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Dipak Sharma-Poudyal, Timothy C. Paulitz, and Lindsey J. du Toit

Thirty-five onion genotypes were evaluated for resistance to stunting caused by Rhizoctonia solani anastomosis group 8 (AG 8) in a growth chamber set at 15 ± 1 °C. The trial was repeated. Resistance to R. solani AG 8 was defined as a lack of significant difference in plant height, root length, and/or total dry biomass between inoculated and noninoculated plants of the same genotype. Plant height was not reduced significantly by R. solani AG 8 for 14 and 7 of the 35 genotypes in Trials 1 and 2, respectively. In Trial 1, plant height reduction caused by R. solani ranged from 24% for the cv. Lasalle to 62% for the experimental line R14882, and in Trial 2 plant height reduction ranged from 22% for the experimental line PX07713218 to 53% for the cv. Montblanc. However, the extent of reduction in plant height caused by R. solani did not differ significantly among genotypes in either trial. Onion root length was not reduced by R. solani AG 8 for 26 and 18 of the 35 genotypes in Trials 1 and 2, respectively. The degree of reduction in root length ranged from 9% (R14889) to 76% (Sterling and SN232) in Trial 1, and 14% (SN325) to 74% (Sterling) in Trial 2. Onion dry biomass was not reduced by R. solani AG 8 for 19 and 7 of the 35 genotypes in Trials 1 and 2, respectively, and ranged from 18% (Elbrus) to 69% (Sterling) in Trial 1, and 29% (SN232) to 79% (Sterling) in Trial 2. The reduction in onion root length and total biomass did not differ significantly among onion genotypes in Trial 1, but differed among genotypes in Trial 2. Of the 35 genotypes evaluated, 3, 16, and 3 demonstrated partial resistance to R. solani AG 8 for plant height, root length, and total biomass, respectively, in both trials. Only four genotypes displayed partial resistance to R. solani AG 8 for at least two of the three growth parameters: plant height, root length, and biomass of PX07713218 were unaffected by the fungus in either trial; and R14885, R14888, and SN307 displayed partial resistance in both trials for two of the three parameters, and in one of the two trials for the third parameter. These four genotypes could be used in onion breeding programs to develop cultivars partially resistant to stunting caused by R. solani AG 8.

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Beiquan Mou, Steven T. Koike, and Lindsey J. du Toit

The entire U.S. Department of Agriculture (USDA) spinach (Spinacia oleracea L.) germplasm collection (338 accessions) and 22 commercial cultivars were evaluated for resistance to leaf spot caused by Stemphylium botryosum in a greenhouse trial with two replications in 2004. The resistant and susceptible accessions identified as well as the 22 commercial cultivars were included in a second test in 2005 with four replications to confirm the results. No genotype was completely resistant (immune) to the disease. However, there were significant differences in disease incidence (percent of plants with leaf spot) and severity (percent diseased leaf area) among the genotypes tested. Two accessions from Turkey, PI 169685 and PI 173809, consistently had low disease incidence and severity ratings. Two Spinacia tetrandra and four Spinacia turkestanica accessions screened in these public germplasm tests were all susceptible. None of the commercial cultivars tested consistently had low disease incidence or severity. There was no significant correlation between disease incidence/severity and leaf type (smooth, semisavoy, or savoy). In addition to the public germplasm evaluated, 138 proprietary spinach genotypes (breeding lines and cultivars) were obtained from seed companies and screened along with 10 accessions from the USDA germplasm collection for resistance to Stemphylium leaf spot and Cladosporium leaf spot (caused by Cladosporium variabile) in a greenhouse in both 2004 and 2005. Significant differences in severity of leaf spot were observed among the genotypes for both diseases. For each disease, there was a significant positive correlation in severity ratings of the genotypes between the 2004 and 2005 trials. Information on the relative resistance (or susceptibility) of the spinach germplasm evaluated in this study should be useful for plant breeders to develop leaf spot-resistant cultivars.

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

Xanthomonas hortorum pv. carotae (Xhc) causes bacterial blight of carrot (Daucus carota L.), is endemic in the primary regions of carrot seed production, and is readily seed-transmitted. Genetic resistance to Xhc is not well documented in commercially available carrot cultivars, and there has been little public research on screening for resistance. Carrot PI lines (n = 66), public inbred lines (n = 2), and commercial cultivars (n = 17) were assessed for response to Xhc in a greenhouse in 2012 based on the incidence and severity of bacterial blight symptoms after inoculation as well as Xhc population [colony-forming units (CFU)/g dry foliage] detected by dilution plating onto XCS agar, a semiselective medium for Xhc. Severity of bacterial blight averaged 8.8% ± 0.4% (mean ± se) with a range of 0% to 50%, and size of the Xhc population detected on the foliage averaged 8.16 × 109 ± 1.07 × 109 CFU/g (range, 1.38 × 104 to 3.28 × 1011 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 with an additional two PI lines, 12 cultivars, two inbred lines, and 12 carrot wild relatives. In the 2013 trial, severity of foliar blight 6 weeks post-inoculation ranged from 0% to 90% (11.8% ± 0.4%), and Xhc population ranged from 4.90 × 104 to 1.30 × 1011 CFU/g dry foliage (1.00 × 1010 ± 5.29 × 108 CFU/g) for individual plants. Spearman’s correlation coefficient between the Xhc population detected and severity of bacterial blight was highly significant in the 2012 and 2013 trials (r = 0.52 and 0.62, respectively, at P < 0.0001). PI lines 418967, 432905, and 432906 were the most resistant based on Xhc population detected and could be used to develop resistant cultivars. Of the 12 carrot wild relatives screened, only Ames 7674 and SS10 OR displayed a relatively low severity of bacterial blight and population of Xhc on the foliage.

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Maria I. Villarroel-Zeballos, Chunda Feng, Angela Iglesias, Lindsey J. du Toit, and James C. Correll

Verticillium dahliae is a pathogen of spinach (Spinacia oleracea) during spinach seed crop production but not in vegetative leafy spinach crops, because plants remain asymptomatic until bolting has been initiated (conversion from vegetative to reproductive growth). The objectives of this research were to evaluate a set of USDA spinach germplasm accessions for resistance to Verticillium wilt and to determine the range in incidence of natural seed infection/infestation in a subset of the USDA spinach accessions screened for resistance. A total of 120 Spinacia spp. accessions from the USDA North Central Regional Plant Introduction Station spinach germplasm collection and 10 commercial S. oleracea hybrids were screened for resistance to V. dahliae in Trials 1, 2, and 3 in 2006, 2007, and 2008, respectively, in greenhouse evaluations. Each accession was grown in either V. dahliae-infested or non-infested potting mix and rated weekly on a seven-step scale from 1 week before bolting to 4 weeks after bolting. A wide range of disease severity ratings was observed among the accessions evaluated. Most of the accessions were highly susceptible. There was no evidence of qualitative resistance but some showed greater levels of quantitative resistance than others. Plants in soil infested with V. dahliae senesced faster and had significantly reduced biomass compared with plants in non-infested soil of the same accession. In addition, in Trial 2 (2007), 34% (20 of 59) of the seed samples assayed of the accessions were infested or infected with V. dahliae; and in Trial 3, (2008) 16% (21 of 130) of the seed samples of the USDA accessions evaluated were infested or infected with V. dahliae, V. tricorpus, or Gibellulopsis nigrescens (formerly known as V. nigrescens). These results are valuable for characterizing potential genetic variability within spinach germplasm for resistance to V. dahliae.