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Insect-borne prokaryote-associated diseases in tropical and subtropical perennial crops

Periodic Reporting for period 2 - TROPICSAFE (Insect-borne prokaryote-associated diseases in tropical and subtropical perennial crops)

Reporting period: 2018-11-01 to 2020-04-30

The project addresses three insect-borne prokaryote-associated diseases of perennial crops (palm, citrus and grapevine) grown in tropical and subtropical areas due to ‘Candidatus Phytoplasma’ species associated with “lethal yellowing” (LY) in palms and “yellows” in grapevine (GY), and ‘Ca. Liberibacter’ species associated with “huanglongbing” (HLB) in citrus. The focus is on Africa (Ghana, South Africa), America (Mexico, Chile, Guadeloupe, Jamaica, Cuba), and Europe (Spain and Italy). The general objective is to provide innovative tools and solutions to manage and reduce the impact of these harmful diseases affecting and threatening these and other tropical and subtropical agricultural relevant crop species
Pathogen identification. ‘Candidatus Phytoplasma palmicola’-related strains were detected in coconut palms in Africa (Ghana) and in three alternative plant host species; morevoer three putative insect vectors were identified. In Jamaica 16SrIV-A subgroup phytoplasmas were detected in coconut palms and in two alternative host species; Oecleus mackaspringii was described as new insect species in coconut palm infected areas. In Mexico LY the 16SrIV-A, -B, and –D subgroup phytoplasmas were detected in coconut palms and in several alternative plant host species. LY phytoplasmas were detected also in two plants species, hosting the nymphs of Haplaxius crudus (LY insect vector), and in five insect species. Transmission of LY with adults of H. crudus to healthy young palms was achieved. In Cuba in palms phytoplasmas belonging to 16SrIV-A and other ribosomal groups were identified; the 16SrIV group phytoplasmas were detected also in citrus species, in Pritchardia pacifica and in two insect species. Phytoplasmas detected in grapevine were: ‘Ca. P. solani’ (Italy, Chile, Spain, South Africa); ‘Ca. P. fraxini’, (Italy, Chile); ‘Ca. P. asteris’ (Italy, Chile, South Africa); “flavescence dorée” (Italy, Spain); ‘Ca. P. pruni’-related (16SrIII-J) (Chile). In South Africa ‘Ca. P. asteris’ was detected in grapevine, in Mesembryanthemum crystallinum, and in two putative insect vectors. In Chile eight alternative host plant species for the 16SrIII-J phytoplasma were identified, and six insect vectors or potential vector were determined. In Italy samples from 13 plant species and from 11 insect vectors or putative vectors were determined as infected by the phytoplasmas detected also in grapevine. Surveys in citrus confirmed the presence of ‘Candidatus Liberibacter africanus’ in South Africa and ‘Ca. L. asiaticus’ in Guadeloupe and Cuba; these pathogens were not detected in Chile and in Spain. In South Africa, Cuba and Spain neither alternative host plant species nor potential insect vectors of HLB, besides the known vectors Diaphorina citri in Cuba, and Guadeloupe and Trioza erytreae in South Africa and in Spain, were identified. Management. In Ghana and in Mexico coconut varieties showing promising LY resistance are under evaluation, and in Mexico resistant germplasm was transferred as in vitro plants to Cuba and Jamaica. The seasonal trend of insect vector populations was determined in Mexico and Cuba in order to verify the LY disease potential cycle. Tamarixia dryi from South Africa was introduced in the Canary Islands (Spain), and showed good dispersion and parasitism efficacy against T. erytreae. Predictive models to estimate the invasion risk of T. erytreae in Europe demonstrates that the psyllid would probably establish in all the citrus growing regions. In Cuba the elimination of symptomatic trees at a regional scale resulted in the best HLB management strategy, and the efficacy of kaolin against D. citri kept the infestation level very low during two years. The seasonal trend of D. citri in nine citrus orchards in several regions of Cuba was different according to locality and management strategies. In Guadeloupe, despite the relatively low abundance of D. citri in some orchards under an integrated pest management (IPM) program, the HLB levels and mortality of the trees were very high. In South Africa a management plan for ‘Ca. P. asteris’ and its vector Mgenia fuscovaria (showing several peaks in the year) has been developed. In Italy an experimental vineyard obtained with the F1 crossing population between genotypes with different GY susceptibility was infected using insect vectors. The genotyping of the F1 population showed that approximately 20% of the progeny was self-crossed and 188 samples were selected for the further studies together with progenies of other key varieties. Detection tools. LAMP diagnostic systems were developed for detecting LY phytoplasmas in plants and insects, and specific quantitative PCR (qPCR) assays resulted able to differentiate among the LY-associated phytoplasmas, also at the subgroup level. A ‘Ca. L. africanus’ specific LAMP assay was developed. LAMP and qPCR assays for the specific detection of the South African ‘Ca. P asteris’ strain were successfully applied, however the qPCR assay was incompatible with the dipstick method that is the more convenient for on-site applications. The phytoplasma detection was also obtained in grapevine, and in other plant and insect species following an ELISA protocol with a recombinant antiserum, and on phytoplasma infected periwinkles and colonies with an IFAS (immunoflourescence assay) protocol with an antiserum produced from phytoplasma colonies. Economic evaluation. The methodology to be applied in the selected case studies was prepared. The studies will be performed in the absence and in the presence of the disease with selected management on three case studies. The method estimates the impact of the proposed solution comparing the economic results of existing pest management strategies with the changes in the selected variables after the introduction of the innovation. The economic variables will be collected mainly from the farmers and supported by the scheme of financial statement available online
Tackling these diseases in an effective manner would determine positive effects on the social and economic environment, especially in the poorest areas. Filling the gaps about alternative host plants and insect vectors will allow the validation of a sustainable and eco-friendly IPM management adapted to the studied crops in tropical and subtropical areas. One important issue will be to verify the establishment and spread of T. dryi, main parasitoid of T. erytreae, in mainland Europe after its release in the fall 2019. Evaluation of the economic and social sustainability of the innovations and strategies proposed will improve the social life of farmers and small farmers. The economic and social effects of the innovation and the effects on the market for the specific crops in the targeted countries will be elucidated. This analysis will ensure that economic and social effects of the innovation and strategies are evaluated not only with reference to farmers and agricultural families, but also to all the subjects that are affected by the innovation (workers, village communities, rural population, and marketing sector)
Alien pests
Consortium KoM