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  • Author or Editor: Rebecca Creamer x
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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.

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Kaolin reflectant treatments have been shown to reduce stress due to the environment, pests, and pathogens in many plants. We tested the effect of kaolin on yield, beet curly top virus (BCTV) incidence, and physiological parameters (measured as hyperspectral reflectance) of field-grown chile pepper (Capsicum annuum L.) in southern New Mexico. Curly top incidence was significantly lower in kaolin-treated chile blocks than untreated blocks. Peppers treated with the kaolin-reflectant showed significantly less water stress and higher photochemical reflectance than untreated plants during active growth periods. Treated plants had significantly higher levels of chlorophyll a and higher reflectance than untreated plants. Yield from treated plants was not significantly different from that from untreated plants. We did not detect any deleterious effects on peppers due to application of kaolin. Kaolin treatments suppressed beet curly top virus on chile and reduced water stress parameters during the hottest months of the growing season, suggesting that it would be useful in New Mexico chile production in years with moderate disease pressure.

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Identification of resistant or tolerant onion (Allium cepa L.) cultivars is crucial for the development of integrated management strategies for Iris yellow spot virus (IYSV). Exclusively vectored by onion thrips (Thrips tabaci), IYSV is a potentially devastating tospovirus of onion that has been confirmed to be present in 15 countries all over the world. In this study, 18 winter-sown onion entries were screened for IYSV symptom expression over two seasons. Over the growing season, straw-colored, necrotic lesions typical of IYSV infection were observed and rated for disease severity. Entries, NMSU 03-52-1, NMSU 04-41, NMSU 04-44-1, and ‘NuMex Jose Fernandez’, exhibited fewer symptoms than many other entries tested. ‘Caballero’, NMSU 04-57-1, NMSU 04-78-1, and ‘Cimarron’ exhibited more symptoms. Disease progression over time was rapid for entries exhibiting more symptoms and slow for entries exhibiting fewer symptoms. Enzyme-linked immunosorbent assay (ELISA) optical densities correlated poorly with the severity of disease symptoms. Trends in the disease progression over time emphasize the importance of rating IYSV symptoms late in the crop's development and to search for delayed disease progression rather than early symptom expression to determine IYSV susceptibility.

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