In Mexico the main onion producing states (Allium cepa L.) are Baja California, Guanajuato, Michoacan, Chihuahua and Zacatecas; 44 398 ha with onion during the agricultural season of 2012 in which a production of 1 238 000 tonnes was reached; for the same crop cycle, 4 142 ha were planted with that vegetable in the state of Zacatecas, where 162 204 tonnes were obtained, which represented an average yield of 39.1 t ha^-1 (^{Osuna and Ramírez, 2013}; ^{Reveles-Hernández et al., 2014}).

The onion culture cycle in Zacatecas begins with the establishment of seedlings between September and November for varieties or short-day hybrids that will be transplanted between december and march while seedlings for intermediate day varieties or hybrids are established between December and February to be transplanted from March to May (^{Reveles-Hernández et al., 2014}). The production of onion seedlings in this region is performed under open sky conditions with little protection against thrips infestations, which may facilitate infection by viruses such as the yellow spot of the iris yellow spot virus (IYSV) that is disseminated efficiently through this insect (^{Gent et al., 2004}).

The transmission of IYSV is performed by trips Thrips tabaci Lind. and Frankliniella fusca (Hind.) (^{Srinivasan et al., 2012}); in Zacatecas the occurrence of the first one has been reported whereas the presence of the second has been mentioned in Mexico (^{Johansen, 2002}; ^{Velásquez et al., 2009}). The presence of IYSV in commercial onion plots in Zacatecas was detected by serological means in the 2010 crop cycle (^{Velásquez-Valle and Reveles-Hernández, 2011}), however, the importance of onion seedlings with low employment of production technology as potential sources of viral inoculum in the state of Zacatecas has not been investigated. Consequently, the objective of the present work was to detect the presence of IYSV in onion seedlings in Zacatecas, Mexico and to determine the correlation between the absorbance readings and the detection frequency of IYSV.

The seedlings randomly selected in the central region of Zacatecas were sampled; a variable number of seedlings (between 10 and 30) were collected randomly in each of the varieties or hybrids present in each seedling. In some seedlings it was possible to obtain the name of the varieties or hybrids planted; otherwise, only the type of onion (white, purple or industrial) was recorded. The seedlings collected were transferred to the Phytopathology Laboratory of the Zacatecas Experimental Field (INIFAP). Each seedling was carefully examined for lesions caused by IYSV (straw-colored diamond-shaped spots, occasionally with green “islands” within the injury). A portion of foliar tissue from each sampled seedling was analyzed for the presence of IYSV following the protocol mentioned by ^{Velásquez-Valle and Reveles-Hernández (2011)} and consisted in using the enzymatic immunoabsorption technique called the double antibody sandwich (DAS) to be developed in two days, using the antibody to the IYSV coat protein conjugated to the alkaline phosphatase enzyme (Agdia Inc.).

As positive and negative controls, the method used was the positive and negative controls (extract of onion leaves) method (Agdia Inc.). The absorbance reading was performed on the BioTek ELx 800 spectrophotometer at a wavelength of 405 nm. The optical density values that exceeded three times the mean value of negative controls were considered positive for IYSV. The mean and standard deviation of the absorbance readings obtained from the IYSV positive samples in each nursery were calculated to compare that variable between nurseries.

Between january and march 2015, 30 onion seedlings located in the municipalities of Calera de V.R. and Villa de Cos, Zacatecas (6.7 and 93.3% respectively). Although no IYSV lesions were found in any of the seedlings collected, the DAS-ELISA results indicated that the pathogen was present in 90% of the seedlings visited, which may be partially caused by the seedling production system which does not prevent the infestation of the seedlings by T. tabaci, a vector of IYSV present in this area. On the other hand, it has been suggested that the incubation period of IYSV (from infection to expression of symptoms) is 30 days (^{Hsu et al., 2010}), which would prevent the recording of symptoms during sampling.

In addition, the practice of repeatedly cutting the foliage of the seedlings with the aim of strengthening their subterranean development could have eliminated the infected portions by preventing the symptoms from being recorded. The presence of IYSV in asymptomatic seedlings could hamper their management in onion seedlings since, generally, the producers opt to fight the vector until severe damage is observed due to their feeding habits, that is to say when viral transmission and dissemination could have reached a considerable number of seedlings. The three seedlings without plants serologically positive to IYSV had varieties or hybrids whose bulb was white; it is likely that a low natural population of viruliferous thrips has been responsible for the low viral concentration in these seedlings.

The Sierra Blanca variety was obtained in relatively close seedlings between them in the community of Sierra Vieja, belonging to the municipality of Villa de Cos, but isolated from the rest of the sampled seedlings, thus presenting a similar viral detection frequency, 73.3 and 80%. The Mata Hari variety (purple onion) was sampled in five seedlings in Villa de Cos and presented a wide range of pathogen incidence, from 13.3 to 73%. These ranges of variation in the detection of the virus in the same variety or hybrid could reflect, in turn, the variability in the management practices of the vector thrips of the IYSV or possible differences in the ability of virus transmission by the vectors caused by the availability of the pathogen in weeds or other hosts (Table 1).

Table 1 Frequency of detection of IYSV, variety/hybrid or onion type, absorbance reading and location of onion seedlings sampled between January and March 2015 in Zacatecas, Mexico.  

The frequency of detection of the virus varied between 8.3 and 100%, 60 and 80%, and 23 and 96.7% in seedlings belonging to the types of white, industrial (orange or brown) and purple onion, respectively (Table 1). The leaf color of onion plants has been associated with resistance to thrip infestation (^{Díaz-Montano et al., 2012}), however, during the nursery stage the color of the foliage is not clearly different between the types of onion. On the other hand, producers of onion seedlings cut the foliage of seedlings in order to produce more roots; consequently, foliage color is often closer to pale green that might be more attractive to vectors.

According to ^{Mohseni-Moghadam et al. (2011)} found a strong correlation between the visual readings of IYSV infection severity in plants of 13 onion accessions and the absorbance readings obtained by means of ELISA, however, in the present work, where the variables to be related were the incidence and absorbance readings, the majority of the latter provided broad values of standard deviation that prevented a clear separation between seedlings with different incidence of IYSV, however, 20, 53.3 and 16.7% of the sampled seedlings had readings of absorbance greater than 1.31, between 0.5 and 1.3, and between 0.1 and 0.49 respectively. The mean incidence of IYSV was 55.1, 48.1 and 67.7% for those where the absorbance reading was greater than 1.31, between 0.5 and 1.3 and between 0.1 and 0.49, respectively. Coincidentally, the lowest mean absorbance value (0.268 ± 0.122) also obtained the lowest incidence value (8.3%) among IYSV-positive seedlings. In general, high values of absorbance reading did not always correspond to higher values of virus detection frequency; on the contrary, it was observed that with the samples obtained in seedlings 27 to 30, where values of viral detection ranging from 73.3 to 96.7% were found at relatively low values of the absorbance reading (0.4 ± 0.054 a 0.47 ± 0.104); a similar phenomenon is reported by ^{Resende et al. (2000)} in tomato plants showing slight symptoms of TSWV infection and high absorbance values, although susceptible cultivars showed TSWV symptoms to be related to viral multiplication (Table 1).