A fold dilution series was then prepared for each plant sampled. The foliage in the buffer solution in each flask was then separated in a strainer, rolled in a paper towel, placed in a paper bag, and dried in an oven for 3 to 7 d to record foliar dry weight. After 3 to 5 d, the XCS agar du Toit et al. Yellow, mucoid, raised colonies typical of this pathogen du Toit et al. Seeds of the 46 carrot accessions listed in Tables 2 and 3 were planted into a 2.
The accessions included 12 carrot wild relatives from the collection of Dr. Christianson; four cultivars one developed for resistance to Xhc and three selected for susceptibility to Xhc from Bayer Cropscience Vegetable Seeds; 13 PI lines selected from the trial eight partially resistant and five highly susceptible to Xhc based on the Xhc population size detected as well as foliar severity rating 6 wpi ; two PI lines that had abnormal foliar growth in the trial that impeded adequate assessment of reaction to inoculation with Xhc PI and ; eight commercial open-pollinated and hybrid cultivars from the trial; an additional four proprietary cultivars from Bayer CropScience Vegetable Seeds selected for resistance one line or susceptibility three lines to bacterial blight; and the two inbred lines evaluated in Plants were grown in the same manner as the trial.
Nutritional deficiency symptoms were observed on the foliage of some lines 4 weeks after planting. Analysis of the potting medium by Soiltest Farm Consultants, Inc. Accession, species, and source of carrot wild relatives and cultivars evaluated in a greenhouse screening for resistance to Xanthomonas hortorum pv. The same inoculation protocol used in the trial was followed in the trial, except that The foliage of each plant was cut 6 wpi as described for the trial.
Some plants had senesced before sampling, resulting in less than 25 plants sampled for those lines. The cut foliage was harvested, washed, and a dilution series plated like in the trial. The and data were analyzed using SAS Version 9. Carrot lines were treated as a fixed effect and replications as a random effect in the analysis of variance. The log-transformed data for all lines and cultivars tested in each of and did not meet the assumptions of normally distributed residuals and homogeneous variances for parametric analysis. Other transformations also did not satisfy these assumptions.
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Bacterial blight symptoms were observed on at least one, and typically all, inoculated plants of each accession evaluated in the trial, demonstrating success of the inoculation protocol for establishing the disease. No symptoms were observed on plants of the non-inoculated inbred lines 4 and 5 wpi.
Symptoms typically began as chlorotic lesions that developed into angular, water-soaked lesions and eventually brown or black, necrotic lesions. However, there were variations in symptoms among entries including purple discoloration along the leaf margins and purple symptoms that turned necrotic e. Disease severity varied extensively among accessions at each of the weekly ratings Table 1 ; Fig.
A Xanthomonas hortorum pv. Citation: HortScience horts 50, 3; Similarly, correlation coefficients for foliar disease ratings 4, 5, and 6 wpi were significant for all 66 PI lines and for the 17 cultivars Table 4. Bacterial populations recovered from the foliage of inoculated plants ranged from log 10 4.
Bacterial blight symptoms did not appear until Xhc populations were greater than log 10 6.
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The pathogen was not detected on eight of the 15 non-inoculated inbred plants. Three control plants had log 10 4. A Population of Xanthomonas hortorum pv. The black dot for each group of entries represents the mean, the horizontal line in each box represents the median, the upper and lower ends of each box represent the first and third quartiles, respectively, and the whiskers represent the range in pathogen population size detected. Correlation coefficients of pathogen population size vs. Several PI lines were outliers because of limited mean bacterial population sizes detected on the foliage despite relatively severe foliar disease ratings, i.
Foliar symptoms were not observed on any plants of PI at 4 and 5 wpi. This PI line had the least severe foliar blight 6 wpi and ranked the third least in bacterial population recovered from the foliage log 10 7. PI had the least Xhc recovered from foliage log 10 6. In contrast, PI had the greatest average disease severity 6 wpi PI had the greatest population of Xhc of all the entries log 10 Bacterial blight symptoms were observed 6 wpi on at least one, and typically all, inoculated plants of all 42 carrot accessions evaluated in the trial.
Bacterial blight was not observed on two plants of each of PI and Nunhems at 3 and 4 wpi. Like in the trial, severity of foliar symptoms varied among entries Table 3 ; Fig. Additionally, slightly different symptoms were observed for some accessions compared with In particular, lesions on plants of the carrot wild relative Ames were chlorotic, did not develop water-soaking, and had relatively distinct margins, unlike lesions on most other entries.
Disease severity measured 3, 4, and 6 wpi varied among all accessions evaluated in Table 3 ; Fig. For all 42 accessions, foliar disease ratings 2, 3, 4, and 6 wpi averaged 1. The eight putatively resistant PI lines selected from the trial averaged severity ratings of 0. In comparison, the 12 commercial cultivars averaged 1.
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PI and , the two PI lines screened again in because of abnormal growth impeding adequate evaluations in , averaged foliar blight severities at 6 wpi of 6. Christianson, averaged 3. Likewise, correlation coefficients of foliar disease ratings 3, 4, and 6 wpi were highly significant when calculated separately for the 15 PI lines, 12 cultivars, and 12 carrot wild relatives Table 5. Xhc was recovered from every inoculated plant in The size of the Xhc population detected on inoculated plants 6 wpi averaged log 10 9.
The pathogen was not detected on eight of 50 non-inoculated control plants.
For 18 of these control plants, Xhc population averaged log 10 4. Similar to the screening, bacterial blight symptoms did not appear until the Xhc population was greater than log 10 6. The eight resistant PI lines selected in averaged log 10 7.
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The putative resistant and susceptible PI lines selected from the screening had similar reactions to Xhc in the screening, although average disease severity ratings for some PI lines were greater in than in In , plants of the cultivar Nunhems had the least severe mean foliar blight rating 6 wpi 0. Like in the screening, the resistant selection PI had the least Xhc recovered from foliage log 10 6. This PI had the second least severe foliar blight 6 wpi 1.
Among the resistant PI lines selected, PI had the narrowest interquartile range in and averaged log 10 7. The other six resistant selections, PI lines , , , , , and , averaged log 10 9. The carrot wild relatives were screened only in The black dot for each group represents the mean, the horizontal line in each box represents the median, the upper and lower ends of each box represent the first and third quartiles, respectively, and the whiskers represent the range in pathogen population size detected.
Among the five susceptible PI lines selected, PI had the greatest disease severity rating 6 wpi This PI had a similar mean population of Xhc recovered log 10 9. Nunhems had the greatest mean Xhc population detected log 10 Of these five highly susceptible PI lines, PI had the narrowest interquartile range of log 10 9. The other four, PI lines , , , and , had average Xhc populations ranging from log 10 9. Results of this greenhouse study in which carrot accessions were inoculated with Xhc indicate that significant differences in resistance exist among PI lines from the USDA NPGS carrot germplasm collection.
Symptoms of bacterial blight were observed on almost all inoculated plants of the carrot accessions tested in and , and Xhc was recovered from every inoculated plant, demonstrating that none of the accessions had complete resistance to this pathogen. However, the severity of bacterial blight symptoms and the amount of Xhc recovered from the foliage varied significantly among the accessions tested. The greatest level of resistance was identified in four PI lines , , , and as well as two carrot wild relatives Ames and SS10 OR. Based on disease severity ratings and size of the Xhc population detected on the foliage, PI and PI had comparable reactions to Xhc as Nunhems , a proprietary cultivar specifically developed for resistance to Xhc.
Nunhems had the least severe bacterial blight in this study, although PI and PI had statistically similar bacterial blight severity ratings. Significantly less Xhc was recovered from the foliage of these two PI lines, in addition to PI and PI , than from foliage of the other 11 commercial carrot cultivars tested.
The smallest mean Xhc population was recovered from plants of PI , although plants of Nunhems , PI , and PI had statistically similar populations. Based on correlation coefficients for foliar disease ratings vs.
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Rating groups of plants for severity of bacterial blight were more efficient than evaluating individual plants, particularly for the resistance screening with a large number of entries. Regardless, an adequate number of replications of entries is necessary for accurate assessment of the reaction of accessions to Xhc. Carrots can also be cut in thin strips and added to rice, can form part of a dish of mixed roast vegetables or can be blended with tamarind to make chutney.
Related Carrots and Related Vegetable Umbelliferae (Crop Production Science in Horticulture, Volume 10)
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