to 50% of cocoa yield losses.

CSSTBV spreads through mealybug vectors, including Pseudococcidae and Homoptera species. These scale insects feed on the sap of T. cacao, transmitting the virus as they move from host to host. Inside the host, the virus moves through the host via plasmodesmata, hijacking the plant’s cellular machinery to replicate while using RNA silencing genes to to turn off the plant’s defences.

Symptoms of CSSD only become observable after several years of infection, creating challenges for disease management. Symptoms can be seen on the leaves, stem, roots and pods of T. cacao. Most symptoms occur on mature leaves, including yellow clearing along veins, chlorotic vein banding, swelling of the stems and roots, along with pod shape distortion and discolouration. In younger leaves, red-vein banding may be observable.

The repercussions of CSSD extend far beyond agricultural yield, profoundly affecting livelihoods of smallholder cocoa farmers. In Ghana and Côte d’Ivoire, households have reported spending less on children's education and health, as well as modern health services, underscoring the urgency for comprehensive control measures.

Tree removal is a highly effective way to reduce further spread, however this has been met with resistance from farmers after a failed eradication attempt through tree removal in 1946. Current efforts have instead focussed on managing the mealybug populations through biological control, use of insecticides, and removal of plant debris to reduce populations. Efforts are also underway to breed resistant T. cacao varieties to safeguard future cocoa production.

If you’re interested in reading more about CSSD, I’ve summarised a few articles below. If you’re a subscriber, scroll down to access my online database which includes full summaries of these, and all other Robigalia featured articles!

Cocao swollen shoot disease papers

Molecular Surveillance, Prevalence, and Distribution of Cacao Infecting Badnavirus Species in Côte d’Ivoire and Ghana

• This study on CSSD in Ghana and Côte d'Ivoire reveals the prevalence and distribution of various badnavirus species affecting cacao production. Using PCR detection assays, the research identified CSSTBV as the most widespread, while cacao swollen shoot CE virus (CSSCEV) was mainly found in border regions. These findings offer insights for improving disease management and diagnostics in West African cacao plantations.

New insights in the evolutionary history of cacao-infecting badnaviruses

• This study investigates the evolutionary history and genetic diversity of cacao-infecting badnaviruses. Through phylogenetic analyses, ancestral state reconstructions, and molecular dating, the researchers provide evidence for an American origin of these viruses before their introduction to Africa.

Effectiveness of the eradication campaign of cocoa swollen shoot virus disease in Ghana: the extension and implementation problem

• Despite over 70 years of efforts by the Ghanaian government to eradicate the devastating cocoa swollen shoot virus disease (CSSVD), the disease persists as a major threat to cocoa production. This study reveals that low farmer knowledge about the virus and its spread, as well as failure to follow recommended treatment procedures, have significantly hampered eradication efforts.

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Until next week,

Alyssa