First Report of Albugo ipomoeae-panduratae Causing White Rust on Ivyleaf morningglory (Ipomoea hederacea) in Iowa

Ivyleaf morningglory (Ipomoea hederacea L.) is a prevalent weed in Iowa. This weed was found in corn field with approx. >50% incidence and from 10-40% severity of white pustules in Boone County during summer 2022, 2023, and 2024. The weed was observed with irregularly yellow-necrotic spots on the adaxial surface of leaves with white pustules on the abaxial side. Initially, the adaxial surface of the infected leaf exhibited yellow irregular spots that gradually turned brown and necrotic (Fig. S1A). On the yellow irregular spots of the abaxial surface, tiny, powdery, and chalky-white-cream sori appeared (Fig. S1B). Sori were observed individually or in clusters in somewhat as in concentric rings (Fig. S1C-D). Fresh, symptomatic leaves were collected for microscopic and molecular identification. If sori remained unopened, a sterile syringe was used to scrape sporangia onto a glass slide with blue lactophenol to inspect under a light microscope. Sporangia were hyaline, cylindrical-rectangular, 15.8 to 20.9 µm long (mean 17.9 µm, SE 0.46 µm), and 12.8 to 17.6 µm wide (mean 15.6 µm, SE 0.6 µm; Fig. S1E) (n = 20). Microscopic observations indicated that the causal agent was Albugo sp. Voucher specimens (accession nos. 458429 & 458430) were deposited in the Ada Hayden herbarium from Iowa State University. High amounts of sporangia were collected using a sterile elongated flock swab 6 in HydraFlock (Puritan Medical Products, Guilford, ME) for DNA extraction and pathogenicity test. Fungal genomic DNA was isolated separately from sporangia sori by Qiagen DNeasy Plant Mini kit (Qiagen, Valencia, CA). For molecular identification, two samples from different plants were processed (ALBUG_03 and ALBUG_04). The internal transcribed spacer (ITS1-O/LR0 primers) region and cox-2 (cox2-F/cox2-RC4 primers) gene were amplified by PCR (Kim et al. 2019; Choi et al. 2015). In both cases, amplicons (600 bp ITS region; 840 bp cox-2 region) were purified by PCR cleanup ExoSAP-IT express (Invitrogen, Waltham, MA) and further subjected to Sanger sequencing. Both sequences showed similarity to A. ipomoeae-panduratae in ITS (99.87% with reference strain DQ643920) and cox-2 (100% with reference strain AY913804) region. A phylogenetic tree was built with sequences of these two genes of Albugo species and related genera (Fig. S2). Both isolates-sequences were molecularly identical, then the isolate ALBUG_03 strain from this study, was uploaded as representative in GenBank as Albugo ipomoeae-panduratae (PQ452335 and PQ489431). For the pathogenicity test, ivyleaf morningglory seeds were surface disinfested with ethanol 70% for 30 s and NaOCl (0.8%) for 1 min, rinsed twice in sterile water and sown into autoclaved compost:perlite mix (50:50). Zoospores were produced from a sporangiospores- suspension of sterile water (10⁵/ml) as described by Pidskalny & Rimmer (1985) with slight modifications. Suspension was shocked at 4°C for an hour and placed in the dark at 25°C. After 24 h, the zoospores were inoculated on 14 days old-healthy plants by spraying the suspension with 0.1% Tween 20. After the inoculation, plants were kept inside a plastic bag for 3 days and then removed. Plants were placed in a growth chamber at 25°C and 70% RH. Symptoms resembling the original white rust disease appeared at 10 DAI, and sporangia were seen under microscope; controls remained symptomless (Fig. S1F). White rust has been reported previously in Iowa and nearby states; however, to our knowledge, this is the first report of Albugo ipomoeae-panduratae being identified as the causal pathogen for white rust on ivyleaf morningglory in Iowa. It is important to document the actual identity of this oomycete, as this pathogen has a broad host range within Ipomoea and Convolvulus genera plants, including important food crops such as sweet potato (I. batatas).