First report of Bipolaris zeicola causing leaf spot on Northern Wild Rice (Zizania palustris L.) in Minnesota

Authors: CASTELL-MILLER, CLAUDIA - University Of Minnesota; Samac, Deborah; KIMBALL, JENNIFER - University Of Minnesota

Submitted to: Plant Disease
Publication Type: Research Notes
Publication Acceptance Date: 12/19/2024
Publication Date: 3/1/2025
Citation: Castell-Miller, C.V., Samac, D.A., Kimball, J.A. 2025. First report of Bipolaris zeicola causing leaf spot on Northern Wild Rice (Zizania palustris L.) in Minnesota. Plant Disease. 109(3):720. https://doi.org/10.1094/PDIS-10-24-2252-PDN.
DOI: https://doi.org/10.1094/PDIS-10-24-2252-PDN

Interpretive Summary: Northern Wild Rice is an aquatic grass species, native to North America, that is used to produce a gourmet grain. Many leaf spot diseases occurs annually in commercial fields and can cause significant grain losses. In 2022, leaves with dark-round to oval spots were collected in two fields near Waskish and Gully, Minnesota. A fungal pathogen was isolated and characterized using morphological characteristics and DNA sequences. The fungus was identified as Bipolaris zeicola, a known pathogen of corn and rice. It was shown to be pathogenic on Northern Wild Rice. This is the first report of the fungus causing disease on Northern Wild Rice in Minnesota. Because several Bipolaris species cause similar symptoms, identification of each species is imperative for implementing efficient disease management.

Technical Abstract: Northern Wild Rice (NWR; Zizania palustris L.) is an aquatic Poaceae species, endemic to North America. Minnesota is one of the largest NWR grain producers. A complex of fungal diseases occurs annually in commercial fields, where Bipolaris spp. predominate, and can cause significant grain losses. In 2022, leaves with dark-round to oval spots were collected in two fields near Waskish (48° 09' 41" N, 94° 30' 45" W) and Gully (47° 46' 06" N, 95° 37' 29" W), Minnesota. Leaf pieces with lesions were surface sterilized with 10% household bleach (6% NaOCl) for 15 s, rinsed two times in sterile ddH2O, blotted, and placed in moist chambers. After 7 to 10 days at 21°C (±1), single sporing was done to generate isolate PI-O-56 (Gully) and RR-N-33 (Waskish). Colonies on PDA had fluffy or cottony aerial mycelia, mostly gray, with darker sectors in some cultures, and with a gray to white upraised edge. Conidia were slightly curved or straight, tapering toward the ends, pluriseptate with 4 to 9 cells (mode = 4), initially light brown becoming dark brown, 47.1 µm (±9.6) x 10.9 µm (±1.8) (n = 200). Conidiophores were flexuous and unbranched. These characteristics were consistent with Bipolaris zeicola. Molecular identification of the two isolates was done by amplification and BLASTn sequence analysis of the Internal Transcribed Spacers rDNA (ITS) (primers ITS4/ITS5); glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (primers gdp1/gdp2), and the elongation factor-1a (EF1 a) (primers EF983/ EF2218R). The sequences were identical for both isolates and deposited in GenBank under the accessions PQ421579,PQ449034,PQ449036 (PI-O-56) and PQ421580,PQ449035,PQ449037 (RR-N-33) for ITS, GAPDH, and EF1 a, respectively. The ITS sequence had 100% identity (583/583 bp) to B. zeicola CBS 127731 (MH864760.1), GAPDH had 100% identity (594/594 bp) to CBS 317.64 (LT715783.1), and EF1a had 100% identity (898/898 bp) to MF193877. A phylogenetic tree constructed by concatenating the ITS and GAPDH sequences showed both isolates in the B. zeicola clade, a known pathogen of corn and rice. Pathogenesis was tested by spray inoculation of leaves using the isolate PI-O-56 on nine plants/replicate at the phenological stage of booting to initiation of flowering using 2.5 ml/plant of a conidial suspension obtained by washing spores from water agar plate cultures. Five plants/replicate were sprayed with sterile water as a control. Plants were pre-misted continuously for 15 min and then at 2 min on/30 min off for 48 h. The experiment was repeated twice, and two replicates were used per experiment with the same results. Brown spots of 1 to 2 mm, round to oblong, were observed 3 days after inoculation that enlarged to 5 to 9 mm, with some larger lesions on the edges in a few leaves. Some spots had a tan center with a dark border. Tissue discoloration around the lesions was often observed. No symptoms were observed on controls. Re-isolation of the fungus was done from three independent lesions and identity confirmed by spore and colony morphology and ITS sequencing, completing Koch’s postulates. This is the first report of B. zeicola infecting NWR in Minnesota. Because several Bipolaris spp. cause similar lesions on NWR plants early in the season, identification of each species is imperative for implementing efficient control with the NWR integrated disease management system.