Studies on the pathogenesis of cacao swollen shoot virus to enhance the theobroma cacao l. Quarantine procedures with appendicies

Richardson, Megan

Abstract

Theobroma cacao (cocoa) is an economically important crop and the source of one of the world’s most consumed products, chocolate; an industry valued at USD 103 billion per annum. Pests and diseases pose a major threat to cocoa production, affecting yield and resulting in a loss of up to 38% of the annual global harvest. To further the understanding of these diseases, as well as developing strategies for detection and prevention, are imperative in protecting the livelihoods of small-hold cocoa farmers worldwide.

Cocoa Swollen Shoot Disease (CSSD) has been described as one of the most important diseases affecting West African cocoa production. The causative agent is a Badnavirus named Cacao Swollen Shoot Virus (CSSV). CSSV is vectored by mealybug species in a semi-persistent manner, although there are still major gaps in the understanding of modes of transmission including location of viral particles in planta. Symptoms of the disease on T. cacao are only identifiable three years post-infection; this long latency period makes it difficult for farmers to control spread and increases processing times in quarantine centres, where symptoms must be observed to confirm viral presence.

This project aimed to improve on knowledge of CSSV pathogenicity and explore new methods for detection which can be easily implemented in quarantine centres such as the International Cocoa Quarantine Centre (ICQC, Reading), which serve to stop the spread of cocoa diseases. A method of CSSV detection using samples of T. cacao stems has been developed. CSSV DNA can be extracted from stem exudate and detected by qPCR and end-point PCR. This method has the potential to improve quarantine processes for detection of low levels of CSSV DNA from budwood samples. During this project, novel badnaviral sequences were discovered in T. cacao. A High-Resolution Melt assay was developed which successfully differentiates between four different badnaviral insert types in T. cacao. The mendelian inheritance of these integrated sequences was determined using this method to examine samples from multiple crosses of T. cacao plants containing different insert types. This assay can be implemented in quarantine centres as a quick and straightforward method of detection, and differentiation between CSSV positive and negative samples. Finally, fluorescent in situ microscopy was used in order to identify the location of CSSV in planta. Using a novel CSSV antibody and an indirect immunofluorescent staining strategy, CSSV was successfully detected in the spongy mesophyll tissues of T. cacao leaves. This finding provides valuable insight into the mealybug-based transmission of this virus, suggesting that the rate of transmission is linked to the extended periods of time the vectors are known to probe these cells during the feeding process.

Citation

Richardson, M. Studies on the pathogenesis of cacao swollen shoot virus to enhance the theobroma cacao l. Quarantine procedures with appendicies. (Thesis). University of the West of England. Retrieved from https://uwe-repository.worktribe.com/output/11404419

• Thesis Type: Thesis
• Deposit Date: Nov 3, 2023
• Publicly Available Date: Apr 29, 2024
• Public URL: https://uwe-repository.worktribe.com/output/11404419
• Award Date: Apr 29, 2024