SUGARBEET.CERCOSPORA.2009.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 6 May 09. Azoxystrobin was applied in a band in furrow at planting to control Rhizoctonia damping-off and crown and root rot. The nursery was inoculated on 9 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 19 Aug, 26 Aug, 2 Sep and 9 Sep, with the peak of the epidemic occurring around 9 Sep. The field was sprayed four times with phenmedipham, desmedipham, triflusulfuron methyl, and clopyralid (15 and 27 May and 4 and 16 Jun), once with S-metolachlor (25 Jun), and a cultivation was performed on 24 Jun to control weeds. The beet crop was thinned by hand. Bolting beets were removed throughout the season, after which some annual materials could not be rated as there was not sufficient remaining leaf tissue.

The moderate night time temperatures in the summer of 2009, combined with good moisture, contributed to a moderate leaf spot epidemic. The Beetcast leafspot advisory daily severity values accumulated in the Frankenmuth area from 15 May to 15 Sep were 129. Disease severity peaked by early September, after which regrowth started to outpace new disease development, so that disease ratings remained constant or decreased after that rating, thus ratings are not given after this date. At our 9 Sep rating, means of the resistant and susceptible internal control for the entire nursery (including two additional experiments) were 2.5 and 5.3 (scale of 0-10), respectively, across the nursery. At the peak of the epidemic in 2008 (2 Sep), these means were 2.4 and 5.2 respectively. Means of contributor lines in the entire nursery (including three additional tests) in 2009 ranged from 2.3 to 6.3. 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. Three accessions (PI 518429, PI 540691, and PI 604208) had ratings that were not significantly different from the resistant control at all rating dates. Two additional accessions, (PI 540583 and PI 546401) had average ratings that were not significantly different from the resistant control at four of the five rating dates. PI 546401, however, had very few plants and lost a great deal of tissue from removal of seed stalks from bolting, so ratings may not be reliable for this accession. Only seven accessions (PI 518322, 518417, 540675, 540678, 540679, 546433, and 550718) as well as the two control varieties did not require removal of seed stalks during the course of the ratings. Of these, only the resistant control was rated as resistant to Cercospora leaf spot. These data, and more information on the accessions evaluated, are available through the USDA-ARS GRIN database at . Roots were collected from PIs with low disease ratings to be included in crosses with sugar beet germplasm.

For a .doc file with the disease index data