Field evaluation of crown rust in annual and perennial ryegrasses

Crown rust (Puccinia coronata [Pers.] Cda.) is the most prevalent disease found on annual ryegrass (Lolium multiflorum L.) of the southeastern USA. Determination of crown rust resistance is difficult because of the erratic nature of infections at most locations. A dependable field evaluation method for rating commercial annual ryegrass cultivars and experimental lines was needed. As crown rust is present at Gainesville, FL each year, a rust nursery was designed to evaluate rust resistance and susceptibility of annual ryegrass, The nursery also appears to be effective for perennial ryegrass (L. perenne L.). Evaluation consists of periodic rust ratings of the seed crop and regrowth of part of each plot after cutting, Results from the nursery indicate early and late buildup of rust, including the date of 90% heading (start of flowering), and buildup of rust on regrowth under high rust spore load for all ryegrass genotypes. A rust index based on rust ratings at the 90% heading date for two or more growing seasons is proposed to distinguish highly resistant, resistant, susceptible and highly susceptible ryegrass cultivars.