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Poster: Diseases of Plants: Disease Detection & Diagnosis


Two new races and several novel strains of the spinach downy mildew pathogen Peronospora farinosa f. sp. spinaciae
C. FENG (1), J. Correll (1), K. Saito (1), K. Kammeijer (2), S. Koike (3) (1) University of Arkansas, U.S.A.; (2) University of California, U.S.A.; (3) University of California Cooperative Extension—Monterey County, U.S.A.

Downy mildew disease, caused by Peronospora farinosa f. sp. spinaciae (Pfs), is the most economically important disease of spinach. Current high-density spinach production provides conducive conditions for disease development, particularly for the increasing acreages of organic spinach. The use of resistant cultivars to manage downy mildew exerts strong selection pressure on the pathogen and many new races of Pfs have emerged in recent years threatening spinach production worldwide. To monitor race diversity of Pfs, downy mildew samples were collected from major spinach production areas and tested for race identification with a standard set of differentials. Two new races (races 15 and 16, sanctioned by the IWGP) and several novel strains have been identified in the past 3 years. The disease reactions of Pfs15 (isolate UA4712) was identical to that of race 4 except the race 4 could not overcome the RPF9 locus resistance and race 15 could. The RPF1, 2, 4, and 6 loci provided resistance to Pfs15. Pfs16 (isolate UA201519B) could overcome the resistance of RPF2, 4, 5, 9, and 10 but not RPF1, 3, and 6. One novel strain (UA1014APLP) could overcome all known spinach resistant loci, but it only infects cotyledons and not true leaves of some cultivars. None of the 360 USDA spinach germplasm accessions tested has resistance to Pfs16 or UA1014APLP. Improved resistance is needed for protection of spinach to the highly variable spinach downy mildew pathogen.