Detection of <i>Cercospora beticola</i> and <i>Phoma betae</i> on Table Beet Seed using Quantitative PCR

Abstract

<i>Cercospora beticola</i> and <i>Phoma betae</i> are important pathogens of table beet, sugar beet, and Swiss chard (<i>Beta vulgaris</i> subsp. <i>vulgaris</i>), causing Cercospora leaf spot (CLS) and Phoma leaf spot, root rot, and damping-off, respectively. Both pathogens may be seedborne; however, limited evidence is available for seed infestation by <i>C. beticola</i>. Due to the limitations of culture-based seed assessment methods, detection of these pathogens was investigated using PCR. A <i>P. betae</i>-specific quantitative PCR assay was developed and used in conjunction with a <i>C. beticola</i>-specific assay to assess the presence of pathogen DNA in 12 table beet seed lots. DNA of <i>C. beticola</i> and <i>P. betae</i> was detected in four and eight seed lots, respectively. Plate tests and BIO-PCR confirmed the viability of each pathogen; however, competitive growth of other microbes and low incidence limited the frequency and sensitivity of detection in some seed lots. The results for <i>P. betae</i> support previously described infestation of seed. Further investigation of <i>C. beticola</i>-infested seed lots indicated the ability of seedborne <i>C. beticola</i> to cause CLS on plants grown from infested seed. Detection of viable <i>C. beticola</i> on table beet seed demonstrates the potential for pathogen dispersal and disease initiation via infested seed, and provides valuable insight into the epidemiology of CLS. Surveys of commercial table beet seed are required to determine the frequency and source of <i>C. beticola</i> seed infestation and its role as primary inoculum for epidemics, and to evaluate the effectiveness of seed treatments.