FIELD-EVALUATION OF CROWN RUST IN ANNUAL AND PERENNIAL RYEGRASSES

Crown rust (Puccinia coronata [Pers.] Cda.) is the most prevalent dise ase found on annual ryegrass (Lolium multiflorum L.) of the southeaste rn USA. Determination of crown rust resistance is difficult because of the erratic nature of infections at most locations. A dependable fiel d evaluation method for rating commercial annual ryegrass cultivars an d experimental lines was needed. As crown rust is present at Gainesvil le, FL each year, a rust nursery was designed to evaluate rust resista nce and susceptibility of annual ryegrass, The nursery also appears to be effective for perennial ryegrass (L. perenne L.). Evaluation consi sts of periodic rust ratings of the seed crop and regrowth of part of each plot after cutting, Results from the nursery indicate early and l ate buildup of rust, including the date of 90% heading (start of flowe ring), 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 distinguis h highly resistant, resistant, susceptible and highly susceptible ryeg rass cultivars.