First Detection of <i>Pseudocerradoa paullula</i> Causing Aroid Leaf Rust on Swiss Cheese Plant (<i>Monstera deliciosa</i>) in Virginia

Monstera deliciosa Liebm. (Araceae) is a monocotyledonous plant that is native to tropical forests of southern Mexico to Panama. It is widely grown as an ornamental in the United States because of its easy maintenance and attractive, fenestrate leaves. On May 10th, 2024, at a nursery and garden center in Henrico County, Virginia, four M. deliciosa plants in 3.8 L containers were observed with necrotic spots surrounded by a yellow halo on the leaves (Fig. 1A). Uredinia were present in the center of the lesions with dense, reddish-brown sporulation mostly on the abaxial surface of the leaves (Fig. 1B). Urediniospores with pedicels were golden brown in color, globose, echinulate, with two opposite germ pores, averaging (28) 25.2 x 25 (23) µm (n = 40) in size and a wall thickness of 1.5 to 2 µm (n = 40) (Fig. 1F - K). Telia were not present. The host, symptoms, and urediniospore size was comparable to reports of Pseudocerradoa paullula (Syd. & P. Syd.) M. Ebinghaus & Dianese from South Carolina (22.9 to 27.9 μm), Florida (24 to 31 μm), and Japan (24.8 to 29.3 μm) (Ebinghaus et al. 2022; Sakamoto et al. 2023; Urbina et al. 2023; Yang et al. 2023). Urediniospores from the infected plants were collected with a sterile needle and DNA was extracted using a Qiagen DNeasy PowerLyzer Microbial Kit (Germantown, MD) according to the manufacturer's instructions. PCR and sequencing of the small ribosomal subunit (SSU) and large ribosomal subunit (LSU) gene regions was performed with primer sets NS1/Rust18SR and LRust1R/LR3 (Beenken et al. 2012; Vilgalys and Hester 1990). The resulting 1,630bp and 638 bp sequence fragments of the SSU and LSU loci from strain GS24-AE50 were deposited into the NCBI Genbank database under accessions PQ059898 and PQ059897, respectively. A pairwise alignment of the SSU gene shared 1,363/1,366 (99%) nucleotides with the P. paullula voucher (ON887197) from Florida. A Genbank nBLAST analysis of the LSU gene shared 636/638 (99%), 636/638 (99%), and 592/600 (99%) nucleotides with vouchers from M. deliciosa from South Carolina (OQ746460), Florida (ON887197) and Japan (OK509070) (Sakamoto et al. 20222; Urbina et al. 2023; Yang et al. 2023). Koch's postulates were fulfilled by spraying four, healthy, non-wounded M. deliciosa plants to run-off with a urediniospore suspension (1 x 106 spores/ml distilled water, 20 ml per plant) that was collected from the original infected plants. An additional four, healthy control plants were sprayed with distilled water only. After 6 weeks in a greenhouse at 22 ± 2°C with ≥85% relative humidity under an 8-h photoperiod, uredinia in the center of lesions identical to those on the original symptomatic plants developed on 12 out of 20 leaves from the inoculated plants, while all the leaves from the control plants remained asymptomatic (Fig.1C - E). Urediniospores collected from the inoculated plants were morphologically identical to the urediniospores from the original infected plants with 100% LSU sequence homology to accession PQ059897. Globally, P. paullula has been reported from Australia, China, Japan, Malaysia, the Philippines, and the United States, where the pathogen was detected at the port of Los Angeles in 2014, Florida in 2019, and South Carolina in 2023 (Sakamoto et al. 2023; Shaw et al. 1991; Urbina et al. 2023; Yang et al. 2023). Although the pathogen is not known to be established in Virginia, the recent surge of reports suggests that the pathogen's distribution is expanding. The impact of aroid leaf rust on M. deliciosa production is unclear, but it has the potential to reduce the aesthetic and commercial value of plants under favorable conditions.