Beta PIs from the USDA-ARS NPGS evaluated for resistance to Cercospora beticola, 2014

Authors: Hanson, Linda; Goodwill, Tom; McGrath, Jon

Submitted to: Plant Disease Management Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2015
Publication Date: 3/20/2015
Citation: Hanson, L.E., Goodwill, T.R., McGrath, J.M. 2015. Beta PIs from the USDA-ARS NPGS evaluated for resistance to Cercospora beticola, 2014. Plant Disease Management Reports. 9:FC001.

Interpretive Summary: Cercospora leaf spot is a destructive fungal disease of beet and occurs in most beet production areas. One important method of managing the disease is the use of varieties with resistance to the pathogen, Cercospora beticola. In continuing efforts to identify sources of resistance to Cercospora, 30 Plant Introdcutions (PIs) from the USDA-ARS National Plant Germplasm System were tested for their response to the leaf-spotting fungus, Cercospora beticola. Plants were inoculated July 10 and the disease was severe enough to start rating by mid August. Significant variability was detected in disease severity between different entries. One entry was not significantly different from the resistant control at all four rating dates. However, it was an annual under Michigan growing conditions, which can be an issue when attempting to use as a resistance source for biennial commercial beets. Five additional PIs were not significantly different from the resistant control at the first three rating dates, but had higher disease severity ratings by the peak of the epidemic. Only five of the PIs did not produce seed stalks (i.e. were biennial) during the study. One PI produced seed and died before the end of the rating period so could not be effectively assessed for disease response. Materials identified in these screenings may be useful sources for resistance breeding.

Technical Abstract: Thirty Plant Introductions (PIs) from the USDA-ARS National Plant Germplasm System were evaluated for resistance to Cercospora beticola in an artificially produced epiphytotic at the Saginaw Valley Research and Extension Center near Frankenmuth, Michigan. Internal controls included a susceptible, '12N0050', and a resistant germplasm 'EL50/2'. The field was planted May 5. The nursery was inoculated once with a liquid spore suspension of Cercospora beticola on July 10. Visual evaluations on the plot, producing a disease index (DI) on a scale from 0 (no disease) to 10 (plant completely defoliated), were made on four dates, with the peak of the epidemic occurring around the fourth date. After the fourth rating, regrowth of several PIs was visible, so ratings started to go down. At our fourth evaluation, means of the resistant and susceptible internal control were 2.4 and 5.8, respectively, across the nursery. In 2013, these means were 3.1 and 5.2, respectively. Means of the PIs ranged from 3.3 to 6.0. An analysis of variance on the disease indices determined that there were significant differences among entries (P=0.05) on all dates of evaluation. One accession was not significantly different from the resistant control at all four ratings. Five additional accesions were not significantly different from the resistant control at the first three ratings, but had significantly higher disease severity ratings by the peak of the epidemic. All but five of the accessions required removal of seed stalks from at least one replicate during the season, meaning they had an annual growth habit. One accession produced seed stalks and died before the end of the rating. Some of the materials may be useful sources for resistance breeding, but the annual growth habit makes incorporation into a biennial commercial beet a challenge.