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First report of rust caused by Puccinia paullula on Monstera deliciosa and M. adansonii in Japan.

Monstera deliciosa Liebm. and M. adansonii Schott. (Araceae) have been cultivated for the commercial production in Hachijo Island, Tokyo, Japan, since the 1950s. A rust disease producing uredinial sori was found on the leaves of M. deliciosa and M. adansonii in the fields on Hachijo Island in February 2021. Symptoms were first observed on the leaf surfaces as small chlorotic spots, which expanded and became brown to reddish-brown, and produced uredinia with abundant urediniospores. The disease occurred on the whole island including the farm land, symptom incidence ranged from 5 to 30%. To confirm the pathogenicity of this rust fungus, ten plant species of Araceae (M. deliciosa, M. adansonii, Alocasia macrorrhizos, Anthurium andreauum, Dieffenbachia maculata, Epipremnum mirabile, E. pinnatum, Philodendron scandens, Spathiphyllum sp., Syngonium podophyllum) were inoculated with urediniospores obtained from infected samples on M. deliciosa and M. adansonii. Urediniospores were suspended in distilled water (1 × 106 conidia/ml) and sprayed on healthy plants. The inoculated plants were kept in a dark chamber at about 25°C for 48 h, and then transferred to a greenhouse. After 40 days, uredinia were reproduced only on M. deliciosa and M. adansonii. Morphological characteristics of the urediniospores obtained from inoculated monstera plants matched those obtained from the field plants and used as inoculum, thus fulfilling Koch's postulates. No symptoms were observed on the other plant species inoculated. For identification of the rust fungus, dry specimens obtained from both naturally infected plants and inoculated plants were used for morphological observations. Urediniospores with pedicels emerging from the stomata were densely aggregated, globose, and 24.8-29.3 µm (n = 30). Their walls were brown, echinulate, and 1.4-2.2 µm thick. Telia were found in the specimens collected in July and August. The teliospores with pedicels emerging from the stomata were two-celled, ellipsoid, and 21.3-27.5 × 19.5-23.4 µm. Their walls were pale yellow, smooth, and 1.0-1.9 µm thick. Morphological differences among the specimens on M. deliciosa and M. adansonii were not observed. These morphological characteristics were consistent with the description of P. paullula (Sydow and Sydow 1913). For molecular identification, the large subunit (LSU) rDNA and cytochrome oxidase III (cox3) were amplified. LRust1R, LR3, and LR5 (Vilgalys and Hester 1990) primers were used for sequencing of LSU region. Cox3_F (5'-GTTCAGTATGTTATTTTAACG-3') and cox3_R (5'-ATAGGAATAGTCAAACAACATC-3') primers were constructed here based on the P. paullula sequences (KX999927) for the cox3 region. The sequences were deposited in the GenBank as accession numbers OK509070, OK509071, LC663719 and LC663720. BLAST analysis showed that LSU and cox3 sequences shared 98.8% (902/913 nts) and 100% (632/632 nts) identities with P. paullula, KX999886 and KX999927 (Marin-Felix et al. 2017). This is the first report of a rust disease of M. deliciosa and M. adansonii caused by P. paululla in Japan. The rust fungi has been reported as the pathogen on the species of Amorphophallus, Rhaphidophora and Monstera (Sydow and Sydow 1913, Shaw 1991, Chen et al. 2009). The occurrence of this disease should be monitored because it can reduced yield and commercial value of monstera plants. All the specimens used in the experiments were deposited in the National Museum of Nature and Science, Japan (TNS-F82068-82077).