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Fusarium basal rot (FBR) caused by Fusarium oxysporum f. sp. cepae (FOC) is a major threat to onion (Allium cepa L.) production and marketing worldwide. Finding new sources of FBR-resistance to develop synthetic cultivars is a priority for onion breeders. As there are no FBR-resistant short-day onion cultivars, 85 U.S. National Plant Germplasm System plant introduction onion accessions that originated from 23 different countries were screened for their FBR susceptibility. To compare FBR susceptibility of these accessions at their seedling and mature bulb stages, a susceptible check cultivar, NuMex Crimson, a partially resistant check cultivar, Serrana and its selected population, Serrana-sel, were included in the study. The seedling screening was performed after inoculating silica-sand media with a virulent FOC isolate ‘CSC-515’ at 1.0 × 104 macroconidia g−1 of sand. Each entry was evaluated twice in growth chambers, and percent survival was adjusted to the number of seedlings that germinated in the uninoculated trays. Mature bulbs produced in the field were screened by inoculating transversely cut basal plates with potato dextrose agar plugs containing spores of the same isolate at 3.0 × 104 macroconidia mL−1. FBR severity and incidence were then calculated after 20 days of incubation. Significant variation was found among the accessions for FBR-susceptibility (p< 0.001) at both the seedling and the mature bulb stages. Two sets of 18 accessions were identified either for their higher seedling survival or higher mature bulb FBR-resistance compared to the checks. Among them, PI 256326 (‘Baia Periforme’, the originator cultivar of ‘Serrana’) had a higher seedling survival than both the checks, and a lower mature bulb severity than the susceptible check. Another accession, PI 656956 (‘S015’), exhibited higher seedling survival than the susceptible check and a low FBR severity (4.3 on a 1 to 9 scale) and incidence (41.7%). These two accessions, which were known previously for their high intra-population heterogeneity and root or bulb resistance for FBR, respectively, show promise for incorporating FBR-resistance into short-day onion cultivars. The cultivar rankings could vary in future studies with a range of FOC isolates due to a high cultivar × isolate interaction as observed in past studies.
Subhankar Mandal; Christopher Cramer. Screening of USDA Onion Germplasm for Fusarium Basal Rot Resistance. Horticulturae 2021, 7, 174 .
AMA StyleSubhankar Mandal, Christopher Cramer. Screening of USDA Onion Germplasm for Fusarium Basal Rot Resistance. Horticulturae. 2021; 7 (7):174.
Chicago/Turabian StyleSubhankar Mandal; Christopher Cramer. 2021. "Screening of USDA Onion Germplasm for Fusarium Basal Rot Resistance." Horticulturae 7, no. 7: 174.
Fusarium basal rot (FBR) of onion, caused by the soil-borne fungus Fusarium oxysporum f.sp. cepae (FOC), results in a substantial storage loss of marketable bulbs. Seedling and field screening methods, which were used to generate FBR resistant long-day and intermediate-day onion cultivars, were found to be ineffective at increasing FBR resistance in short-day onion cultivars. This study attempted to improve the FBR resistance of seven commercial short-day onion cultivars and two check cultivars when evaluated at their mature bulb stage. Mature bulbs were artificially inoculated with 1 cm diameter potato dextrose agar plugs containing conidia of a virulent FOC isolate, CSC-515, at a concentration of 3 × 104 spore mL−1, after transversely cutting the basal plate tissue. Incubated bulbs, which had few or no FBR symptoms, were selected after 20 d using visual scoring, from 1 (no disease) to 9 (>70% basal plate is infected), and combined in seed production cages to produce the selected generation of a cultivar. Multiple cycles of phenotypic recurrent selection were conducted, and the resultant populations were compared with their respective original populations for FBR severity and incidence, from 2016 to 2019, using the same conidial inoculation method. A variable amount of progress was achieved in reducing FBR severity and incidence in the seven cultivars, with maximum improvement in the most advanced selected populations. FBR development in the advanced selected populations differed between mature bulbs of each entry and was influenced by yearly environmental variation. The progress of FOC infections was slower in resistant bulbs when compared to susceptible bulbs. These results indicated a partial or quantitative resistance against FBR. The partial FBR resistant cultivar populations could be used to develop synthetic short-day FBR resistant cultivars after multi-locational and multi-seasonal field trials. These populations could also be used to study the mechanism(s) of FBR resistance in onion, which has yet to be determined.
Subhankar Mandal; Christopher Cramer. Improving Fusarium Basal Rot Resistance of Onion Cultivars through Artificial Inoculation and Selection of Mature Bulbs. Horticulturae 2021, 7, 168 .
AMA StyleSubhankar Mandal, Christopher Cramer. Improving Fusarium Basal Rot Resistance of Onion Cultivars through Artificial Inoculation and Selection of Mature Bulbs. Horticulturae. 2021; 7 (7):168.
Chicago/Turabian StyleSubhankar Mandal; Christopher Cramer. 2021. "Improving Fusarium Basal Rot Resistance of Onion Cultivars through Artificial Inoculation and Selection of Mature Bulbs." Horticulturae 7, no. 7: 168.
Development of resistant cultivars for Fusarium basal rot (FBR), a fungal disease caused by the soil-borne fungus Fusarium oxysporum f.sp. cepae (FOC), is a major breeding goal for onion breeding programs around the world. Currently, the selection of FBR-resistant bulbs is based on a visual interval or category scale, which is an entirely subjective method. This study aimed to develop an objective approach using digital image analysis to quantify symptom development in the basal plate of dormant bulbs. Digital image analyses were performed after artificially inoculating dormant bulbs of eighty-five United States Department of Agriculture (USDA) Allium cepa accessions with a virulent FOC isolate, ‘CSC-515’. An analysis with confocal microscopy identified bright blue–green autofluorescence from FOC-infected tissue, effectively differentiating diseased from healthy tissue. Visual scoring of the FBR symptom was aided by stereo fluorescence microscopic images captured using a green fluorescence protein dual filter to quantify accurately FBR severity in the basal plate tissue. An automatic stepwise image segmentation method was developed that was relatively more accurate than a visual estimation. This method exhibited comparable reliability and precision to visual scoring, but it tended to underestimate FOC infection. To our best knowledge, this is the first comprehensive study to investigate the potential use of image analysis as a viable alternative to conventional visual scoring for FBR symptom development. This method could be used for developing resistant cultivars for onion breeding programs in the near future.
Subhankar Mandal; Christopher Cramer. Comparing Visual and Image Analysis Techniques to Quantify Fusarium Basal Rot Severity in Mature Onion Bulbs. Horticulturae 2021, 7, 156 .
AMA StyleSubhankar Mandal, Christopher Cramer. Comparing Visual and Image Analysis Techniques to Quantify Fusarium Basal Rot Severity in Mature Onion Bulbs. Horticulturae. 2021; 7 (6):156.
Chicago/Turabian StyleSubhankar Mandal; Christopher Cramer. 2021. "Comparing Visual and Image Analysis Techniques to Quantify Fusarium Basal Rot Severity in Mature Onion Bulbs." Horticulturae 7, no. 6: 156.
Onions are one of the most important vegetable crops worldwide. However, their production faces many challenges. Genetic improvement is one mechanism to address those challenges. In this review, we discuss recent research pertaining to the diseases Fusarium basal rot and Iris yellow spot, the insect pest onion thrips, onion pungency, and dormancy. Recent research for screening onion bulbs for Fusarium basal rot resistance has resulted in improved screening techniques and germplasm exhibiting less disease when inoculated with the disease-causing pathogen. Improved screening methods have resulted in germplasm exhibiting fewer and less severe Iris yellow spot symptoms when onion thrips and conducive environmental conditions are present. Onion germplasm with less and differing compositions of epicuticular wax on their leaves have shown a nonpreference for thrips feeding and have the potential for developing thrips tolerant cultivars. Conventional breeding efforts and genetic manipulation of the genes producing alliinase and lachrymatory factor synthase has resulted in low pungency, tearless onions. In long-day onions, an annual generation time has been achieved by artificially breaking bulb dormancy early while ensuring proper vernalization has been completed. Genetic improvement of these and many other onion traits will continue and result in better production in the future.
Christopher Cramer; Subhankar Mandal; Suman Sharma; Seyed Nourbakhsh; Irwin Goldman; Ivette Guzman. Recent Advances in Onion Genetic Improvement. Agronomy 2021, 11, 482 .
AMA StyleChristopher Cramer, Subhankar Mandal, Suman Sharma, Seyed Nourbakhsh, Irwin Goldman, Ivette Guzman. Recent Advances in Onion Genetic Improvement. Agronomy. 2021; 11 (3):482.
Chicago/Turabian StyleChristopher Cramer; Subhankar Mandal; Suman Sharma; Seyed Nourbakhsh; Irwin Goldman; Ivette Guzman. 2021. "Recent Advances in Onion Genetic Improvement." Agronomy 11, no. 3: 482.
The development of Fusarium Basal Rot (FBR)-resistant onion cultivars through field and seedling screening approaches faces tremendous challenges due to non-uniform distribution of the disease pathogen and possible multiple mechanisms of host–plant resistance. This study compared the efficiencies of these two methods for increasing FBR resistance of short-day onion after a single selection cycle. Asymptomatic plants or bulbs of seven onion cultivars were selected using a seedling screen performed in a growth chamber or a field screening of mature bulbs. Original and selected populations were evaluated for their responses to FBR stress thereafter employing the same two methods used for screening. The field screening of mature bulbs was found unreliable in both selection and evaluation, likely due to a non-random distribution of the FBR pathogen and variable environmental factors present in the field. The seedling screening successfully increased FBR resistance in the selected cultivar populations revealed by a seedling evaluation. From the results, it is recommended to use a consistent method for both screening and evaluation to make the most selection progress.
Subhankar Mandal; Ashish Saxena; Christopher Cramer; Robert Steiner. Comparing Efficiencies of Two Selection Approaches for Improving Fusarium Basal Rot Resistance in Short-Day Onion after a Single Cycle of Selection. Horticulturae 2020, 6, 26 .
AMA StyleSubhankar Mandal, Ashish Saxena, Christopher Cramer, Robert Steiner. Comparing Efficiencies of Two Selection Approaches for Improving Fusarium Basal Rot Resistance in Short-Day Onion after a Single Cycle of Selection. Horticulturae. 2020; 6 (2):26.
Chicago/Turabian StyleSubhankar Mandal; Ashish Saxena; Christopher Cramer; Robert Steiner. 2020. "Comparing Efficiencies of Two Selection Approaches for Improving Fusarium Basal Rot Resistance in Short-Day Onion after a Single Cycle of Selection." Horticulturae 6, no. 2: 26.