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Revista mexicana de fitopatología
versión On-line ISSN 2007-8080versión impresa ISSN 0185-3309
Rev. mex. fitopatol vol.35 no.2 Texcoco may. 2017
https://doi.org/10.18781/r.mex.fit.1701-1
Scientific articles
Alternate hosts of Iris yellow spot virus and trips on onion crops in Morelos and Michoacan, Mexico
1Postgrado en Fitosanidad-Fitopatología, Colegio de Postgraduados, Campus Montecillo, km 36.5 Carr. México-Texcoco. Montecillo, Estado de México, C. P. 56230.
2Campo Experimental Zacatepec, INIFAP. km 0.5 Carretera Zacatepec-Galeana. Col. Centro Zacatepec, Morelos, C. P. 62780.
3Departamento de Horticultura, Universidad Autónoma Chapingo, km 38.5 Carr. México-Texcoco. Chapingo, Texcoco, Estado de México, C. P. 56230.
4Universidad Autónoma Chapingo, Centro Regional Morelia (CRUCO), Perif. Paseo de la República 1000, Lomas del Valle, Morelia, Michoacán, C. P. 58170.
5Postgrado en Fitosanidad-Fitopatología, Colegio de Postgraduados, Campus Montecillo, km 36.5 Carr. México-Texcoco. Montecillo, Estado de México, C. P. 56230.
The main goal of this research was to identify alternate hosts of Iris yellow spot virus (IYSV) and to establish the range of hosts of the putative vector(s) in onion (Allium cepa) producing regions of Morelos and Michoacan, Mexico. In 10 localities of both states, onion crops and weeds were sampled in the presence and absence of the crop. Onion plants were analyzed by RT-PCR and the weeds by DAS-ELISA. The weeds were identified at species level, the thrips collected from weeds were established colony for identification by PCR, with specific primers that amplify a segment of the cytochrome oxidase I (COI) gene. A total of 295 weeds grouped in 56 species (23 families) were analyzed and identified. All the weeds were negative for IYSV. Thrips were detected on 75 weeds grouped in 17 species. Thirty-three thrips populations were analyzed (22 from Morelos and 11 from Michoacan). Sequencing indicated identity with Thrips tabaci with a homology greater than 97 %. Weeds Ricinus communis and Acalypha ostryifolia were found with high number of T. tabaci. In onion, the presence of IYSV was confirmed with RT-PCR in the ten field plots sampled. This is the first report of IYSV in Michoacan state.
Key words: IYSV; Tospovirus; Thrips tabaci; COI PCR; Ricinus communis; Acalypha ostryifolia
El objetivo de la investigación fue identificar hospedantes alternos de Iris yellow spot virus (IYSV) y establecer el rango de hospedantes del putativo vector(es) en regiones productoras de cebolla (Allium cepa) de Morelos y Michoacán, México. En 10 localidades de ambos estados se muestrearon cultivos de cebolla y arvenses, en presencia y ausencia del cultivo. Las plantas de cebolla se analizaron por RT-PCR y las arvenses por DAS-ELISA. Las arvenses se identificaron a nivel especie, los trips colectados de las mismas se establecieron colonias para su identificación por PCR, con iniciadores específicos que amplifican un segmento del gen de citocromo oxidasa I (COI). Se analizaron e identificaron 295 arvenses agrupadas en 56 especies (23 familias), todas resultaron negativas para IYSV. Se detectaron trips en 75 arvenses agrupadas en 17 especies. Se analizaron 33 poblaciones de trips (22 de Morelos y 11 de Michoacán). La secuenciación indicó identidad con Thrips tabaci con una homología superior a 97 %. Las arvenses Ricinus communis y Acalypha ostryifolia registraron el mayor número de T. tabaci. En cebolla se confirmó la presencia de IYSV con RT-PCR en las 10 parcelas muestreadas. Este es el primer reporte de la presencia de IYSV en el estado de Michoacán.
Palabras clave: IYSV; Tospovirus; Thrips tabaci; PCR COI; Ricinus communis; Acalypha ostryifolia
Onion is a highly economically important crop in Mexico, and it can be affected by several diseases, including those caused by a virus that cause considerable losses and are difficult to handle. The Iris yellow spot virus (IYSV) of the Tospovirus genus, causal agent of the disease known as white spot in onion, is one of the most important viruses in plants of the genus Allium (Brewster, 2008), due to its wide geographical distribution and range of hosts (Ghotbi et al., 2005; Sampangi and Mohan, 2007; Smith et al., 2011). In Mexico its presence has been reported in the states of Zacatecas (Velásquez-Valle and Reveles-Hernández, 2011) and Morelos in onion plantations (Ramírez-Rojas et al., 2016), and in Guanajuato, in garlic plantations (Pérez-Moreno et al., 2010). The species Thrips tabaci is the main vector of IYSV. On the other hand, alternate hosts of the virus and its vector play a crucial role in the epidemiology of the disease caused by IYSV, particularly if these insects complete their biological cycle in these plants (Hsu et al., 2011). This highlights the importance of performing studies related to the vector, alternate hosts, the virus, and interactions amongst them, in order to provide knowledge to help design better disease management strategies, since this type of interactions could influence the appearance of new types of viruses (Rodríguez et al., 2007). Likewise, the ability of trip vectors to grow on non-cultivated plants and permanently subsist in the field may be epidemiologically significant, particularly in the secondary spreading of the virus (Chatzivassiliou et al., 2007; Syller, 2012). In Morelos, IYSV is important due to its high incidence (100 %) and severity (above 90 %) with which it appears in the production areas of the entity (Ramírez-Rojas et al., 2016). On the other hand, in Michoacan, IYSV has practically gone unnoticed by onion farmers despite its presence in the crop. This contrast in the intensity of the disease in both states could be due to different environment-related factors, the main and alternate hosts (weeds), the virus, vectors, and others. Based on this, the aim of this investigation was to identify alternate Iris yellow spot virus (IYSV) hosts and establish the range of hosts of the putative vector(s) in onion-producing regions of Morelos and Michoacan, Mexico, and generate information that contributes to explain these differences in epidemic intensity in both entities.
Materials and Methods
Collecting and identifying weeds
Weeds were collected from 10 fields used for the planting of onion in the municipalities of Puente de Ixtla (one field), Ayala (two fields), Axochiapan (one field) and Emiliano Zapata (one field) in Morelos, as well as in Vista Hermosa (two fields) and Tanhuato (three fields) in Michoacán. The samples were taken in December 2014 (Morelos), January (Morelos and Michoacan), July (Morelos), October 2015 (Morelos and Michoacán) and January 2016 (Morelos and Michoacan) in the presence and absence of the crop. The collection of samples was based on four criteria: weeds previously reported as IYSV hosts (Sampangi et al., 2007; Weilner and Bedlan 2013; Schwartz et al., 2014), plants with putative symptoms to virosis (mosaics, chlorosis, foliar deformities, and white stains), plants near onion plants with viral symptoms, and the most abundant weeds at the time of collecting samples. Plants in advanced phenological stages were preferred. These were places in plastic bags with moist paper and transported to the laboratory, where they were identified with taxonomical codes (Espinosa and Sarukhán, 1997; Rzedowski et al., 2001; Rzedowski et al., 2004; Vibrans and Tenorio, 2012) up to the level of species.
Detecting IYSV in onion
To confirm the presence of the virus, onion plant samples were taken from the same fields from which weeds were taken. The collection of samples was aimed at plants (five plants) with typical IYSV symptoms, considering the presence of chlorotic or long white wounds with or without a green islet in the center (Gent et al., 2006; Weilner and Bedlan, 2013). RNA was extracted from symptomatic plants using the method by TRIzol® Reagent, following manufacturer instructions (life technologiesTM). Later, the pathogen was identified using the reverse-transcription technique of the polymerase chain reaction (RT-PCR), with specific primers that amplify a strip of 750 pb of the gene, which codifies for the protein of the nucleocapsid of the virus: IYSV-For (TGG YGG AGA TGY RGA TGT GGT) and IYSV-Rev (ATT YTT GGG TTT AGA AGA CTC ACC) (du Toit et al., 2007). The reaction mixture for PCR has a final volume of 25 µL: 2.5 µL of buffer PCR 5X, 1.25 µL of MgCl2 (25 mM), 0.5 µL dNTP’s (10 mM), 0.5 µL for each primer (10 mM), 0.5 µL de Taq polymerase (5U/µL) and 2 µL of cDNA. The conditions for the PCR were an initial denaturalization at 94 °C for 2 min, followed by 40 cycles at 94 °C for 30 s, 51 °C for 30 s and 68 °C for 1 min and a final extension of 68 °C for 7 min. The products of PCR were viewed in a photodocumentor by electrophoresis in a 1 % agarose gel, previously dyed with ethidium bromide. From the products amplified, four samples (two from Michoacán and two from Morelos) were purified using Wizard® SV Gel and PCR Clean-Up System and sent for sequencing to the company Macrogen Inc. in South Korea, and the sequences were compared in the GenBank NCBI (National Center for Biotechnology Information) data bank. Only one IYSV isolate, obtained in Michoacán, was inoculated in plants indicating Nicotiana benthamiana (Kritzman et al., 2001), in order to have a backup of the isolate for later use. To verify the presence of the virus in inoculated plants, RNA was extracted from inoculated tissue and RT-PCR.
Detecting IYSV in weeds
Due to the number of weeds collected, the serological test DAS-ELISA was used, with the specific antibody for IYSV following indications by the manufacturer (Agdia®). Samples of each weed collected, consisting of leaves of the lower, middle, and lower sections, were analyzed in duplicate. For weeds with symptoms, a sample composed of diseased and healthy tissues, was taken. The positive control was used for the virus, and the negative control (healthy onion tissue) was acquired from Agdia®. The samples were measured in an ELISA plate reader at a wavelength of 405 nm (A405). The reaction was considered positive if the absorbance value was three times greater to the average of the negative control (Sutula et al., 1986).
Presence of thrips
Both sides of the weed leaves collected were checked for thrips. In plants in which these insects were found, using a fine-tip brush, adult or inmature insects (depending on each case) were collected to form colonies, and were deposited individually in plastic jars with holes covered in cloth to let air in, and inside them, green beans (Phaseolus vulgaris) for them to feed on. The dehydrated green beans were replaced with fresh ones twice a week. The jars with thrips were kept under laboratory conditions with a photoperiod of 12 h, an average temperature of 24 ±1 °C and a relative humidity of 42 % until an increase of individuals was observed for their identification using PCR.
DNA extraction and PCR in thrips
From the colonies established, total DNA was taken using CTAB from 15 adults, and its quality and concentration was determined using a spectrophotometer. The PCR was carried out with primer that amplify a fragment of 710 pb of the gene cytochrome oxidase I (COI): LCO1490 (5’GGT CAA CAA ATC ATA AAG ATA TTG G3’) and HCO2198 (5’TAA ACT TCA GGG TGA CCA AAA AAT CA3’) (Folmer et al., 1994). The PCR was carried out in a volume of 25 µL consisting of: 1.25 µL of MgCl2 (25 mM), 0.5 µL of dNTP’s (10 mM), 0.5 µL of each primer (10 mM), 0.5 µL of Taq polymerase, 2.5 µL 10X PCR buffer, 4 µL of DNA (10-50 ng/µL) and water. The PCR was carried out under the following conditions: 94 °C for 4 min for the initial denaturalization, followed by 30 cycles at 94 °C for 1 min, 50 °C for 1 min, 72 °C for 1 min, and a final extension at 72 °C for 5 min. The amplified products were viewed in electrophoresis in agarose gel at 1 % containing ethidium bromide. The products of the PCR were purified using Wizard® SV Gel and PCR CleanUp System and were sequenced in both directions. The sequences were analyzed using the program Chromas LITE version 2.1.1, and afterwards, consensus sequences were created from each sample using the program Vector NTI 8 and they were compared with those deposited in the genetic sequences data bank (GenBank).
Results
Collection and identification of weeds
A total of 212 weeds were collected in the state of Morelos (33 weeds in 2014, 150 in 2015 and 29 in 2016) and 83 in Michoacan (67 weeds in 2015 and 16 weeds in 2016), for a total of 56 species belonging to 23 families (Table 1). Out of these, 41 species were identified as belonging to 18 families for Morelos and 26 species from 14 families in Michoacan. Ricinus communis, Acalypha ostryifolia, Parthenium hysterophorus and Tithonia tubiformis were the most abundant in the state of Morelos, whereas Amaranthus hybridus, Parthenium hysterophorus, Sonchus oleraceus and Chenopodium murale were the most frequent in Michoacan.
Table 1. Table 1. Species of weeds gathered and analyzed for IYSV by DAS-ELISA, and presence of thrips in weeds in the onion-producing regions of the states of Morelos and Michoacan, Mexico.
Ay: Ayala, Mor.; Pi: Puente de Ixtla, Mor.; Ez: Emiliano Zapata, Mor. Ax: Axochiapan, Mor.; Xo: Xochitepec, Mor.; Vh: Vista Hermosa, Mich.; Ta: Tanhuato, Mich; I: winter (December 2014 and/or January 2015 and/or January 2016); V: summer (July 2015); O: Autumn (October 2015); Np: Not present.
Detecting IYSV in onion
Using the RT-PCR technique, we identified IYSV in the 10 onion fields; the plants presented well-developed bulbs. The virus was not found in young plants (before the bulb was formed). The presence of the pathogen was confirmed in both the state of Morelos and Michoacan. The symptoms were pale white, elongated wounds, and some plants presented green islets in the middle of them; these symptoms were most frequent in the state of Michoacan (Figure 1). The four samples sequenced (two from Morelos and two from Michoacan) displayed an identity of 99 % with the gene of the nucleocapsid for IYSV. The plants inoculated in N. benthamiana with the isolate from Michoacan presented symptoms of chlorotic stains in inoculated and non-inoculated leaves, as well as necrosis. The presence of IYSV was confirmed in N. benthamiana tissue by RT-PCR, since it was inoculated by the virus (Figure 2).
Figure 1. Symptoms caused by the Iris yellow spot virus in onion leaves: a) initial and advanced wounds collected in the state of Morelos, and b) initial and advanced wounds with the presence of a green islet in the middle, collected in the state of Michoacan.
Detecting IYSV in weeds
An analysis was carried out on 295 plants from both stated in different collection times (212 from Morelos and 83 from Michoacan) and none of them were positive for Iris yellow spot virus for DAS-ELISA, except for the positive control. Only 102 weeds (34 %) showed putative symptoms for virosis such as mosaics, yellowing, foliar deformity, and chlorotic spots; however, no symptoms were caused by IYSV. The greatest number of weeds with symptoms was collected in Morelos. The remaining plants were asymptomatic (Figure 3).
Presence of thrips
Out of the 295 different weeds observed, 75 (25.4 %) presented thrips, out of which 61 came from the state of Morelos and 14 from Michoacan. These insects were observed in 17 species of weeds belonging to the families Euphorbiaceae, Asteraceae, Amaranthaceae, Cruciferae, Convolvulaceae, Malvaceae, and Acanthaceae. The thrips were mostly found in R. communis (30 of 31 plants analyzed); 27 plants were from the state of Morelos and three from Michoacan, followed by Acalypha ostryifolia, Amaranthus hybridus and Parthenium hysterophorus, although in less frequency. On the other hand, out of these 75 weeds, in the state of Morelos, 19 weeds were gathered when the onion plantation was established, and 42 in the absence of the crop. In the case of Michoacan, 11 weeds with thrips were collected in the absence of the crop and only three in its presence (Figure 4). It is worth highlighting that an increase was observed in the thrips populations in fields with onion crops, mainly in advanced phenological stages (formation of the bulb).
DNA extraction and PCR
Seventy-five colonies of thrips from the same number of plants collected in Morelos and Michoacan, were established. Out of the total number of colonies, only 33 increased their populations (25 from Morelos and eight from Michoacan). In all cases, the expected amplicon of 710 pb was obtained, and showed an identity of 97 to 100 % for Thrips tabaci (Table 2).
Table 2. Thrips colonies collected from different weed species and identified by PCR COI in onion-producing regions of the states of Morelos and Michoacan.
Ay: Ayala, Mor.; Pi: Puente de Ixtla, Mor.; Ez: Emiliano Zapata, Mor. Ax: Axochiapan, Mor.; Vh: Vista Hermosa, Mich.; Ta: Tanhuato, Mich; I: winter (January 2016); V: summer (July 2015); O: autumn (October 2015).
Discussion
The Iris yellow spot virus is an agriculturally important virus worldwide due to the damage it causes to species of the genus Allium (Bag et al., 2009). To this day there have been numerous hosts infected naturally by this virus (Cosmi et al., 2003; Sampagni et al., 2007; Evans et al., 2009b; Evans et al., 2009a; Hsu et al., 2011; Smith et al., 2012; Weilner and Bedlan, 2013; Schwartz et al., 2014; Szostek and Schwartz, 2015; Karavina and Gubba, 2017), some of which were found in this study in the municipalities visited. In the state of Michoacan, samples were taken of S. oleraceus, S. nigrum, C. album, L. serriola, Amaranthus sp. and P. oleracea, along with R. communis, Amaranthus sp. and P. oleracea in Morelos, which have been reported as IYSV hosts naturally when analyzed by DAS-ELISA (Schwartz et al., 2014; Karavina and Gubba, 2017). The greatest diversity and frequency of weeds was observed in Morelos, unlike Michoacan, since most of the fields sampled in Michoacan had plastic mulch, controlling the weeds in the fields, along with the farmers weeding and/or spraying herbicides more frequently than in Morelos (data not shown).
Despite the information reported internationally of alternate hosts with the presence of the virus, the range of hosts in Mexico is still unknown. Up to now, the presence of IYSV by DAS-ELISA has only been reported in Amaranthus spp., Bidens odorata, Brassica campestris, Chenopodium spp., Eruca sativa, Malva parviflora, Medicago sativa, Sisimbrio spp. and S. oleraceus in the onion-producing areas of Zacatecas; however, the authors do not specify the number of plants evaluated or positive samples for the virus (Velásquez-Valle et al., 2013). In the case of the state of Morelos Ramírez-Rojas et al. (2016) reported a total incidence and a severity of over 90 % of the virus in onion-producing areas, yet they did not evaluate the presence of the virus in weeds. It is worth pointing out that although our study also confirmed the presence of the virus in the onion crop, both in Morelos and Michoacan during the autumn-winter cycles of 2014 and 2015, the analysis was also carried out in the weeds related to the crop and no samples were positive for IYSV.
One of the possible causes to explain why the virus was not found in the weeds analyzed is that it is a localized virus, i.e., it is not systemic, making it more difficult to detect when performing serological tests such as DAS-ELISA (Gent et al., 2006), and even RT-PCR. This has been shown in the case of onion, where its distribution of the virus in the plant is heterogenous, which can give false negatives, leading to an underestimation of its incidence (Hsu et al., 2010). On the other hand, the symptoms caused by the IYSV do not tend to be consistent and clear, therefore, when taking plant tissue samples for their analysis with RT-PCR and/or DAS-ELISA results may be inconsistent (Krauthausen et al., 2012). This expression of symptoms is influenced by the period of incubation, the phenology of the plant at the moment of infection, weather conditions, the level of stress in the plant, as well as in the cultivar (Bag et al., 2012).Likewise, Ochoa et al. (1996) point out that obtaining negative samples in weeds does not imply that they are not a reservoir for the virus. It is also possible that during the weed collection dates, the viral content was not detectable with ELISA, since the time of inoculation by thrips was unknown. In addition to this, to date there is no knowledge of the times of acquisition and transmission of IYSV by T. tabaci in the weeds (Smith et al., 2011). Using RT-PCR, our study also evaluated the most abundant weeds, along with chili pepper, tomato, and tomatillo plants (data not published) that were planted after the harvest of onion, although none amplified the band for IYSV. We must consider that the tissue chosen for the analysis may not have contained the virus. However, its systemic behavior has been reported in Nicotiana benthamiana, transmitted mechanically (Kritman et al., 2001; Bag et al., 2012). These data correspond with our results in the same host, where it showed systemic symptoms after the IYSV inoculation, chlorotic stains in young leaves (non-inoculated area), and necrosis followed by the death of the plant. So far there are no records of IYSV being transmitted through seeds (Kritzman et al., 2001), although its presence has been observed in onion seedbeds (Velásquez-Valle et al., 2016) in Zacatecas; it is a source of inoculants in these seedbeds and has increased the interest of the study of the ecology of this virus in our entity, since the behavior of the virus in different hosts, vectors, and weeds, can vary.
In regards to thrips, both states showed the greatest presence of insects in weeds in the absence of the crop, and mainly inmature states were observed in the vegetative sections of plants, which indicates that these plants can be thrips reservoirs. In fields with onions, few weeds with thrips were gathered, despite the onion plants having large populations of these insects, mainly in advanced phenological stages of the crop. Szostek and Schwartz (2015) recorded the presence of IYSV vector thrips in the absence of the crop in different weeds. This information can back up our data, since larger thrips populations were observed in the absence of the crop. It has been suggested that in the absence of the crop the thrips move to nearby plants and reproduce in them, whether in the vegetative sections or preferably in the inflorescences (Milne and Walter, 1998). Although several weeds have not been reported to host IYSV, it is known that some of them are thrips hosts, therefore it is important to consider them in the field as part of an epidemiological study for the management of the disease (Smith et al., 2011). The abundance of weeds with thrips in the state of Morelos (13 weed species), compared to the state of Michoacan (7 weed species), can be an important explanation of the problem caused by this disease in this entity, since weeds host thrips with a possible potential of being viral transmitters. The way in which weeds are eliminated in Michoacan may be the main factor behind the low thrips populations in onion plants (data not published), even under the low records of weeds with thrips populations in Michoacan (14 weeds), in comparison with the state of Morelos (61 weeds).
In R. communis and A. ostryifolia alone, the greatest presence of thrips in immature and adult stages was observed, mainly in the state of Morelos. In the case of R. communis, this result differs from a report by Schwartz et al. (2014), who found this species hosts IYSV, but not T. tabaci. Although they are different entities, this weed cannot be discarded as a possible reservoir of the virus and the insect, since our data showed it is the main host for T. tabaci. Despite the virus not being found in R. communis, its study must not be underestimated in these and other weeds, mainly where the insect, the only IYSV vector under field conditions, procreates.
Only T. tabaci has been reported as highly transmitting of the virus (Gent et al., 2004; Diaz-Montano et al., 2011). In Mexico it is found in onion plantations (Velásquez-Valle et al., 2011), but only García- Rodríguez et al. (2014) confirmed the presence of the virus in T. tabaci in garlic plantations. All the thrips collected from the different weeds that managed to increase their population corresponded to T. tabaci; however, if they were carriers of the virus or not was not determined, although these plants are considered important thrips reservoirs, which can be potential in the spreading of IYSV in onion plantations (Jones, 2005). The transovaric transmission of IYSV has not yet been proven in thrips, therefore the individuals of each generation must acquire the virus to be able to transmit it (Weilner and Bedlan, 2013), increasing the interest in the behavior of the ecology of the virus.
As mentioned earlier, many of these vector insects can reproduce successfully in diverse unplanted species. These data indicate that thrips can remain indefinitely in a particular area, even in the absence of onion plants. An adequate control of weeds before the increase of thrips populations could reduce their migration (Chatzivassiliou et al., 2007) or their movement to voluntary onion plants (Ghotbi et al., 2005) which can act as an IYSV reservoir outside the crop’s cycle, increasing the possibility of early infections in the following cycle (Hsu et al., 2011). Our data are backed up by what we mentioned earlier, concerning the handling of weeds in the state of Michoacan in contrast to the state of Morelos, which may explain the prevalence of high thrips populations of Morelos, since they have more alternate hosts in the absence of onion plants, their main host. In the state of Morelos there is not an adequate management of weeds, and in many of them, the presence of T. Tabaci in immature stages was found. We also found large thrips populations in onion plants.
Our data suggest that the difference in the intensity of the disease observed in both states can be due to the diversity and frequency of weeds found in Morelos, with the consequence of higher thrips populations. Also, in the state of Michoacan, a better management of weeds and crops was observed, such as rotation, with the consequential reduction in weeds and thrips, as well as low populations of this insect in the crop. Regardless of our study not determining the presence of the virus in the thrips obtained from the weeds, it is known to be the main vector, and considering the wide range of T. tabaci hosts in these entities, it can be important in the epidemiology of IYSV.
Our work has confirmed the presence of IYSV in the state of Michoacan, where it has not been reported before. Likewise, the presence of T. tabaci in several weeds of both states suggests the importance as hosts of the insect in the absence of the crop, mainly those located on the edges of the fields and the possibility of being IYSV hosts in the future. Taking this into account, having a broad knowledge of these interactions would help obtain better crop management strategies. Likewise, limiting studies to just the species of weeds in which the vector reproduces, or carrying out mechanical transmission tests on these weeds, as well as virus transmission tests on the thrips collected from the weeds, we could obtain concise data to understand the virus epidemic in onion crops in an efficient way.
Conclusions
This study did not find the Iris yellow spot virus in the species of weeds collected in fields with or without onion crop in Morelos and Michoacan; however T. tabaci was identified as the only species in 13 species of weeds collected in Morelos and seven collected in Michoacan as alternate hosts of the IYSV putative vector. These weeds play an important role in the intensity of the disease, since they are T. tabaci reservoirs, and are more frequent in Morelos than in Michoacan.
Acknowledgements
The main author wished to thank CONACyT for the scholarship granted for the completion of his doctoral studies. To the trusteeship Num. 167304 granted for the scientific research and technological development of the Colegio de Postgraduados, as well as the collaboration of M. C. Juan Carlos Delgado Castillo, specialist in weeds of the Plant Health Program, SAGARPA, Guanajuato.
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Received: January 17, 2017; Accepted: March 19, 2017
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