Beta PIs from the USDA-ARS NPGS evaluated for resistance to Cercospora beticola, 2011.Thirty Plant Introductions (PIs) from the USDA-ARS National Plant Germplasm System (NPGS) Beta Collection [garden beet, sugar beet, leaf beet, fodder beet (Beta vulgaris L), and wild beet (Beta spp.)] were evaluated for resistance to Cercospora beticola in an artificially produced epiphytotic environment (Ruppel, E.G., and J.O. Gaskill. 1971. Techniques for evaluating sugarbeet for resistance to Cercospora beticola in the field. J. Am. Soc. Sugar Beet Technol. 16:384-389). A randomized complete-block design, with three replications was used to evaluate germplasm at the Saginaw Valley Research and Extension Center (SVREC) near Frankenmuth, MI. Internal controls included a susceptible check, ‘CE’ (kindly provided by Syngenta Seeds), and a resistant check, ‘ACH355’ (kindly provided by Crystal Beet Seed). Single-row plots 4.5 m long, with 51 cm between rows were planted on 5 May 11. Azoxystrobin was applied in a band in furrow at planting and again on 15 Jun to control Rhizoctonia damping-off and crown and root rot. The nursery was inoculated on 7 Jul with a liquid spore suspension of Cercospora beticola. Visual evaluations on the plot with a disease index (DI) on a scale from where 0=no symptoms, 1=a few scattered spots, 2=spots coalescing or in large numbers on lower leaves only, 3= some dieback on lower leaves, but leaves not entirely dead, 4-8 are increasing amounts of dead and diseased tissue, 9= mostly dead with few remaining living leaves with large dead patches, and 10=all leaves dead. Evaluations were made on 10 Aug, 17 Aug, and 24 Aug, with the peak of the epidemic occurring around 24 Aug. The field was sprayed five times with phenmedipham, desmedipham, triflusulfuron methyl, and clopyralid (23 May, 31 May, 13 Jun, 25 Jun, and 5 Jul), once with S-metolachlor (15 Jun), and a cultivation was performed on 14 Jun to control weeds. The beet crop was thinned by hand. Bolting beets were removed throughout the season.
The high night time temperatures in the summer of 2011, combined with high humidity and low rainfall, contributed to a moderate leaf spot epidemic. The Beetcast leafspot advisory daily severity values accumulated in the Frankenmuth area from 1 May to 24 Aug were 171. Disease severity peaked by late August, after which regrowth started to outpace new disease development, so that disease ratings for several accessions remained constant or decreased after that rating, thus ratings are not given after this date. At our 24 Aug rating, means of the resistant and susceptible internal control for the entire nursery (including two additional experiments) were 3.5 and 5.9, respectively, across the nursery. At the peak of the epidemic in 2010 (19 Aug), these means were 3.2 and 5.5 respectively. Means of contributor lines in the entire nursery (including three additional tests) in 2011 ranged from 2.0 to 7.8. An analysis of variance (PROC GLM - SAS) on the disease indices (visual evaluation scores) determined that there were significant differences among entries (P<0.05) on all dates of evaluation. One accession, PI546425, had average ratings that were not significantly different from the resistant control on the first and third rating dates. Only 6 accessions (PI 518352, PI 518391, PI 518419, PI 540675, PI 546397, and PI518326) and the two control varieties did not require removal of seed stalks during the course of the ratings. These data, and more information on the accessions evaluated, are available through the USDA-ARS GRIN database at . For a .doc file with the disease index data ">SUGARBEET.CERCOSPORA.2011.HANSON