Puccinia coronata f. sp. avenae (a heteroecious macrocyclic rust) is the causative agent of crown rust, the most harmful disease that affects oats worldwide. The uredial and telial phases occur on oats and other grasses, including all species of oats (Avena spp.), Secale cereale, Hordeum vulgare, Lolium spp., Festuca spp. and Bromus inermis, among others; the spermagonial and aecial phases occur on Rhamnus bushes, the alternate host Enhanced SEM. Image width: 183 micrometers. Magnification: 546x if the image is printed 10 cm wide.