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- Author or Editor: Christopher S. Cramer x
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.
Iris yellow spot (IYS) disease, caused by Iris yellow spot virus (IYSV), results in irregular and diamond-shaped, chlorotic, and necrotic lesions on the leaves and seedstalks of onions (Allium cepa L.). These lesions reduce leaf photosynthetic area and ultimately reduce onion bulb size and yield from larger bulb classes. IYSV is vectored by onion thrips (Thrips tabaci L.) that are difficult to control under certain environmental conditions. Currently, no onion cultivar is resistant to the disease symptoms, virus, and/or thrips. Twenty-one cultivars and 17 germplasm lines were evaluated in the field for IYS disease severity and thrips densities at multiple times during the season as well as leaf color, waxiness, and axil openness of these entries. Plants were grown under conditions that favored thrips populations (high temperatures, low moisture, and no insecticidal spray applications), IYSV presence and distribution, and IYS development. Plants of New Mexico State University (NMSU) 07-10-1 had fewer thrips than several entries later in the season in both 2009 and 2010. Several entries exhibited a lower number of thrips per plant early or later in the season; however, these results were not consistent across years and were not associated with a particular foliage characteristic. Lighter leaf color and/or a lesser amount of epicuticular wax did not always result in the fewest number of thrips per plant as has been reported in the literature. Plants of NMSU 09-58 tended to exhibit fewer and less severe IYS symptoms early in the season as compared with plants of other entries.
Current emphasis of onion breeding programs has been to develop male-sterile, maintainer, and pollinator inbred lines for the production of hybrid cultivars. Five short-day, male-sterile inbred lines from the New Mexico State Univ. Onion Breeding Program were crossed to four short-day, pollinator inbred lines in all possible combinations. In addition, six intermediate-day male-sterile inbred lines were crossed with seven intermediate-day, pollinator inbred lines in all possible combinations. The resulting hybrid lines from all crosses were evaluated for maturity, bolting resistance, pink root resistance, Fusarium basal rot resistance, percentage of marketable bulbs, marketable yield, average bulb weight, and percentage of bulbs with single centers. The average performance among male-sterile and among pollinator lines within each group was determined by averaging over hybrid lines that pertained to the respective male-sterile or pollinator line. Among the short-day inbred lines, NMSU 97-28-2 and NMSU 97-109-2 as female parents produced the best hybrid lines, while NMSU 96-17-1 and NMSU 96-51-1 as male parents produced the best hybrid lines. The best hybrid combinations among the short-day parents were NMSU 97-28-2 × 96-17-1 and 97-46-2 × 96-51-1. Among the intermediate-day inbred lines, NMSU 96-196-2 and 96-300-2 as female parents produced the best hybrid lines, while NMSU 96-280-1, NMSU 96-274-1 and 96-395-1 as male parents produced the best hybrid lines. Some of the best intermediate-day hybrid combinations included NMSU 96-300-2 × 96-335-1 and NMSU 96-300-2 × 96-274-1.
Iris yellow spot virus (IYSV) causes a foliar disease in onion (Allium cepa L.) that results in a reduction in bulb size. Currently, no IYSV-tolerant or -resistant cultivar exists and a genetic source for tolerance or resistance has not been identified. Because other disease control methods are limited, host plant resistance offers the best hope to combat this disease. In this study, 13 winter-sown onion entries were screened for iris yellow spot (IYS) symptoms during the 2007 and 2008 cropping seasons. Twenty plants from each plot were observed and rated weekly during the growing seasons for straw-colored, necrotic lesions, typical of IYSV infection. Collected plant samples were assayed for IYSV by enzyme-linked immunosorbent assay (ELISA) and reverse-transcription polymerase chain reaction (RT-PCR). Visual rating was done using a scale of 1 to 9 with 1 representing no symptomatic tissue and 9 representing more than 50% tissue damage. Two different plant sampling methods were used in disease rating to determine their effect on mean severity and to correlate disease severity with absorbance values. Of the entries tested, plants of NMSU 05-33-1 exhibited a delay in symptom expression and lower IYSV levels relative to plants of other entries. Plants of ‘Denali’ and ‘Gelma’ appeared to be more susceptible to IYSV than plants of other entries. Plant selection within the plot over time did not influence disease rating values. When the same plants were rated and sampled for IYSV using ELISA, there was a strong, positive correlation between rating and absorbance values.
Iris yellow spot is an economically important disease of onion that reduces bulb size and yield and is difficult to control. The disease is spread by Thrips tabaci (onion thrips) and disease symptoms are exacerbated by hot, dry climatic conditions that also favor rapid thrips multiplication and make control programs less effective. Currently, no onion cultivar is resistant to the disease and/or thrips. Certain onion foliar characteristics have shown nonpreferential feeding activity by thrips and may be the first step in developing Iris yellow spot (IYS)-tolerant onion cultivars. Seventy-five onion PI accessions from the U.S. germplasm collection were evaluated for leaf color, waxiness (bloom), and axil pattern; thrips number per plant; and IYS disease severity under conditions that favored thrips and disease buildup. Plants of PI 289689 were less attractive to thrips and had a lower number of thrips per plant than plants of most other accessions. These plants were rated as having light green to green-colored foliage and a relatively low amount of epicuticular leaf wax. Plants of PIs 239633 and 546192 generally exhibited less severe IYS disease symptoms than those of other accessions. Individual plants, that exhibited less leaf area exhibiting IYS disease symptoms, were selected at bulb maturity from 22 different accessions with PI 546140 producing the largest number of selected bulbs. Physiological plant development, environmental conditions, and tolerance to plant stress may influence the degree of disease symptom expression. Further work that examines the role of plant maturity and host plant tolerance to stress with respect to disease expression is needed.
In January 2002, an organic vegetable garden on the New Mexico State Univ. (NMSU) main campus was initiated to expose students to organic production practices and agricultural business management. The project named, OASIS (Organic Agriculture Students Inspiring Sustainability), is funded by a USDA Hispanic Serving Institution Grant and operated as a Community Supported Agriculture (CSA) venture. Students enroll in an organic vegetable production class during spring and fall semesters to help manage and work on the project. The CSA model of farming involves the sale of shares to members who receive weekly allotments of the farm's output. The objectives of the project are to provide students with a multi-disciplinary experiential educational opportunity, to investigate the feasibility of small scale organic drip irrigated farming in the Chihuahuan desert, to demonstrate the CSA model to the local community, to trial vegetable varieties, and to provide a site where faculty can conduct research or student laboratory exercises. This is the first organic vegetable garden on the NMSU main campus, the first organic vegetable production class, and the first CSA venture in southern New Mexico. The project has grown about 230 varieties of vegetables, herbs, and flowers in the first two years of production, and has grossed at total of $32,000 in revenues from both years on 2/3 of an acre of land. In the first year, 32 members purchased 18.5 full share equivalents, and in 2003, 69 members purchased 39.5 full share equivalents.
Fusarium basal rot (FBR), caused by Fusarium oxysporum f.sp. cepae, is a soilborne fungal disease that affects bulb onions (Allium cepa) worldwide. Winter-sown onion cultivars that are resistant to FBR are lacking. The goal of this project was to screen winter-sown onion germplasm for FBR resistance using a mature-bulb field screening at harvest and after 4 weeks in storage. The project was conducted for 2 years, and in each year, 22 winter-sown onion lines were grown in a field known to produce a high incidence of FBR-infected bulbs. At maturity, the basal plates of 20 randomly selected bulbs were cut transversely and each plate was scored for disease severity on a scale of 1 (no diseased tissue) to 9 (70% or more diseased tissue). Bulbs were stored and scored again at 4 weeks after harvest. Severity and incidence increased in storage for both years. NMSU 99-30, `NuMex Arthur', and `NuMex Jose Fernandez' showed the lowest disease severities and incidences in both years. For fields that produce a high incidence of FBR-infected bulbs, these cultivars could be grown with less loss to FBR at harvest and after storage than more FBR-susceptible cultivars. When developing FBR-resistant cultivars, breeding lines should be evaluated over multiple years and bulbs should be stored for 4 weeks before being screened.
Black mold, caused by the fungus Aspergillus niger, is one of the major postharvest onion (Allium cepa) diseases causing qualitative and quantitative losses. To screen autumn-sown onion cultivars for black mold resistance, 12 cultivars were sown in 2004 and 2005 in Las Cruces, NM. Percent sporulated area, disease severity, and disease incidence were recorded after 2 and 4 weeks of storage. ‘Texas Early White’ exhibited less disease symptoms than other cultivars tested. For all cultivars, disease symptoms in terms of severity and incidence increased as bulbs were stored for longer periods of time.