Location: Plant Genetic Resources Research
Project Number: 8060-21000-027-05-S
Project Type: Non-Assistance Cooperative Agreement
Start Date: Aug 15, 2018
End Date: Mar 20, 2020
The United States is the fourth largest producer of dry onion (Allium cepa L.) bulbs in the world. However, production of this specialty crop has been badly affected by several fungal diseases. One of the most devastating fungal diseases is Fusarium basal rot (FBR), caused by Fusarium oxysporum f. sp. cepae (FOC), which is prevalent in all onion-growing regions across the U.S., as well as in the world. A 60% yield reduction has been observed in FOC infested organic soils of the Midwestern U.S. In Michigan, the combined average loss of both from field and storage was estimated to be 23% with most severe losses occurring in storage. FBR ranks as the second most important disease in terms of economic loss in the state of New Mexico that accounts for 33% of the total summer non-storage onions in the US. Although long-day and intermediate-day onion cultivars have exhibited some FBR resistance, commercially-available, short-day resistant cultivars have yet to be developed. A seedling inoculation screening of several short-day accessions has been utilized in the past; however, moderate resistance was found in only one accession. With two different suggested host plant resistance mechanisms, one at the seedling and one at the mature bulb stage, a screening of mature bulbs for FBR resistance would be informative. The New Mexico State University (NMSU) onion breeding program has developed an artificial inoculation technique for onion that can be performed at the mature bulb stage in order to screen for FBR-resistant bulbs. In this method, an objective scoring technique utilizing digital image analysis has been proposed to substitute for the current visual disease severity rating in order to improve the accuracy of detecting FBR sysmptoms. Additionally, a high-throughput biochemical screening of FBR-resistant onion bulbs has been proposed that will utilize steroidal saponins, which are pre-existing secondary metabolites in Allium sp. that exhibit antifungal activity. With a fungal attack, these chemicals have shown increased levels which could be an effective indicator to screen FBR-resistant bulbs of onion. Therefore, this study proposes to evaluate short-day accessions that have not been evaluated for FBR resistance, by utilizing both a seedling inoculation screening and an artificial inoculation mature bulb screening (AIMBS) methods along with improving the scoring technique through digital image analysis and screening based on antifungal steroidal saponins.
Depending upon the availability of seeds, 85 short-day onion accessions were selected from the Allium cepa collection for the two screenings and digital image analysis study. Additionally, fourteen short-day accessions, which are either reported to be resistant or susceptible to FBR, have been selected for the saponin study. Seedling inoculation screening: Inoculum preparation and inoculation will follow established procedures. In five separate trays, 50 seeds of six (four accessions, one susceptible check, one resistant check) entries will be sown 1.5 cm deep in each tray. The initial temperature of the growth chambers will be 22C and then raised to 28C after two weeks. After four weeks, the number of living seedlings will be counted and the percent survival will be determined by expressing survival relative to seedling survival in the uninoculated tray. During whole experiment, a 16 h light period will be maintained. Artificial inoculation mature bulb screening (AIMBS): Due to limited seed amounts and low germination rates, seeds of all accessions will be sown during Aug. 2018 in black plastic trays containing soilless mix before being transplanted to the field once plants are large enough in Oct. 2018. The study will be arranged as a randomized complete block design with three replications per accession. During the summer of 2019, mature bulbs will be harvested, when 80% plant tops have lodged. Individual bulbs will be inoculated by placing a 1 cm diameter PDA plug on the top of transversely cut (section thickness 0.25 – 0.30 mm approx.) basal plate. All the inoculated bulbs of one entry will be placed in a single black plastic bulb crate with the inoculated surface facing upwards. Disease incidence will be calculated as the percentage of inoculated bulbs exhibiting FBR symptoms. Digital image analysis for AIMBS method: Out of the 85 accessions that will be scored visually using the AIMBS method, a subset of the accessions will also be scored using the digital image analysis technique. Visual scoring will also be done. Regression analysis will be performed to measure relative precision of the two methods of FBR scoring between intra- and inter-scorer. Identification of steroidal saponins as a potential biochemical marker: Modification of a published method will be used to extract and concentrate onion saponins using a C18 Solid Phase Extraction (SPE). Results from both the screenings, digital image analysis and steroidal saponin study will be entered in the Germplasm Resources Information Network (GRIN) database.