First Report of Powdery Mildew on Rescuegrass (Bromus catharticus) Caused by Blumeria graminis in Tennessee

Bromus catharticus Vahl, or rescuegrass, is a South American member of the brome grass group that has been spread around the world, including introduction into the United States, where it has become naturalized in many areas including agricultural fields. The plant grows either as a short-lived perennial or annual depending on location and produces copious seeds. Rescuegrass was listed in an index as infected by Blumeria graminis (DC.) Speer (powdery mildew) in Virginia (Roane 2004), but no reports exist in which the pathogen, disease symptoms, and signs were described and in which Koch’s postulates were completed. Powdery mildew-like symptoms were seen on rescuegrass growing along roadsides in Knoxville, Tennessee, in late May and June 2016 and 2017. White, powdery masses in lens-shaped lesions were abundant on the adaxial surfaces of leaves. By mid-July, the lesions had expanded to two-to-threefold the initial size and had become crusty and rust colored. The mostly ellipsoid-shaped conidia (n = 30) borne in chains measured 38.4 µm (33.2 to 44.2 µm) × 17.1 µm (14.9 to 20.2 µm), which is slightly longer and wider than described by Braun and Cook (2012). Foot cells (n = 10) were 32.2 µm (20.9 to 41.8 µm) long with bulbous central swellings. Conidia surfaces were covered by short spines that appeared as small black “dots” under oil immersion. Conidia germinated in two ways: first, by a short, thin subapical germ tube that did not produce appressoria; and second, by a longer and wider germ tube that eventually formed nipple-shaped appressoria either singly or in opposition. Chasmothecia were not observed at any time during the two growing seasons. The morphological data agree with the description for B. graminis (Braun and Cook 2012). Lesions were excised, and total gDNA was isolated, amplified with ITS1 and ITS4 (White et al. 1990), and the amplicons of the plant and fungus processed according to the method of Trigiano et al. (2016). The sequence for B. graminis was entered into GenBank (MF508973) and was 100% identical to B. graminis accessions AB273567.1 and AB000935.1, both taken from B. catharticus. The sequence for rescuegrass was ≥99% identical to accessions (KF713189.1 and DQ676867.1) and was also entered in GenBank (MF508972). Plants were grown from seeds in a greenhouse for 7 months. On June 21, 2017, after B. graminis signs were evident on roadside B. catharticus plants, stems with infected leaves were cut and then brushed against leaves of 12 irrigated, disease-free, containerized plants, which along with six uninoculated control plants were covered with bags for 3 days. By 14 days after inoculation in the greenhouse, B. graminis signs were observed on 6 of the 12 inoculated plants, whereas all controls were free of powdery mildew signs or symptoms. Signs and symptoms observed were identical to those of the original disease. Our conclusion is that B. graminis is the causal pathogen of powdery mildew of rescuegrass. Because B. graminis has an extensive host range (Braun and Cook 2012), rescuegrass may serve as a reservoir for primary inoculum to infect other species, including agronomic crops, in the Poaceae.