First Report of Exserohilum rostratum as Pathogen of Rice Brown Spot in Malaysia

Leaves with brown spot symptoms were sampled from rice planting areas in the Malaysian peninsula, including Kelantan, Penang, Kedah, Perak, and Selangor, over the planting season from November 2012 to December 2013. The disease was observed on the glumes and leaves of rice plants. Mature lesions were brown with a gray or whitish center, whereas younger lesions were small, circular, and dark brown or purplish brown. Thirteen isolates were recovered from 100 diseased leaf samples that were plated on PDA medium. The mycelia were white in early stages but after 3 to 4 days, they turned gray or dark gray. The conidiophores were cylindrical, dark brown or olivaceous but paler toward the apex, 5 to 10 µm thick, septate, simple, and geniculate type. Conidia size ranged from 22 to 66 µm in length and 8 to 14 µm in width, and were brown or olivaceous, rostrate, ellipsoidal to narrowly obclavate, and sometimes straight or slightly curved with a thick wall along the conidia except for the small subhyaline region near the apex and the area at the basal cell where the hilum protrudes as a truncate cone or darkened cylinder. The conidia were 12 distoseptate, with a darker and thicker wall at the basal of each conidium. The pathogen was identified as Exserohilum rostratum based on description stated by Cardona and González (2008) and Sivanesan (1987). To confirm the morphological findings, PCR amplification was carried out using universal primers ITS4 (5′-TCCTCCGCTTATTGATATGC-3′) and ITS5 (5′-GAAGTAAAAGTCGTAACAAGG-3′) and yielded a 520-bp product. BLAST analysis showed that the sequence was 100% identical to published sequences of Setosphaeria rostrata, the teleomorph of Exserohilum rostratum (Accession Nos. KJ887577.1, KJ830936.1, JX966631.1, and KF897860.1). This sequence was deposited into NCBI GenBank as KM606587. Pathogenicity tests were done in the laboratory using a modified detached leaf method (Cardona and Gonzalez, 2008). The length of the leaf used was changed to 10 cm instead of 5 cm, and the isolate was grown for 20 days instead of 5 days before being inoculated on the leaf. The isolate was grown on PDA, and mycelium was excised as discs using a cork borer. Healthy leaves of variety MR219, 10 cm in length, were surface-sterilized with 10% alcohol and placed on a moistened paper towel placed in a petri dish. A disk of mycelium was placed onto the leaves in an inverted position and incubated at 28 ± 2°C for 10 days. Leaves sprayed with sterile distilled water served as control. The experiment was conducted in three replicates. Brown spot symptoms developed on the inoculated leaves after 3 to 4 days. Initially, the symptoms were elliptical lesions that were orange-brown and after 6 days the lesions became brown with a gray or whitish center similar to those observed in the field samples. No symptoms were observed on the control leaves. The information will contribute to accurate disease diagnosis and broadens the geographical area of the disease occurrence by this pathogen. Rice is a food staple in Malaysia and brown spot is an emerging disease that is cause for concern. When not managed, yield losses of up to 45% have been reported (Sparks et al. n.d.). © 2016, American Phytopathological Society. All rights reserved.