Canale, Maria Cristina

A novel phytoplasma species, ′Candidatus Phytoplasma zeae′, is proposed based on ecological distinctiveness, vector specificity, whole-genome comparisons, and community consensus. This phytoplasma is associated with maize bushy stunt (MBS) disease in corn (Zea mays) and is transmitted exclusively by Dalbulus maidis and D. elimatus, two leafhopper species endemic to the Americas. , and has been reported in Brazil, Colombia, Mexico, Peru, and several U.S. states. Here we sequenced and assembled the genome of MBS phytoplasma strains from Brazil, and U.S. to describe and propose this new species. Although the 16S rRNA gene sequence of the proposed reference strain, MBSP–BRRS, shares >99% identity with that of ′Ca. Phytoplasma asteris′, key nucleotide polymorphisms distinguish ′Ca. P. zeae′ from other 16SrI-related phytoplasma species. Average nucleotide identity (ANI) and average amino acid identity (AAI) values between ′Ca. P. zeae′ and ′Ca. P. asteris′ are 97.70–98.00% and 96.65–96.88%, respectively, both near the established species delineation thresholds. Comparative genomic analyses revealed unique gene clusters in ′Ca. P. zeae′ associated with amino acid transport, defence mechanisms, and protein turnover, which may contribute to its specialization in corn. The ecological profile of ′Ca. P. zeae′, including its narrow host range and restricted geographic distribution, supports its recognition as a novel species under Rule c of the IRPCM guidelines. The designation ′Candidatus Phytoplasma zeae′ is therefore proposed by members of the research community who have studied this pathogen for over a decade, with the MBSP-BrazilRS strain serving as the reference.

Although ‘Candidatus Liberibacter asiaticus’ (Las) is a major pathogen associated with citrus huanglongbing (HLB), some characteristics of transmission by the psyllid vector Diaphorina citri are not fully understood. We examined the latent period and persistence of transmission of Las by D. citri in a series of experiments at 25°C, in which third-instar psyllid nymphs and 1-week-old adults were confined on infected citrus for an acquisition access period (AAP), and submitted to sequential inoculation access periods (IAPs) on healthy citrus seedlings. The median latent period (LP50, i.e., acquisition time after which 50% of the individuals can inoculate) of 16.8 and 17.8 days for psyllids that acquired Las as nymphs and adults, respectively, was determined by transferring single individuals in 48-h IAPs. Inoculation events were intermittent and randomly distributed over the IAPs, but were more frequent after acquisition by nymphs. A minimum latent period of 7 to 10 days was observed by transferring groups of 10 psyllids in 48-h IAPs, after a 96-h AAP by nymphs. Psyllids transmitted for up to 5 weeks, when submitted to sequential 1-week IAPs after a 14-day AAP as nymphs. The long latent period and persistence of transmission are indirect evidences of circulative propagation of Las in D. citri.