Recent Advances in Onion Genetic Improvement

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.

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Improving Fusarium Basal Rot Resistance of Onion Cultivars through Artificial Inoculation and Selection of Mature Bulbs

Horticulturae ◽

10.3390/horticulturae7070168 ◽

2021 ◽

Vol 7 (7) ◽

pp. 168

Author(s):

Subhankar Mandal ◽

Christopher S. Cramer

Keyword(s):

Recurrent Selection ◽

Quantitative Resistance ◽

Field Trials ◽

Basal Plate ◽

Screening Methods ◽

Short Day ◽

Basal Rot ◽

Fusarium Basal Rot ◽

Maximum Improvement ◽

Multiple Cycles

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.

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An Artificial Inoculation Method to Select Mature Onion Bulbs Resistant to Fusarium Basal Rot

HortScience ◽

10.21273/hortsci15268-20 ◽

2020 ◽

Vol 55 (11) ◽

pp. 1840-1847

Author(s):

Subhankar Mandal ◽

Christopher S. Cramer

Keyword(s):

Basal Plate ◽

Artificial Inoculation ◽

Economic Damage ◽

Screening Methods ◽

Field Inoculation ◽

Inoculation Method ◽

Short Day ◽

Onion Bulbs ◽

Basal Rot ◽

Fusarium Basal Rot

Fusarium basal rot (FBR) of onion, which is caused by Fusarium oxysporum f.sp. cepae (Hanzawa) Snyder & Hansen (FOC) results in a substantial loss of marketable bulbs worldwide. One of the main reasons for the lack of FBR-resistant short-day cultivars is the unreliable screening methods available for the mature bulb stage when significant economic damage occurs. The objective of this study was to develop an artificial inoculation method with better quantification of inoculum for an effective selection of FBR-resistant mature onion bulbs. Mature bulbs of seven New Mexican short-day onion cultivars, along with susceptible and tolerant controls, were selected and evaluated for FBR resistance using mycelial and conidial inoculation methods, respectively. Transversely cut basal plates of mature bulbs were inoculated artificially with mycelia or conidia (12 × 105 spores/mL in 2014 and 3 × 105 spores/mL in 2015 embedded in potato dextrose agar plug) of a virulent FOC isolate ‘CSC-515’. Mature bulb evaluation using a visual rating scale (1 = no disease; 9 = >70% basal plate infected) revealed a high degree of FBR severity and incidence irrespective of the genetic background of the cultivars, minimizing the chance of disease escape, which is a significant problem in field inoculation. An attempt to inoculate intact basal plates postharvest resulted in minimal disease development, suggesting that mechanical resistance was conferred by the dry outer layer of the basal plate. The high selection pressure conferred by the conidial inoculation method developed in this study can effectively screen FBR-resistant onion bulbs to replace an unreliable field screening. Concentrations of the conidia lower than 3 × 105 spores/mL are recommended to detect subtle genetic differences in FBR resistance among the onion cultivars and their selected population.

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Fusarium basal rot: profile of an increasingly important disease in Allium spp.

Tropical Plant Pathology ◽

10.1007/s40858-021-00421-9 ◽

2021 ◽

Author(s):

Dung Le ◽

Kris Audenaert ◽

Geert Haesaert

Keyword(s):

Disease Management ◽

Cultural Practices ◽

Current Knowledge ◽

Management Strategies ◽

Future Directions ◽

Basal Rot ◽

Major Constraint ◽

Fusarium Basal Rot ◽

Chemical Pesticides ◽

Important Disease

AbstractFusarium basal rot (FBR) is a soil-borne disease that affects Allium species worldwide. Although FBR has long been recognized as a major constraint to the production of economically important Allium species, information that could support disease management remains scattered. In this review, the current knowledge on the causal agents, symptomology and epidemiology, impact, and management strategies of FBR is synthesized. We highlight that FPR is associated with different complexes of several Fusarium species, of which Fusarium oxysporum and F. proliferatum are the most prevalent. These pathogenic complexes vary in composition and virulence, depending on sites and hosts, which can be challenging for disease management. Research to improve disease management using chemical pesticides, resistance cultivars, biocontrol agents, and cultural practices has achieved both promising results and limitations. Finally, research needs and future directions are proposed for the development of effective FBR management strategies.

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Evaluation of Green Manure Amendments for the Management of Fusarium Basal Rot (Fusarium oxysporumf.sp.cepae) on Shallot

International Journal of Agronomy ◽

10.1155/2014/150235 ◽

2014 ◽

Vol 2014 ◽

pp. 1-6 ◽

Cited By ~ 1

Author(s):

Assefa Sintayehu ◽

Seid Ahmed ◽

Chemeda Fininsa ◽

P. K. Sakhuja

Keyword(s):

Green Manure ◽

Agricultural Research ◽

Disease Incidence ◽

Limiting Factors ◽

Lepidium Sativum ◽

Ethiopian Mustard ◽

Basal Rot ◽

Fusarium Basal Rot ◽

Manure Amendments

Shallot (Allium cepaL. var.ascalonicum) is the most traditional vegetable crop in Ethiopia. Shallot is susceptible to a number of diseases that reduce yield and quality, among which fusarium basal rot (FBR) caused byFusarium oxysporumf.sp.cepae (Foc)is one of the most important yield limiting factors in Ethiopia. The present study was designed to evaluate the effectiveness ofBrassicacrops for the management of shallot FBR on shallot. The experiments were carried out at Debre Zeit Agricultural Research Center using cabbage (Brassica oleracea), garden cress (Lepidium sativum), Ethiopia mustard (B. carinata), and rapeseed (B. napus). The evaluations were done underin vitroand greenhouse conditions. Underin vitrotest condition it was confirmed that extracts of Ethiopian mustard and rapeseed showed higher inhibition on the growth ofFocpathogen compared to control. Data on seedling emergence, plant height, plant stand, disease incidence, severity, cull bulbs, and bulb weight were collected in greenhouse experiment. The green manure amendments of rapeseed and Ethiopian mustard significantly reduced disease incidence by 21% and 30% and disease severity by 23% and 29%, respectively. However the plant emergency was not significantly different among treatments in greenhouse test. These results indicated that Ethiopian mustard and rapeseed crops have potential as green manure for the management of FBR disease of shallot crop.

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First Report of Iris yellow spot virus on Onion (Allium cepa) in Texas

Plant Disease ◽

10.1094/pd-90-1359b ◽

2006 ◽

Vol 90 (10) ◽

pp. 1359-1359 ◽

Cited By ~ 7

Author(s):

M. E. Miller ◽

R. R. Saldana ◽

M. C. Black ◽

H. R. Pappu

Keyword(s):

Disease Incidence ◽

Iris Yellow Spot Virus ◽

Yellow Spot ◽

N Gene ◽

Nucleotide Sequence Analysis ◽

Rt Pcr ◽

Onion Thrips ◽

Spot Virus ◽

Das Elisa ◽

Bulb Yield

Iris yellow spot virus (IYSV; family Bunyaviridae, genus Tospovirus) has emerged as a potentially devastating and widespread virus of onion. IYSV was first reported in the United States from Idaho in 1993 and has since spread to many of the onion-producing areas (1). In South America, the most recent reports of the virus on onion were from Peru and Chile (2,4). In 2005, onion plants in Uvalde County, Texas exhibited necrotic lesions on leaves typical of IYSV and disease incidence approached 100% in some fields with yield loss and quality problems. Five of six plants tested were positive for IYSV with double antibody sandwich-enzyme linked immunosorbent assay (DAS-ELISA; Agdia Inc., Elkhart, IN). In 2006, similar lesions were observed on onion plants in Uvalde County and approximately 400 km south in Hidalgo and Cameron counties. Infection points generally started as a single plant near the edge of fields and spread to plants in a 3- to 4-m area after 1 to 2 weeks. Early-season disease incidence was low in onions grown for bulbs and transplants, <10% in 2006. Disease incidence increased in some fields until the crop was harvested. Leaves of symptomatic plants were tested for IYSV and Tomato spotted wilt virus (TSWV) using DAS-ELISA, and 18 of 23 samples from the Hidalgo County area and 12 of 21 samples from the Uvalde County area were positive for IYSV. All samples tested for TSWV from these counties were negative. Virus infection in some ELISA-positive plants was verified by reverse transcription-polymerase chain reaction (RT-PCR) using primers derived from the small RNA of IYSV. The primers flanked the IYSV nucleocapsid (N) gene (5′-TAA AAC AAA CAT TCA AAC AA-3′ and 5′-CTC TTA AAC ACA TTT AAC AAG CAC-3′ (3). RT-PCR gave a PCR product of expected size (approximately 1.2 kb). The DNA amplicon was cloned and sequenced (GenBank Accession No. DQ658242). Nucleotide sequence analysis confirmed the identity of the amplicon as that of IYSV N gene and sequence comparisons with known IYSV N gene sequences showed 95 to 98% sequence identity. The primary vector of IYSV, onion thrips (Thrips tabaci), is a widespread and destructive pest of onion in south Texas. The year-to-year incidence of IYSV and the severity of the disease will probably depend on the onion thrips population levels. Bulb yield reduction could be severe during years with high thrips populations. More research is needed to determine the impact of IYSV on bulb yield in Texas, the relationship between IYSV incidence and T. tabaci population levels, and oversummering hosts. To our knowledge, this is the first known report of IYSV in Texas. References: (1) D. H. Gent et al. Plant Dis. 88:446, 2004, (2) S. W. Mullis et al. Plant Dis. 90:377, 2006, (3) H. Pappu et al. Arch. Virol. 151:1015, 2006. (4) M. Rosales et al. Plant Dis. 89:1245, 2005.

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Comparing Efficiencies of Two Selection Approaches for Improving Fusarium Basal Rot Resistance in Short-Day Onion after a Single Cycle of Selection

Horticulturae ◽

10.3390/horticulturae6020026 ◽

2020 ◽

Vol 6 (2) ◽

pp. 26

Author(s):

Subhankar Mandal ◽

Ashish Saxena ◽

Christopher S. Cramer ◽

Robert L. Steiner

Keyword(s):

Environmental Factors ◽

Random Distribution ◽

Single Cycle ◽

Field Screening ◽

Selection Cycle ◽

Short Day ◽

Basal Rot ◽

Fusarium Basal Rot ◽

Consistent Method ◽

Screening Approaches

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.

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Managing Onion Thrips can Limit Bacterial Stalk and Leaf Necrosis in Michigan Onion Fields

Plant Disease ◽

10.1094/pdis-07-18-1271-re ◽

2019 ◽

Vol 103 (5) ◽

pp. 938-943 ◽

Cited By ~ 1

Author(s):

A. S. Grode ◽

E. Brisco-McCann ◽

P. Wiriyajitsonboom ◽

M. K. Hausbeck ◽

Z. Szendrei

Keyword(s):

Cultural Practices ◽

Disease Incidence ◽

Insect Pest ◽

Management Program ◽

Thrips Tabaci ◽

Onion Thrips ◽

Leaf Necrosis ◽

Positive Correlations ◽

Thrips Population ◽

Reduce Disease Severity

Onion thrips (Thrips tabaci) is a major insect pest of onion and it has been identified as a likely vector of Pantoea agglomerans (bacterial stalk and leaf necrosis), a relatively new pathogen to Michigan’s onion industry. Our objective was to develop an integrated insect and disease management program by examining the efficacy of bactericides and insecticides alone and in combination to limit bacterial stalk and leaf necrosis caused by P. agglomerans. We also examined the association of onion thrips and disease incidence in the field, because thrips are known to transmit this pathogen. In the pesticide trial, insecticides reduced both thrips abundance and bacterial stalk and leaf necrosis incidence whereas bactericides alone did not reduce disease severity. Positive correlations among thrips population density, numbers of thrips positive for P. agglomerans, and bacterial stalk and leaf necrosis incidence in onion fields were determined. This study suggests that onion thrips feeding can facilitate the development of bacterial stalk and leaf necrosis in Michigan’s commercial onion fields, and results from the pesticide trials indicate that thrips feeding damage is positively correlated with disease incidence. Therefore, in order to reduce bacterial stalk and leaf necrosis incidence in onion, management efforts should include reducing onion thrips populations through the use of insecticides and other cultural practices.

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Spatial and Temporal Distribution of Iris yellow spot virus and Thrips in Colorado Onion Fields

Plant Health Progress ◽

10.1094/php-2010-0820-01-rs ◽

2010 ◽

Vol 11 (1) ◽

pp. 11 ◽

Cited By ~ 2

Author(s):

Howard F. Schwartz ◽

David H. Gent ◽

Scott M. Fichtner ◽

Rajiv Khosla ◽

Linda A. Mahaffey ◽

...

Keyword(s):

Soil Ph ◽

Management Practices ◽

Disease Incidence ◽

Risk Index ◽

Iris Yellow Spot Virus ◽

Yellow Spot ◽

Onion Thrips ◽

Spot Virus ◽

Negatively Associated ◽

Sodium Percent

Iris yellow spot virus and its onion thrips vector (Thrips tabaci) are yield limiting pests of onion in the western United States. This two-year project investigated the relationship of iris yellow spot to thrips populations, soil properties, and grower management practices in six onion fields during 2005 and 2006 in Colorado. Thrips density (average number of thrips per plant per day) was negatively associated with soil pH (r = -0.61). The incidence of iris yellow spot at harvest was positively associated with organic matter, zinc, potassium, percent sodium, percent hydrogen, copper, iron, and soil pH. Thrips-days had a clear non-linear association with disease incidence at harvest. Onion plant population was positively associated with levels of iron, phosphorus, potassium, and zinc; and negatively associated with calcium, salts, percent potassium, and cation exchange capacity (CEC). Marketable (medium or larger sized bulbs) yield was negatively associated with phosphorus and percent potassium, and positively associated with salts, calcium, and CEC. Results from this project could contribute to the development of a risk index to predict potential risk from Iris yellow spot virus and its onion thrips vector. Accepted for publication 6 July 2010. Published 20 August 2010.

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