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Evaluation of Resistance to Stem Rust in Perennial Ryegrass Grown in Controlled and Field Conditions. R. E. Welty, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, National Forage Seed Production Research Center, 3450 SW Campus Way, Corvallis, OR 97331-7102. R. E. Barker, Research Geneticist, Agricultural Research Service, U.S. Department of Agriculture, National Forage Seed Production Research Center, 3450 SW Campus Way, Corvallis, OR 97331-7102. Plant Dis. 76:637-641. Accepted for publication 28 January 1992. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source, The American Phytopathological Society, 1992. DOI: 10.1094/PD-76-0637.

Six cultivars of perennial ryegrass were evaluated for reaction to Puccinia graminis subsp. graminicola in controlled conditions when plants were 8 or 14 wk old and in the field as mature plants. Plants in controlled conditions were rated for rust infection type (04 scale) after inoculation with urediniospores, and an average stem rust infection index (ASRII) was used to compare cultivars. Plants in the field were assessed for percent incidence of infection and severity (percent modified Cobb scale). The area under the disease progress curve (AUDPC) was used to compare cultivars. Eight-week-old plants of Birdie II and Linn were significantly more resistant to stem rust than were Ovation, Delray, Palmer, and Yorktown II. ASRII were smaller for 14-wk-old plants than for 8-wk-old plants, but cultivars retained the same ranking for infection type. Field assessments of AUDPC showed Birdie II to be the most resistant. Linn was found to be intermediate, followed by Ovation, Yorktown II, Palmer, and Delray. Birdie II and Linn were slow-rusting cultivars. Cultivars ranked similarly in the field and in controlled-inoculation studies. Generally, plants that were resistant at 14 wk also were resistant as adults in the field. However, some plants rated susceptible in the greenhouse varied widely in stem rust reaction and were rated from 0 or trace to 100% susceptible in the field. Based on our results, cultivars should be tested by a double-screen procedure as 14-wk-old plants in controlled conditions and as adult plants in the field. This system would reduce disease escapes in young plants and retain slow-rusting characteristics expressed in the field by adult plants.

Keyword(s): grass seed production, Lolium perenne, slow rusting.