Tomato and Jute Mallow Are Two New Hosts of Papaya Bunchy Top Phytoplasma, a ‘Candidatus Phytoplasma convolvuli’-Related Strain in Nigeria

Nigerian papaya bunchy top (NGPBT) phytoplasma was first identified in diseased papaya plants in Ibadan, Oyo State, Nigeria (Kazeem et al. 2021). The NGPBT phytoplasma is a ‘Candidatus Phytoplasma convolvuli’-related strain and represents a subgroup lineage, 16SrXII-O (accession number MW530522; Kazeem et al. 2021). This note reports that NGPBT phytoplasma can also infect tomato (Solanum lycopersicum) and jute mallow (Corchorus olitorius). Since May 2020, tomato and jute mallow grown in Ibadan have been observed to develop yellowing, little leaf, and stunting symptoms. Because the symptomatic plants occurred approximately 1 km from where the NGPBT disease was reported, and the symptoms of infected plants resembled those of phytoplasma infection, molecular diagnostic assays for phytoplasma detection were deployed. Total DNA was extracted from symptomatic plants, including four tomato plants and three jute mallows, as well as from two asymptomatic tomato and jute mallow plants. The DNA samples were subjected to seminested PCR using phytoplasma 16S rRNA gene-specific primers P1A and P7A, followed by P1A and 16S-SR (Lee et al. 2004). An amplicon of 1.5 kb was obtained from each of the symptomatic plants, while no amplicon resulted from DNA samples of asymptomatic plants or negative controls without DNA templates (water and PCR reagents only). PCR products were cloned into the TOPO TA cloning vector (Invitrogen, Carlsbad, CA), and three clones were chosen for each sample for Sanger sequencing (Psomagen, Rockville, MD). The nearly full-length 16S rRNA gene sequences (1.53 kb) derived from tomato (OP123558) and jute mallow (OP123559) samples were identical. Based on the iPhyClassifier phytoplasma classification web tool (Zhao et al. 2009) and a BLAST search against the NCBI nucleotide database, these strains showed 100% sequence identity in the 16S rRNA gene with the NGPBT phytoplasma (16SrXII-O, MW530522). Moreover, two additional genetic loci, ribosomal protein genes rplV-rpsC and rplO-secY-adk, were also amplified by nested PCR or seminested PCR with specific primers rpStolF/rpStolR followed by rpStolF2/rpStolR (Martini et al. 2007), and SecYF1a (Xll)/MapR-703-a followed by SecYF2a (Xll)/MapR-703-a (Lee et al. 2010). Gene fragments of rplV-rpsC (1,238 bp) and rplO-secY-adk (2,064 bp) were amplified from DNA of diseased papaya, tomato, and jute mallow plants. The obtained sequences were deposited in GenBank (rplV-rpsC: OP123560, OP123562, and OP123563; and rplO-secY-adk: OP123565, OP123567, and OP123568). Multilocus sequence analysis (MLSA) indicated that the sequences of phytoplasmas amplified from three different plant hosts were also identical in rp, secY, and adk genes. The MLSA results demonstrate that tomato and jute mallow are two new hosts of NGPBT phytoplasmas. This is the first time that phytoplasma diseases have been associated with tomato and jute mallow in Nigeria; they were only previously reported in coconut palm and papaya in the country (Kazeem et al. 2021; Osagie et al. 2016). These results suggest that an insect vector(s) for the transmission of the NGPBT phytoplasma is present in the region. Since both tomato and jute mallow are important crops in Nigeria, timely dissemination of emerging disease information is needed to alert growers and extension personnel in the region. Ongoing incidence and prevalence surveys of NGPBT disease have observed more infected papaya and tomato plants in the region than in previous years. A better understanding of the NGPBT phytoplasma disease epidemiology will help devise strategies to control the diseases associated with the NGPBT phytoplasma.