First report of bacterial leaf spot caused by Pseudomonas cichorii on Monstera adansonii in Hawai‘i, USA

Authors: Luiz, Blaine; Villalun, Maryann; Eyre, Melissa; Bushe, Brian; Johnston, Colleen; Keith, Lisa

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/8/2023
Publication Date: 1/15/2024
Citation: Luiz, B.C., Villalun, M.V., Eyre, M.J., Bushe, B.C., Brill, E., Keith, L.M. 2024. First report of bacterial leaf spot caused by Pseudomonas cichorii on Monstera adansonii in Hawai‘i, USA. Plant Disease. 108(1):201. https://doi.org/10.1094/PDIS-06-23-1224-PDN.
DOI: https://doi.org/10.1094/PDIS-06-23-1224-PDN

Interpretive Summary: Monstera, a tropical plant with unique leaf holes often referred to as the Swiss cheese plant, is a popular ornamental house plant that is also sold as cut foliage in Hawai‘i. In January 2022, water soaked, irregular, grey to black leaf spots were observed at a plant nursery in Hilo, Hawai‘i. The bacterium causing the disease was identified and its pathogenicity was proven. This is the first report of bacterial leaf spot caused by Pseudomonas cichorii on Monstera in Hawai‘i. To avoid further spread of this disease, recommended management options include increasing plant spacing, decreasing the amount of watering, avoiding mist irrigation, and carefully removing and discarding diseased leaves.

Technical Abstract: Monstera adansonii is a popular ornamental house plant that is prized for its small size and unique leaf fenestrations. In Hawai‘i it is also sold as cut foliage (combined annual value of ~$21K; USDA NASS 2019). In January 2022, bright yellow chlorotic lesions that progressed to necrotic, greyish-black water-soaked lesions primarily along the margins of fenestrations were observed on M. adansonii foliage at a plant nursery in Hilo, HI. Symptomatic leaf samples were collected and brought to the USDA Agricultural Research Service (ARS) for further examination. Isolations were made from the margins of leaf spots on Luria-Bertani (LB) agar and nearly pure cultures of bacteria containing cream-colored colonies with undulate margins were isolated from diseased tissue. Isolates were established from single colonies. One isolate (BCB001) was transferred to King’s B medium and culture fluorescence was observed under 365 nm UV light. Isolate BCB001, which was gram-negative, oxidase-positive, and produced a hypersensitive reaction in tobacco, was identified as Pseudomonas cichorii based on the LOPAT scheme (Schaad et al. 2001). A partial 16S rRNA gene product (495 bp) using primers Y1 and Y3 (Cruz et al. 2001) was sequenced and compared in GenBank (accession no. OQ875210) and was 100% identical to multiple accessions of P. cichorii in the NCBI database. Bacterial identity was further confirmed using the P. cichorii-specific primers Hrp1a/Hrp2a (Cottyn et al. 2011) to amplify and sequence a 790 bp fragment (accession no. OQ850761), which was identical to accession no. MH396007, a P. cichorii isolate recovered from Thai basil in Hawai‘i. To prove pathogenicity, strain BCB001 was grown on LB agar for 48 h at 27°C and suspended in sterile water at 107-108 CFU/ml. Four healthy, 2-month-old M. adansonii plants produced from cuttings were syringe inoculated with the suspension following the protocol of Wang et al. (2022). Leaves were injected with sterile water using the same methods and acted as negative controls. Plants were placed in clear plastic bags and held at 24°C with 12 h light for 48 hours, after which time the plants were removed from the bags. Leaf spots were not present on any of the control leaves or on noninjected leaves of the plants after five days of incubation. Grey to black, water-soaked leaf spots 0.84 – 15.24 mm in diameter were present on all injected leaves (96% of the injection sites) 2 days post-inoculation (DPI), which were identical to the original diseased samples. At 5 DPI, spots became dark brown to black with a yellow halo, and the affected tissue was completely collapsed. Bacterial colonies were consistently re-isolated from the lesion margins of inoculated plants and morphologically (LB and KMB) and molecularly (Hrp) identified as P. cichorii, thus fulfilling Koch’s postulates. Various Araceae, including Monstera in FL, are known to be susceptible to P. cichorii (Chase, 1990; Wehlburg et al. 1966). However, to the best of our knowledge, this is the first report of bacterial leaf spot caused by P. cichorii on M. adansonii in Hawai‘i. Since M. adansonii is an ornamental plant that is prized for its leaves, leaf spots caused by P. cichorii can reduce the marketability of inventory. Potential management options include increasing plant spacing to improving airflow, decreasing the amount of watering, avoiding mist irrigation to reduce humidity, and carefully removing and discarding diseased leaves from plants to avoid further spread.