Screening carrot germplasm for resistance to Xanthomonas hortorum pv. Carotae

Charels Eric Christianson

Xanthomonas hortorum pv. carotae causes bacterial blight of carrot and is readily seedborne. Genetic resistance is limited in commercial cultivars, and there has been little public research on screening for resistance. Carrot plant introduction (PI) lines (n = 66) from the United States Department of Agriculture (USDA) National Plant Germplasm System, two inbred lines from the USDA Agricultural Research Service, and 17 commercial cultivars were screened for resistance to X. hortorum pv. carotae in a greenhouse in 2012. Evaluations were based on the percentage of foliage that developed symptoms as well as population (CFU/g dry foliage) of the pathogen on the foliage detected on a semi-selective agar medium. Severity of foliar blight ranged from 0 to 50% (mean ± SE of 8.8 ± 0.4%), and the pathogen population ranged from 1.38 x 104 to 3.28 x 1011 CFU/g dry foliage (8.16 x 109 ± 1.07 x 109 CFU/g) for individual plants. Eight putative resistant PI lines and five highly susceptible PI lines selected from the 2012 screening were evaluated again in 2013 along with an additional 2 PI lines, 12 cultivars, 2 inbreds, and 16 carrot wild relatives. Severity of foliar blight 6 wpi ranged from 0 to 90% (11.8 ± 0.4%), and the pathogen population ranged from 4.90 x 104 to 1.30 x 1011 CFU/g dry foliage (1.00 x 1010 ± 5.29 x 108 CFU/g) for individual plants. In 2012 and 2013, Spearman’s correlation coefficients between bacterial populations and symptoms were highly significant (r = 0.5183 and 0.6162, respectively, at P < 0.0001). PIs 418967, 432905, and 432906 were the most resistant based on pathogen population detected. There was little resistance displayed by the carrot wild relatives. Resistance from the selected PIs will be integrated into commerciallyacceptable carrot inbreds (USDA ARS inbred lines A2566 and A0493) for release to public and private carrot breeding programs. Additionally, assays of 25 seed/PI revealed the presence of Alternaria radicina on 56.1%, A. carotiincultae on 1.5%, A. dauci on 51.5%, and A. petroselini on 1.5% of the seed lots received from the USDA NPGS, highlighting the need for treatment of carrot PI seed.

Screening carrot germplasm for resistance to Xanthomonas hortorum pv. Carotae

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