36th International Carrot Conference Abstract

Screening for resistance to Xanthomonas hortorum pv. carotae in Daucus carota

Charles E. Christianson, Stephen S. Jones, and Lindsey J. du Toit

Washington State University Mount Vernon NWREC, Mount Vernon, WA 98273

Xanthomonas hortorum pv. carotae causes bacterial blight of carrot. Genetic resistance to bacterial blight is limited in commercial carrot cultivars, and little public research has been carried out on screening for sources of resistance. Furthermore, disease symptoms typically do not develop until pathogen populations reach 10⁵ to 10⁶ CFU/g dry leaf tissue, complicating evaluations for resistance to X. hortorum pv. carotae infections in the field. Sixty-six plant introduction (PI) lines from the United States Department of Agriculture (USDA) National Plant Germplasm System, two inbred carrot lines from Dr. Philipp Simon’s carrot breeding program at the University of Wisconsin, and 17 proprietary carrot hybrids or open pollinated lines from six vegetable seed companies or breeders were screened for resistance to X. hortorum pv. carotae. Evaluations included percentage of the foliage that developed symptoms six weeks post-inoculation, and population of the pathogen (CFU/g dry leaf tissue) detected on a semi-selective agar medium (XCS) when the foliage was assayed six weeks post-inoculation. Pathogen populations on foliage ranged from 1.38 X 10⁴ to 3.28 X 10¹¹ CFU/g dry leaf tissue for individual plants, with a median of 2.80 X 10⁹ CFU/g dry leaf tissue, and a mean ± standard error of 8.16 X 10⁹ ± 1.07 X 10⁹ CFU/g dry leaf tissue. Severity of symptoms ranged from 0 to 50% of the foliage blighted, with a median of 7% and a mean of 8.84 ± 0.38%. Visual symptoms after 6 weeks and CFU/g dry leaf tissue (arcsine transformed) were significantly correlated (Pearson’s correlation coefficient of r = 0.4685 at P<0.0001). Thirteen PI lines with potential resistance (lowest visual ratings and populations of X. hortorum pv. carotae detected) were selected for a second screening, along with five of the most susceptible PI lines to aid in the study of inheritance of resistance or susceptibility to X. hortorum pv. carotae. The lines that express the most resistance in the second screening will be crossed to inbred carrot lines from Dr. Simon’s program in an effort to incorporate the resistance into a genetic background available for public use. The variation detected among entries in this study suggests it should be possible to develop carrot cultivars with improved resistance to bacterial blight, but resistance is likely to be highly quantitative. Screening a more diverse set of Daucus germplasm may be necessary to identify greater sources of resistance.

Last updated Thursday, 25-Jul-2013 11:53:44 CDT