Molecular identification and characterization of `Candidatus Phytoplasma convolvuli'-related strains (representing a new 16SrXII-O subgroup) associated with papaya bunchy top disease in Nigeria

Authors: KAZEEM,SHAKIRU,A - Nigeria Agricultural Quarantine Service; Inaba, Junichi; Zhao, Yan; ZWOLINSKA, AGNIESZKA - Institute Of Plant Protecton - National Research Institute; OGUNFUNMILAYO, AKINDELE - Nigeria Agricultural Quarantine Service; AROGUNDADE, AROGUNDADE - National Horticultural Research Institute; Wei, Wei

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2021
Publication Date: 6/10/2021
Citation: Kazeem,Shakiru,A, Inaba, J., Zhao, Y., Zwolinska, A., Ogunfunmilayo, A.O., Arogundade, A., Wei, W. 2021. Molecular identification and characterization of `Candidatus Phytoplasma convolvuli'-related strains (representing a new 16SrXII-O subgroup) associated with papaya bunchy top disease in Nigeria. Crop Protection Journal. https://doi.org/10.1016/j.cropro.2021.105731.
DOI: https://doi.org/10.1016/j.cropro.2021.105731

Interpretive Summary: Papaya is a very popular fruit tree and is widely cultivated in tropical and subtropical countries due to its delicious taste and rich content of antioxidant nutrients, B vitamins, minerals and fiber. The market demand for papayas has been increasing steadily in recent years. However, papaya production has been severely impacted by different pathogens including phytoplasmas, which were formerly known as mycoplasma like organisms (MLOs) and were discovered in 1967. They are characterized by small size, pleomorphism, and lack of a cell wall. Starting from 2018, papaya trees exhibiting symptoms such as excessive shoot proliferation at the top of the crown, leaf mosaic and crinkling, and dieback, were observed in Ibadan, Oyo State, Nigeria. The symptoms were similar to previously reported papaya bunchy top disease that was caused by phytoplasmas. In this study, ARS scientists and Nigerian collaborators employed the PCR approach, and identified phytoplasma as the etiological agent of papaya disease occurring in Nigeria. The disease was designated as Nigerian papaya bunchy top disease. Further in silico RFLP analysis based on 16S rRNA gene F2nR2 fragment revealed that the new phytoplasmas represented a new subgroup, 16SrXII-O. This is the first time that papaya phytoplasma disease was reported in Nigeria. The findings have important implications for extension personnel and farmers in developing disease management strategies. The information is also crucial for regulatory agencies to prevent exotic pathogens from entering the United States and other papaya producing countries. The findings will benefit researchers and students interested in the diagnosis of plant pathogens.

Technical Abstract: Beginning in 2018, a new papaya disease occurred in Ibadan, Oyo State, Nigeria. Affected papaya trees exhibited symptoms including excessive shoot proliferation at the top of the crown, leaf mosaic and crinkling, and dieback. The disease was designated as Nigerian papaya bunchy top (NGPBT) disease. Since excessive shoot proliferation was the most characteristic symptom of the disease and it was indicative of phytoplasma infection, a polymerase chain reaction (PCR)-based phytoplasma molecular diagnostic method was employed in this study. Phytoplasmas were detected in diseased papaya samples by semi-nested PCR using phytoplasma 16S rRNA gene-specific primers. Subsequent determination and analysis of the amplified 16S rRNA gene sequences and phylogenetic analysis revealed that NGPBT phytoplasmas were most closely related to 'Candidatus Phytoplasma convolvuli' in the Stolbur (16SrXII) group. Virtual restriction fragment length polymorphism (RFLP) analysis based on the 16S rRNA gene F2nR2 fragment showed that NGPBT strains represented a novel subgroup in the Stolbur phytoplasma group, designated as 16SrXII-O. This is the first report of a phytoplasma disease in papaya in Nigeria. Identification of the NGPBT strains not only broadened the genetic diversity of the Stolbur phytoplasma group but also signaled an expansion of the phytoplasma group's geographic distribution. Notably, two previously reported phytoplasmas associated with papaya diseases in Australia and Taiwan also belong to the 16SrXII group and exhibit symptoms similar to NGPBT disease. It would be interesting to learn how these three diseases relate to each other. Further in-depth study on weed reservoirs and insect vectors of NGPBT phytoplasmas will contribute to a better understanding of pathosystem and epidemiology of the NGPBT disease.