Evaluating Winter-sown Onion Entries for Iris yellow spot virus Susceptibility

Abstract

<jats:p><jats:italic>Iris yellow spot virus</jats:italic> (IYSV) causes a foliar disease in onion (<jats:italic>Allium cepa</jats:italic> 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.</jats:p>

Improved Tolerance for Onion Thrips and Iris Yellow Spot in Onion Plant Introductions after Two Selection Cycles

“…In New York, plants of 'Colorado 6' and 'NMSU 03-52-1' exhibited the fewest number of thrips, and antibiosis and/or antixenosis were suggested as the cause [29]. Long term breeding objectives can be focused on developing semi-glossy onion germplasm with greenish foliage, for sustainable control of onion thrips.Differences for IYS severity exist among cultivars, however, no cultivar has shown resistance to IYS so far [1,24,26,33,[45][46][47]. In New Mexico, NMSU 03-52-1, NMSU 04-41, NMSU 04-44-1, and 'NuMex Jose Fernandez' exhibited the fewest IYS symptoms among 18 winter-sown entries screened [47].…”

“…Differences for IYS severity exist among cultivars, however, no cultivar has shown resistance to IYS so far [1,24,26,33,[45][46][47]. In New Mexico, NMSU 03-52-1, NMSU 04-41, NMSU 04-44-1, and 'NuMex Jose Fernandez' exhibited the fewest IYS symptoms among 18 winter-sown entries screened [47].…”

“…In New Mexico, NMSU 03-52-1, NMSU 04-41, NMSU 04-44-1, and 'NuMex Jose Fernandez' exhibited the fewest IYS symptoms among 18 winter-sown entries screened [47]. Plants of NMSU 05-33-1 showed delayed IYS symptom expression when 13 winter-sown entries were screened [45]. New Mexico State University released eight onion germplasm lines that were selected from plant introduction accessions for reduced IYS symptom expression [48].…”

Selection Progress for Resistance to Iris Yellow Spot in Onions

“…Increased thrips pressure under hot (>30°C) and dry conditions in several onion-growing regions due to quicker life stages and more number of generations can intensify the spread of IYS due to the limited control of thrips even by the use of various insecticides more frequently (Cramer et al, 2014). In addition, an increase in thrips resistance to various insecticides, such as pyrethroids and organophosphates, reduces grower's chemical options to control onion thrips and IYS (Cramer et al, 2014;Diaz-Montano et al, 2011;Gent et al, 2006;Mohseni-Moghadam et al, 2011).…”

“…Several studies reported differences for thrips number, thrips-feeding injury, and IYS for screened onion germplasm but no single onion cultivar has been found to be resistant or immune to IYS (Boateng et al, 2014;Cramer et al, 2014;du Toit et al, 2004b;Gent et al, 2004Gent et al, , 2006Mohseni-Moghadam et al, 2011;Multani et al, 2009;Shock et al, 2008;Singh, 2013). Creamer et al (2004) reported differences among three cultivars for IYS incidence.…”