Beta PIs from the USDA-ARS NPGS evaluated for resistance to Cercospora beticola, 2014.
Thirty Plant Introductions (PIs) from the USDA-ARS National Plant Germplasm System (NPGS) Beta Collection [includes garden beet, sugarbeet, 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 generally 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 Michigan State University Saginaw Valley Research and Extension Center (SVREC) near Frankenmuth, MI. The field had been planted in wheat with clover underseeded in 2013. Internal controls included a susceptible check, 12N0050 (kindly provided by L. Campbell), and a resistant check, EL50/2 (PI 664912). Single-row plots 4.5 m long, with 51 cm between rows were planted on 5 May 14. Moncut (N-[3-(1-methylethoxy) phenyl]-2-(trifluoromethyl) benzamide) was applied in a 14 cm band in-furrow at planting to control Rhizoctonia damping-off and azoxystrobin was applied 5 Jun 14 to manage Rhizoctonia crown and root rot with limited potential to impact leaf spot development. The nursery was inoculated on 10 Jul with a liquid spore suspension (approximately 1 x 103 spores/ml as determined with a hemacytometer) of C. beticola. Inoculum was produced from a mixture of leaves collected from the 2013 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 0-10 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 14, 21, and 28 Aug, and 3 Sep 14, with the peak of the epidemic occurring around 3 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 three times with mixtures of phenmedipham, desmedipham, triflusulfuron methyl, and clopyralid (1, 16, and 28 Jun 14) and once with S-metolachlor (28 Jun 14) to control weed seedlings. 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 moderate night temperatures in the summer of 2014, combined with high humidity and low rainfall, contributed to a moderate leaf spot epiphytotic. Supplemental moisture was applied using an overhead irrigation system 11, 14 and 16 Jul. The BeetCast leafspot advisory in the Frankenmuth area from 1 May to 20 Sep accumulated 208 daily severity values. Disease severity peaked by early Sep, 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 the 3 Sep 14 rating, means of the resistant and susceptible internal controls for the entire nursery (including two additional experiments) were 2.4 and 5.8, respectively, across the nursery. At the peak of the epiphytotic in 2013 (12 Sep), these means were 3.1 and 5.2 for resistant and a moderately susceptible variety (CE, kindly provided by Syngenta seeds), respectively. Means of contributor lines in the entire nursery (including three additional tests) in 2014 ranged from 2.0 to 7.0. 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 604522, was not significantly different from the resistant control at any of the rating dates. Five additional accessions (PI504242, PI518298, PI540610, PI590767, and PI 599351) were not significantly different from the resistant control at three of the four rating dates but had higher ratings higher at the peak of the epidemic. Only five accessions (NSL141985, PI590616, PI590767, PI614828, and PI604521) did NOT require 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.2014.HANSON