Emergence of Puccinia graminis Causing Orchardgrass (Dactylis glomerata) Rust in China

Abstract

Orchardgrass (Dactylis glomerata L.) is one of the most important cool-season perennial forage grasses around the world and has been widely cultivated in China. Since 2007, a rust was observed on orchardgrass in Chongqing municipality and Sichuan province, China. In May 2018, 85% incidence of the rust was observed in an orchardgrass genetic resources field (29.40 ° N; 105.58 ° E; 296.1 m elevation) in Rongchang district, Chongqing. The disease on orchardgrass is characterized by small, light yellow to yellowish-brown, nearly circular uredia (0.75 to 1.03 × 1.21 to 3.84 mm) surrounded by chlorotic spots, appearing on both sides of leaf, leaf sheath, stem, and inflorescence. Urediniospores were subglobose or ellipsoid monospores, yellow or yellow-brown, 17 to 19 × 22 to 28 μm, with echinulate spines on the surface and three or four germ tube pores distributed equatorially. Telia were rarely observed. Teliospores were smooth, yellowish-brown, club-shaped, twin cells, constricted at the septa, with an obvious stalk, 26 to 41 × 16 to 20 μm. Teliospore wall thicknesses were 1 to 2 μm at the side and 5 to 8 μm at the apex. To support the morphological identification, molecular phylogenetic procedures were applied. Genomic DNA was extracted from urediniospores on infected leaves of orchardgrass. PCR was used to amplify the ITS rDNA region, the 5′ end of the nuclear ribosomal large subunit (28S) and partial beta-tub1 gene sequence (Aime et al. 2018; van der Merwe et al. 2007). A comparison with the GenBank sequences revealed that the ITS, 28S, and beta-tub1 gene sequences were 99.84, 99.70, and 99.38% identical, respectively, to those of Puccinia graminis (GenBank DQ417384.1, HQ412648.1, and HQ317589.1). Homology was also confirmed by analysis of a phylogenetic tree constructed by MEGA7. Representative sequences were deposited in GenBank (accession nos. MH638997.1, MK952783, and MK952781). On the basis of morphology and sequence comparisons, the pathogen was identified as P. graminis (Wang and Zhuang 1998). Rust specimens have been kept in the Forage Specimen Room of Southwest University. Pathogenicity was demonstrated by brushing fresh urediniospores from rusted leaves directly onto the leaves of healthy orchardgrass grown in pots. Control plants were inoculated with sterile water. All pots were kept in a plant growth chamber (20°C, 100% relative humidity) for 24 h and left under glasshouse conditions. Initial symptoms of single chlorotic flecks appeared 4 to 5 days after inoculation. On the seventh day, the host epidermis began to bulge, and within 2 weeks, the epidermis ruptured and showed yellow to brown uredia. Control plants remained symptomless. The rust fungus P. graminis is an important pathogen of orchardgrass, which has been found in 30 countries in the world (Farr and Rossman 2019). However, this is the first report of this fungus causing rust disease on orchardgrass in China. It is anticipated that plants would eventually die under heavy infection, so this fungus potentially could pose a significant threat to local orchardgrass production.