Knowledge of disease cycles provides strategies for prevention of their spread and for their management. These diseases are associated with insect-borne prokaryotes and are a serious threat to production of coconut palm, citrus and grapevine. In order to reduce both local losses (thus preventing also social problems) and threats to European agro-industry, TROPICSAFE focused its work on the main and most severe insect-borne phloem restricted bacterial diseases threatening selected tropical and subtropical areas.
Lethal Yellowing - Coconut Palm
Lethal yellowing (LY) is associated with the presence of different phytoplasmas worldwide, and it leads to the death of high percentages of coconut palms, especially in Central America, the Caribbean and Africa.
The epidemiology of LY in palm plantations in Mexico, Jamaica, Cuba and Ghana still presents important knowledge gaps to be filled: only one insect vector (Haplaxius crudus) has been identified in Central America and in the Caribbean, while in West Africa, information on insect vectors and alternative host plants was not available yet.
Wide surveys across Central America, the Caribbean, and West Africa proposed by TROPICSAFE helped to clarify the identity and spread of phytoplasmas in coconut palms in the surveyed countries, and their molecular characterization enabled the determination of disease cycles and epidemiology.
Studying the range of alternative host plant species in different environments by cataloguing and barcoding phytoplasmas solved some of the currently existing knowledge gaps.
Citrus Greening / Huanglongbing - Citrus
Huanglongbing (HLB) disease is transmitted by the psyllids Diaphorina citri and Trioza erytreae, and severely reduces yield and performance of citrus worldwide. Three species of ‘Candidatus Liberibacter’ (CL) are responsible for HLB in Asia, the Americas and Africa. Although it has not been yet detected in the EU, one of its insect vectors was found in 2015 in the Iberian Peninsula, main citrus producing area in Europe, therefore the risk of an epidemic is very high.
Information is still missing on HLB alternative hosts plants and insect vectors in several citrus growing countries, and their distribution in Southern Europe is still unknown.
Studies by means of CL strain characterizations in South Africa, Chile, Cuba and Guadeloupe focused on understanding the pathogen strains associated with severe epidemics. The pathways of HLB regional spread was clarified in the different selected regions. TROPICSAFE provided the use of diagnostic tools, new information about alternative host plants and the possibility to verify the absence of HLB in citrus in Europe.
Grapevine Yellows - Grapevine
Grapevine Yellows (GY) is present in grapevine growing regions all over the world, leading to grapevine and wine losses of up to 60%. This disease is associated with several phytoplasmas and its epidemiological cycles show regional variation due to phytoplasma and insect vector diversity. The most commonly detected phytoplasmas associated with severe GY epidemics are ‘Candidatus Phytoplasma fraxini’, ‘Candidatus Phytoplasma asteris’ and ‘Candidatus Phytoplasma solani’.
Widespread surveys carried out by TROPICSAFE in Chile, South Africa and Spain clarified the identity and spread of these and other phytoplasmas in grapevines in the surveyed countries, and their molecular characterization enabled the determination of their disease cycles and epidemiology.
Gaps in GY disease cycles were addressed, such as the role of known and newly identified insect vectors and alternative host plants in the vineyards from the selected countries.