SUGARBEET.CERCOSPORA.2010.HANSON

Thirty Plant Introductions (PIs) from the USDA-ARS National Plant Germplasm System (NPGS) (garden beet, sugar beet, leaf beet, fodder beet, and wild beet) were evaluated 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 Farm (SVRF) near Frankenmuth, MI. Internal controls included a susceptible check, variety 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 14 Apr 10. Azoxystrobin was applied in a band in furrow at planting and again on 28 May to control Rhizoctonia damping-off and crown and root rot. The nursery was inoculated on 1 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 29 Jul, 5 Aug, 12 Aug, 19 Aug and 26 Aug, with the peak of the epidemic occurring around 19 Aug. The field was sprayed four times with phenmedipham, desmedipham, triflusulfuron methyl, and clopyralid ( and May and and Jun), once with S-metolachlor ( Jun), and a cultivation was performed on 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 2010, 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 15 May to 26 Aug were 156. 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 19 Aug rating, means of the resistant and susceptible internal control for the entire nursery (including two additional experiments) were 3.2 and 5.5 (scale of 0-10), respectively, across the nursery. At the peak of the epidemic in 2008 (9 Sep), these means were 2.5 and 5.3 respectively. Means of contributor lines in the entire nursery (including three additional tests) in 2010 ranged from 3.0 to 7.3. 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. Two accessions (PI 540659 and PI540579) had average ratings that were significantly lower than the susceptible control at the second and third rating dates, but not at the other rating dates, and one of these (PI 540659) was not significantly different from the resistant control at the first rating date. One accessions (PI 540586) had a significantly lower rating than the susceptible control at the third rating date only. Only 10 accessions (PI 518400, PI540672, PI540673, PI540674, PI 540694, PI 540697, PI 540699, PI 546406, PI 546412, and PI 599350) 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