Beta PIs from the USDA-ARS NPGS evaluated for resistance to Cercospora beticola, 2012.
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 (based loosely on Ruppel, E.G. and J.O. Gaskill. 1971. J. Am. Soc. Sugar Beet Technol. 16:384). 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 moderately susceptible check, C869, and a resistant check, EL50/2 (PI 664912). Single-row plots 4.5 m long, with 51 cm between rows were planted on 10 May. Azoxystrobin was applied in a 14 cm band in-furrow at planting to control Rhizoctonia damping-off and crown and root rot. The nursery was inoculated on 11 Jul with a liquid spore suspension (approximately 1 x 103 spores/ml as determined with a hemocytometer) of Cercospora beticola. Inoculum was produced from a mixture of leaves collected from the 2011 inoculated leaf spot nursery at SVREC and naturally infected beets grown on the Michigan State University campus farms in East Lansing, MI. Visual evaluations of 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 22 Aug, 29 Aug, 5 Sep and 12 Sep, with the peak of the epidemic occurring around 12 Sep. An evaluation was attempted subsequently, but several PIs were losing leaves following production of seed stalks and others were showing new leaf growth following defoliation from Cercospora leaf spot, so these ratings were not used. The field was sprayed two times with mixtures of phenmedipham, desmedipham, triflusulfuron methyl, and clopyralid (6 Jun and 15 Jun), once with S-metolachlor (29 Jun) to control weed seedlings, and hand weeding was done as needed to control larger weeds. The beet crop was thinned by hand with the generous help of Michigan Sugar Cooperative. Bolting beets were removed throughout the season.
The high night temperatures in the summer of 2012, combined with high humidity and low rainfall, contributed to a moderate leaf spot epiphytotic. Supplemental moisture was applied using an overhead irrigation system 13, 16 and 17 Jul. The Beetcast leafspot advisory in the Frankenmuth area from 1 May to 20 Sep was 185 cumulative daily severity values. Disease severity peaked by early September, 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. One entry, PI 663876, was not included in analysis as only one plot was available for rating At our 12 Sep rating, means of the resistant and susceptible internal control for the entire nursery (including two additional experiments) were 3.1 and 5.0, respectively, across the nursery. At the peak of the epiphytotic in 2011 (24 Aug), these means were 3.5 and 5.9 for resistant and susceptible, respectively. Means of contributor lines in the entire nursery (including three additional tests) in 2012 ranged from 3.0 to 8.0. 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, PI 504285, was not significantly different from the resistant control at all four ratings. Another accession, PI 504186, was not significantly different from the resistant control at the final two rating dates. In contrast, two accessions, W6 17103 and PI 578086 had average ratings that were significantly higher than the susceptible control at all but the first rating date, and another accession, PI 590582 was significantly higher at the final two rating dates. Twelve accessions (Ames 4219, PI 504186, PI 504285, PI 518307, PI 518339, PI 518360, PI 518365, PI 518367, PI 518411, PI 546523, PI 599352, and PI 590811) required removal of seed stalks from at least one replicate during the season. 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.2012.HANSON