Plant Disease

‘Candidatus Liberibacter solanacearum’ (Lso) haplotype D, transmitted by the carrot psyllid Bactericera trigonica, is a major threat to carrot production, particularly in the Mediterranean Basin and the Middle East. We investigated the impact of plant age at the time of inoculation on symptom development and yield under greenhouse and outdoor conditions. In greenhouse trials, 1- and 2-month-old plants were inoculated with psyllids harboring Lso. Both age groups showed reduced taproot weight compared with controls, but younger plants were more severely affected, exhibiting 90% yield reduction, while 2-month-old plants showed a 30% reduction. Witches’ broom symptoms appeared earlier in plants inoculated at 1 month (5 weeks postinoculation); however, within 2 weeks, plants inoculated at 2 months reached comparable levels of symptom incidence and severity. Additional greenhouse experiments using Lso-free psyllids confirmed that yield losses and disease symptoms were specifically due to Lso infection and not psyllid feeding. In outdoor trials, we tested the effects of inoculation at 7, 11, 15, and 19 weeks postsowing (wps) on disease incidence and yield. Significant yield losses occurred only in plants inoculated at 7 and 11 wps, with up to 45% reduction by harvest time. Symptoms began to appear at 20 wps in plants inoculated at 7 and 11 wps. At 23 wps, symptoms also appeared in plants inoculated at 15 wps, with a similar incidence to that observed in the earlier inoculation groups. Molecular detection of Lso at harvest (23 wps) confirmed that all plants inoculated at 19 wps or earlier were infected. These results demonstrate that carrots are highly susceptible to Lso-induced yield losses at the earlier stages of growth and highlight the importance of managing early season vector pressure to minimize economic damage.

Little ironweed (Cyanthillium cinereum; syn. Vernonia cinerea) is an herbaceous plant in the Asteraceae family (Trang et al. 2024). The plant is distributed in Asia, Australia and Africa, and has been used as a traditional medicine (Trang et al. 2024). In January 2026, little ironweed plants exhibiting phyllody symptoms (Fig. S1) were observed in a grass-covered area (measuring 3 m × 6 m) in Qishan District, Kaohsiung, Taiwan. Around 8% (22/285 plants) of the population showed symptoms. Six symptomatic and four asymptomatic plants were collected. Flower-like tissues from symptomatic plants were collected for DNA extraction with a Synergy 2.0 Plant DNA Extraction Kit (OPS Diagnostics). For asymptomatic plants, capitula were used instead. One DNA sample was extracted for each plant. The samples were first tested with primers specific to the plant’s 26S rDNA (28KJ/28C; Cullings 1992), and all of them produced the target amplicon (~0.69 kb), indicating sufficient quality for polymerase chain reaction (PCR). Nested PCR targeting phytoplasmas’ 16S rDNA was conducted with outer and inner primer pairs P1/P7 and fU5/rU3 (Lorenz et al. 1995). In all PCR assays, negative controls (nuclease-free water) were included. All samples from symptomatic plants amplified the outer and inner amplicons (~1.81 kb and ~0.88 kb), whereas DNA from the asymptomatic plants did not. After assembling sequences of the outer and inner fragments for each symptomatic sample, identical 16S rDNA sequences were found. A representative sequence was deposited in GenBank (1,459 bp; accession no. PZ193724), and BLASTn search against GenBank revealed that it was identical to that of a reference phytoplasma strain belonging to subgroup 16SrII-A (NCHU2014; accession no. CP040925; 100% identity, 100% query coverage; Rodrigues Jardim et al. 2023). A 1,248-bp fragment within the obtained sequence was further analyzed using iPhyClassifier (https://plantpathology.ba.ars.usda.gov/cgi-bin/resource/iphyclassifier.cgi), and its virtual RFLP pattern was the same as 16SrII-A’s reference pattern (similarity coefficient = 1.00; accession no. L33765). The detected phytoplasma’s identity was further confirmed by amplification and sequencing of the elongation factor Tu (tuf) and protein translocase (secY) genes. Fragments containing tuf and secY were respectively amplified by primer sets TUF-II-F1/TUF-II-F2/TUF-II-R1 (for semi-nested PCR; Al-Subhi et al. 2017) and SecYF1(II)/SecYR1(II) (for PCR; Lee et al. 2010). The tuf sequences were obtained from the inner amplicon, whereas sequences of secY were obtained by sequencing followed by primer walking (accession nos. PZ377067 and PZ377068). The detected phytoplasma and the reference strain NCHU2014 (‘Ca. Phytoplasma australasiaticum’-related strain; 16SrII-A; accession no. CP040925) shared identical sequences for both tuf (986 bp; partial) and secY (1,263 bp; complete) fragments. This is the first report of little ironweed infected by a 16SrII-A phytoplasma in Taiwan; this specific host-pathogen association has only been reported in Hainan Province of China (Yu et al. 2024). Because 16SrII-A phytoplasma can infect economic crops in Taiwan (Wu et al. 2026) and little ironweed is widely distributed in the island, it is possible that the weed may represent a potential reservoir host. Thus, monitoring and eradication of symptomatic little ironweed is advisable in crop production areas.

Pear decline, associated to ‘Candidatus Phytoplasma pyri’, has emerged in Chilean pear orchards in recent years. While several Cacopsylla species are potential vectors of ‘Ca. P. pyri’, the disease’s full epidemiological cycle remains uncertain. Cacopsylla bidens, present in Chilean orchards, has recently been reported as a vector. This study conducted year-long surveys in two commercial pear orchards across different Chilean regions, capturing C. bidens in zones with ‘Ca. P. pyri’-infected pear trees. All developmental stages were collected, with peak abundances occurring in March and April. Outside the study zones, C. bidens were found in pine trees but not in adjacent cultivated areas. Two seasonal morphotypes, summer and winter forms, were identified. Molecular analysis detected ‘Ca. P. pyri’ in a high proportion of insects, with maximum infection rates in March and April. These findings advance our understanding of ‘Ca. P. pyri’ spatial and temporal dynamics and their potential role in ‘Ca. P. pyri’ spreading under Chilean field conditions.

In July 2025, two loblolly pines (Pinus taeda L.) growing within the 761-acre coastal Carolina Beach State Park (New Hanover County, NC) were observed with conspicuous growth abnormalities. One tree displayed a large witches’ broom arising from the main stem approximately 7 meters above the ground, immediately below the primary canopy, whereas the second showed extensive proliferation of epicormic shoots bearing needles along much of its 12-meter trunk, accompanied by reduced crown development. Needle tissues were collected from both trees, along with samples from four asymptomatic loblolly pines in the same area for the investigation of the possible phytoplasma presence. Total DNA was extracted using a Qiagen DNeasy Plant Pro Kit according to the manufacturer’s instructions. DNA extracts were screened by phytoplasma-specific real-time PCR (Hodgetts et al. 2009), which resulted in detection of phytoplasmas only in samples from the symptomatic trees. The full-length 16S rRNA gene (1,531 bp) was amplified with PCR primers flanking the gene: Tyr-F2 (5’-CAGTCTGTAAAACTGYTCCG-3’) and Ile-R2 (5’-TATCAGGCGTGTGCTCTAAC-3’). Amplicons of the expected size (~1.9 kb) were purified and sequenced using Oxford Nanopore sequencing. Identical consensus sequences were obtained and deposited in GenBank as strains PES-NC3 and PWB-NC4 (accession numbers PZ173280–PZ173281). BLASTn analysis of the 16S rRNA gene sequence showed the highest identity (1,530/1,531 bp; 99.93%) with a ‘Candidatus Phytoplasma pini’-related strain MDPP4 from Maryland (GenBank acc. no. OR147379), and 98.43% identity with the reference strain of ‘Ca. P. pini’ Pin127S from Spain (subgroup 16SrXXI-A; GenBank acc. no. AJ632155). Virtual RFLP analysis of the F2n/R2 region using iPhyClassifier (Zhao et al. 2009) produced a restriction pattern identical to that of the designated reference strain of subgroup 16SrXXI-B (GenBank acc. no. KU242428), supporting assignment to subgroup 16SrXXI-B. To further characterize the phytoplasma strains, a 1960 bp fragment of the secA gene was amplified from each symptomatic tree as previously described by Valiunas et al. (2015). The secA sequences differed by 10 nucleotides and were deposited in GenBank under acc. nos. PZ184669 (PES-NC3) and PZ184670 (PWB-NC4). BLASTn analysis showed 99.64% identity (1,932/1,939 bp) for PES-NC3 and 99.74% identity (1,934/1,939 bp) for PWB-NC4 to the corresponding locus of ‘Ca. P. pini’ strain MDPP (GenBank acc. no. KU242429). Phylogenetic analyses based on both the 16S rRNA and secA gene sequences indicated that the two strains detected in North Carolina cluster with previously reported strains from Maryland, forming a distinct, well-supported clade that is clearly separated from European ‘Ca. P. pini’ strains. This is the first report of ‘Ca. P. pini’-related strains infecting loblolly pine in North Carolina and only the second documented occurrence of this phytoplasma in the United States of America, following its detection in mountain pine (P. pungens Lamb.) in Maryland (Costanzo et al. 2016). The 16S rRNA gene sequences of the Maryland and North Carolina strains differ by a single nucleotide, whereas both U.S. strains differed from the European reference strain of ‘Ca. P. pini’ by 24 nucleotides across the 1,531-bp 16S rRNA gene sequence. These findings expand the known host range and geographic distribution of phytoplasmas associated with pines and suggest that they may be more widespread in the U.S.A. than previously recognized.

Bambusa multiplex, one of the most acclimatized clumping-bamboo species, has been broadly cultivated in China and has significant economical, ecological and ornamental importance (Yuan et al. 2009). In November 2025, more than 30% of B. multiplex plants exhibiting typical witches’ broom, internode shortening, leaf proliferation and clustering of leaves were observed in a horticultural landscaping field in Zhangjiang city, Guangdong Province, China. To confirm the presence of a phytoplasma, total DNA was extracted from fresh leaves of ten symptomatic and four asymptomatic plants using the CTAB method. Conventional nested PCR was performed using primer pairs P1/P7 followed by R16F2n/R16R2 (Gundersen and Lee, 1996), and expected fragment amplified only from the symptomatic plants. The PCR products were sequenced, and deposited in GenBank under the accession number PX998769. BLAST analysis revealed that the sequence shared 99.60% identity with that of the SPLL subgroup reference strain ‘Candidatus Phytoplasma australasiaticum subsp. Ipomoeae’ o7C (GenBank accession no. GCA_024425275.1, 1,244/1,249 bp) (Rodrigues Jardim et al. 2023). Further virtual RFLP pattern analysis of the 16S rRNA sequence by iPhyClassifier (Zhao et al. 2009) verified that the phytoplasma strain, designated as BMW-ZJ, was a member of the 16SrII-A subgroup with a 1.00 similarity coefficient to the reference phytoplasma strain (GenBank accession no. L33765). Moreover, the secY gene was amplified using primer pairs secYwbF1/secYwbR1 followed by secYwbF2/secYwbR2 (Wang et al. 2025), and the sequence(GenBank accession no. PZ135411) shared 100% identity to several ‘Ca. P. australasiaticum’ strains (NTU2011, BAWM-TWN and BAWM-THA-CLP). Phylogenetic analysis based on the 16S rRNA and secY genes conducted by MEGA 12 (Kumar et al. 2024), confirmed that BMW-ZJ was related to ‘Ca. P. australasiaticum’(Bertaccini et al. 2022; Rodrigues Jardim et al. 2023). Earlier, 16SrII phytoplasma associated with bamboo species Dendrocalamus giganteus (Che et al. 2024), and 16SrI phytoplasma associated with Sasa fortunei (Zhang et al. 2009) were reported in Hainan and Shaanxi Province, China, respectively. To our knowledge, this is the first report a ‘Ca. P. australasiaticum’ -related strain associated with Bamboo Witches’ Broom Disease (BWB) in Guangdong, China. The BWB caused by Ca. Phytoplasma (16SrI, 16SrII, 16SrVIII and 16SrXIV group), has been confirmed in several bamboo species in Southeast Asian countries such as India (Yadav et al. 2016; Ravi et al. 2022), Philippines (Dolores et al. 2023), Indonesia (Prasetya et al. 2020) and China (Zhang et al. 2009; Che et al. 2024). This study highlight the need to identify insect vectors and alternative hosts, which will provide theoretical guide to the control of Bamboo Witches’ Broom Disease.

Chokecherry (Prunus virginiana L.) is a native shrub that is widely found across North America and is a host of ‘Candidatus Phytoplasma pruni’ associated with X-disease of stone fruits. This pathogen is of major concern to U.S. cherry and peach growers, with the most recent and ongoing outbreak in the Pacific Northwest causing major economic harm. To investigate the role of chokecherry as a potential reservoir and source of orchard-associated strains, we surveyed wild chokecherry trees across 12 U.S. states, and compared the strains present to those in commercially cultivated Prunus species. Approximately 47% of chokecherry sampled tested positive for a ‘Ca. P. pruni’-related strain and phylogenetic analysis revealed high strain diversity, including six distinct clades and four newly identified lineages. In contrast, ‘Ca. P. pruni’ strains isolated from cultivated cherry and peach trees exhibited lower diversity and were infected by known disease outbreak-associated strains such as NWX. These data suggest limited transmission between wild and cultivated hosts in the western U.S., with patterns indicating spillover from orchards into nearby chokecherry populations rather than the reverse. However, in states east of the Rocky Mountains, chokecherry likely functions as a reservoir for orchard infections.

Lethal wilt (LW), known as “Marchitez Letal” in Colombia, is an endemic disease affecting oil palms (Elaeis guineensis Jacq.) and a leading cause of crop loss. The disease is characterized by the drying of leaflets from the tip to the base, primarily impacting the lower third of the plant and progressively moving upward. This progression leads to physiological disturbances, including necrosis at the tips of immature inflorescence bracts (spines) and the detachment of bunch fruits, ultimately causing wilting. As a phytosanitary measure, infected palms are eradicated to prevent further spread of the disease. The primary goal of this research was to identify the bacteria associated with LW and to validate a molecular detection method. A 16S amplicon-based analysis was employed to identify and compare the microbial diversity in LW-affected palm tissues with those of healthy plants. Among the 16 OTUs corresponding to different bacterial genera found in all LW samples, taxonomic classification and symptomatology suggested that the bacteria closely associated with LW belong to the genus ‘Candidatus Liberibacter’. Further phylogenetic analysis indicated that these bacteria are part of the Rhizobiaceae family, grouping closely with other species of the genus ‘Ca. Liberibacter’. The concentration of the pathogen in different oil palm tissues was determined using droplet digital PCR and quantitative PCR, expressed in copies/μl in the LW samples. This study represents the first report of ‘Ca. Liberibacter’ sp. being associated with lethal wilt in oil palms of the Arecaceae family in Colombia. The findings from this research could contribute significantly to the development of effective management strategies to prevent crop losses. [Formula: see text] Copyright © 2026 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .