First report of gray leaf spot, caused by Pyricularia oryzae (synonym: Magnaporthe oryzae), in oat (Avena sativa) in Georgia, USA

Authors: SPRATLING, W - University Of Georgia; Sapkota, Suraj; VERMEER, C - University Of Georgia; MALLARD, J - University Of Georgia; ALI, E - University Of Georgia; MARTINEZ-ESPINOZA, A - University Of Georgia; BAHRI, B - University Of Georgia

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2021
Publication Date: N/A
Citation: N/A

Interpretive Summary: In Georgia and the southeastern U.S., oat is predominantly grown as a grain or forage crop due to its exceptional palatability. Diseases such as crown rust (Puccinia coronata var. avenae), loose smut (Ustilago avenae), and barley yellow dwarf virus are common hindrances of oat production in the southeastern US. In this study, we report a new pathogen, Pyricularia oryzae (synonym: Magnaporthe oryzae), causing gray leaf spot disease on oats in Georgia. Knowledge of the expanded geographical distribution of Pyricularia oryzae will provide insight in effective disease control.

Technical Abstract: In Georgia and the southeastern U.S., oat (Avena sativa) is predominantly grown as a grain or forage crop due to its exceptional palatability (Buntin et al. 2009). Diseases such as crown rust (Puccinia coronata var. avenae), loose smut (Ustilago avenae), and barley yellow dwarf virus are common hindrances of oat production in the southeastern US (Haber and Harder 1992). In November 2020, leaf spot symptoms were observed in an oat field planted as forage crop (cv. Horizon XXX) in Screven County, Georgia (GPS: 32°38'57.6"N 81°31'32.178"W). Lesions were oblong, whitish to gray in color, and surrounded by dark brown borders. Symptomatic oat leaves were sampled from the field and cut into '1cm2 sections that were surface sterilized (10% bleach for 2 min, 80% ethanol for 2 min, and rinsed with sterile water three times), plated onto Potato Dextrose Agar (PDA) plates and incubated in the dark at 23°C. To obtain pure cultures, fungal hyphal tips were subsequently transferred onto fresh PDA plates several times. The pathogen was identified as Pyricularia (Magnaporthe) based on conidial morphology (Ellis 1971). Conidia were hyaline, pyriform, 2-septate and displayed a prominent basal hilum. Conidia measured 5.32-10.64 µm (average 8.24 µm) wide by 15.96-29.26 µm (average 25.40 µm) long. To genetically confirm Pyricularia identification, DNA was extracted from a 14-day old pure culture using a cetyltrimethylammonium bromide method (Doyle and Doyle 1987). The internal transcribed spacer (ITS) region of ribosomal DNA, calmodulin (CaM) gene, and '-tubulin (TUB) gene were amplified using ITS5-ITS4 (White et al. 1990), CMD5-CMD6 (Hong et al. 2005) and Bt2a- Bt2b (Glass and Donaldson 1995) primer sets, respectively. Amplicons were Sanger sequenced and blasted against the NCBI database. Results exhibited 100% (ITS), 99.83% (CaM), and 99.61% (TUB) homology with Pyricularia oryzae (GenBank Acc. Numbers LC554423.1, CP050920.1, and CP050924.1, respectively). The ITS, CaM, and TUB sequences of the isolate were deposited in GenBank: XXX, XXX, and XXXX, respectively. In a greenhouse (23°C, 80% relative humidity), Koch’s postulates were carried out by using oat seedlings cv. Horizon 270 grown in Kord sheet pots filled with Sun Gro professional growing mix, and a P. oryzae spore suspension containing 104 conidia ml-1. The spore suspension (10ml) was sprayed with an air sprayer (Paasche Airbrush Company) onto 7 pots of oat seedlings when the seedlings developed two leaves. 7 supplementary pots of oat seedlings of the same cultivar were sprayed with sterile water to act as controls. After inoculation, plants were covered with black plastic bags that had been sprayed with sterile water and incubated overnight in the greenhouse. The bags were removed the next day, and plants were evaluated for symptoms in the following days. 7 days after inoculation, plants displayed symptoms similar to those found in the original field sample. Control plants showed no symptoms. P. oryzae was consistently re-isolated from artificially inoculated symptomatic oat tissues. P. oryzae can infect several graminaceous plants, including agronomically important crops such as rice (Oryza sativa) and wheat (Triticum spp.) (Chung et al., 2020). In Georgia, it has been reported on tall fescue and St. Augustinegrass thus far (Tredway et al., 2005). To our knowledge, this is the first report of gray leaf spot caused by P. oryzae on oat in the state of Georgia, USA.