First report of a rust fungus (Puccinia sp.) infecting lemongrass in Minnesota

Authors: GREATENS, NICHOLAS - University Of Minnesota; KLEJESKI, NICK - University Of Minnesota; Szabo, Les; Jin, Yue; OLIVERA, PABLO - University Of Minnesota

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2023
Publication Date: 6/22/2023
Citation: Greatens, N.J., Klejeski, N., Szabo, L.J., Jin, Y., Olivera, P.D. 2023. First report of a rust fungus (Puccinia sp.) infecting lemongrass in Minnesota. Plant Disease. https://doi.org/10.1094/pdis-10-22-2314-pdn.
DOI: https://doi.org/10.1094/pdis-10-22-2314-pdn

Interpretive Summary: Lemongrass (Cymbopogon citriatus) is a staple of various Asian cuisines and is sometimes used for tea or medicinal purposes. It is grown in gardens and in small-scale production by some communities around the Twin Cities areas in Minnesota. In July 2021, a rust disease was observed on lemongrass in a community garden plot near Mankato (MN), causing necrosis and leaf dieback. Morphological characteristics of uredinia and urediniospores are mostly consistent with Puccinia cesatii. Other spore types were not observed. Sequence analysis showed a high homology with P. cesatii and divergence with P. nakanishikii, another rust pathogen of lemongrass. Pycnidia of a mycoparasitic fungus were observed within uredinia and identified as Sphaelleropsis filum based on ITS sequences. Rust diseases on lemongrass have been reported in Hawaii, California, and Florida. In each case, the fungus was identified as P. nakanishikii. P. cesatii is a common rust fungus in the southwestern U.S. on several weedy grass species, but lemongrass is not known as a host. Research is needed to investigate the source of inoculum and potential impacts of the rust on lemongrass production and on native flora. This research will be useful for gardeners and horticulturists to detect and identify this rust disease on lemongrass.

Technical Abstract: Lemongrass is a staple of some Asian cuisines and is used for tea or medicine. It is not hardy in Minnesota but is grown in gardens and in small-scale production. In July 2021, in a community garden near Mankato, Minnesota, a rust disease was observed on lemongrass (Cymbopogon citriatus). Uredinia were cinnamon-brown on the abaxial surface of leaf blades small (0.5-1.5 mm) and numerous, causing large necrotic lesions and leaf dieback. Urediniospores were finely verrucose, slightly ovular (22-25 x 20-23 µm), thick-walled (2.5-4 µm), with 3-4 roughly equatorial, sometimes scattered germ pores. Clavate paraphyses abundant. Other spore types were not observed. Sequence of ITS had a 99% similarity to Puccinia cesatii. Morphologies of uredinia and urediniospores also fitted the description of P. cesatii. Sequence analysis and morphological examination identified the pathogen to be P. cesatii or a closely related species. The specimen was submitted to the Arthur Fungarium at Purdue University (PUR N24011). P. cesatii is a common rust fungus in the southwestern U.S. on several weedy grass species, but lemongrass is not known as a host. Further research is needed to investigate the potential impact of P. cesatii on lemongrass production and to elucidate phylogenetic relationships of rust fungi infecting lemongrass. The pycnidia of a mycoparasitic fungus were present within uredinia. Blastn searches of ITS sequences identified a 98.3 % (529/538 bp) match with Sphaerellopsis filum (syn. Darluca filum).