First Report of Leaf Spot of Rice Caused by Epicoccum sorghinum in the United States.
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abstract
Brown spot (Cochliobolus miyabeanus), blast (Magnaporthe oryzae) and stackburn (Alternaria padwickii) are common diseases in rice with similar leaf spot symptoms. In August 2021, a leaf spot disease, with symptoms dissimilar to these diseases, occurred on almost 100% of the leaves and sheaths of rice plants (cv. Presidio) in a 1-hectare field in Eagle Lake, Texas. Lesions started as small dark brown spots on lower leaves and sheaths. The spots enlarged to become round or oval (1.5 to 5.0 mm) spots having round ends with gray centers, dark-brown borders or rings, and slight gold halos. The spots on the sheaths were similar to those on the leaf blades, with lesion size ranging from 2 to 5 mm. Pieces of infected tissue were cut from the margin of necrotic lesions, surface disinfected with 1% NaOCl for 2 min followed by 75% ethanol for 30 s and rinsed with sterile distilled water three times. The tissues were then dried on sterilized filter paper, placed on potato dextrose agar (PDA), and incubated at 25 for 7 days. Two isolates (LS36 and LS37) were obtained, and their colonies were initially villose, gray at the center and pale at the margin, and then turned dark gray, with the reverse side becoming scarlet. Chlamydospores were unicellular or multicellular and massively produced in nearly spherical shape (11 to 26 10 to 22 m, n=100). Pycnidia were dark and mostly spheroid (105 to 171 76 to 128 m, n=100). Conidia were unicellular, hyaline, ellipsoidal, with the size of 3.6 to 5.8 1.9 to 2.8 m (n=100). These morphological characteristics were similar to those described for Epicoccum sorghinum (Zhou et al. 2018). The rDNA internal transcribed spacer (ITS), rRNA large subunit (LSU), and translation elongation factor 1 alpha (EF1) gene of an representative isolate (LS37) were amplified (Fell et al. 2000; Wang et al. 2014) and sequenced. The ITS sequence (OK189534) of the isolate was 96.95% identical to E. sorghinum (KX758542); the EF1 sequence (OK236518) was 98.37% identical to E. sorghinum (MN461167); and the LSU sequence (OK189535) was 99.29% identical to E. sorghinum (MK817520, MK817521, and MK817522). Rice plants (cv. Presidio) at heading were inoculated with the two isolates individually by placing a drop of conidial suspension of 1 x 106 conidia/ml or a 2-mm PDA plug of 7-day-old cultures on the wounded or unwounded leaves and sheaths (3 sites/leaf or sheath, 3 plants/treatment). The wound was made by penetrating the epidermis using a 0.5-mm-diameter pin. The plants inoculated with sterilized water or PDA-only plugs served as the controls. The treated plants were placed in a dew chamber at 26 for 2 days and then transferred in a greenhouse (25 to 30). After 5 days, typical symptoms, similar to those observed in the field, developed on all of the inoculated leaves and sheaths, with the wound inoculation inducing more rapid development of symptoms than the unwounded inoculation. No symptoms developed on the controls. The two isolates produced similar symptoms and the fungus was consistently re-isolated from the infected plants and confirmed to be E. sorghinum based on morphological characteristics. The pathogenicity test was repeated twice with similar results. To our knowledge, this is the first report of leaf spot caused by E. sorghinum in rice in the United States. This disease was first reported on rice in China in 2020 (Liu et al. 2020). This research will help identify this new disease from other leaf spot-like diseases and develop management strategies for control of this disease.
