First Report of Fusarium Sheath Rot of Rice Caused by Species Complex in the United States.

Fungal diseases, including sheath rot (), cause significant losses of yield and milling quality of rice (). In August 2021, symptoms like sheath rot were observed on 20% of rice plants (cv. Presidio) in 1-hectare field in Eagle Lake, Texas. Initial lesions occurred on the upper flag leaf sheaths and were oblong or irregular oval, with gray to light brown centers, and a dark reddish-brown diffuse margin. Lesions enlarged, coalesced, and covered a large area of the sheath. Infection led to panicle rot with kernels turning dark brown. Unlike sheath rot, sheath infection also led to inside culm infection with irregular dark brown lesions. Infected tissue pieces were sterilized with 1% NaOCl for 2 min, followed by 75% ethanol for 30 s, washed in sterile H2O three times, air dried and incubated on PDA at 27℃. Fungal isolates were obtained from 15 diseased plant samples and their singled-spored fungal colonies were whitish, loosely floccose and produced light yellow pigmentation. On carnation leaf agar, macroconidia were slightly curved and tapered at the ends, with 3 to 5 septa, and measured 17.5 to 34.3 × 3.1 to 5.0 µm. Microconidia were ovoid, usually with 0 to 1 septum and were 4.0 to 15.5 × 2.5 to 4.5 µm. Spherical shaped chlamydospores were produced in chain. These morphological characteristics were consistent to those described for species complex (O'Donnell et al. 2009), including (Wang et al. 2021) and (Avila et al. 2019). For molecular identification, DNA of a representative isolate was extracted and ITS, LSU, and EF1 of the fungus were amplified using the primers of ITS1/ITS4 (Wang et al. 2014), D1/D2 domain region of LSU (Fell et al. 2000), and EF1 (Wang et al. 2014), respectively, and sequenced. The ITS sequence (OL344049) was 99.61% identical to species complex (FD_01692) in Fusarium-ID database and 99.61% identical to (LC514690, KY523100, MW016539) and (MH979697) in NCBI database. The LSU sequence (OK559512) was 98.77% similar to (MN877913, MN368509) and (MH877332, MH877326); the EF1 sequence (OK570044) was 99.27% similar to (MK278902) in NCBI database. A phylogenetic analysis based on the concatenated nucleotide sequences grouped this isolate in the species complex clade at 100% bootstrap support. To evaluate pathogenicity, a conidial suspension of 1 x 10 conidia/ml or sterilized water (the controls) was injected into the sheaths and young panicles of three rice plants (cv. Presidio) at boot. Treated plants were maintained in a greenhouse at 25 to 30℃. After 3 weeks, typical symptoms, like those observed in the field, developed on the inoculated plants but not on the controls. The same fungus was consistently re-isolated from the diseased plants. To our knowledge, this is the first report of Fusarium sheath rot caused by species complex in rice in the U. S. species complex has been reported to be associated with panicle infection in wild rice () in Brazil (Tralamazza et al. 2021). has also been reported to cause panicle rot in China (Wang et al. 2021). has been reported to cause Fusarium sheath rot in India (Prabhukarthikeyan et al. 2021) and the U. S. (Cartwright et al. 1995). This research demonstrates the potential of different pathogens being involved in causing sheath rot of rice.