Vegetables and viruses in Zacatecas : A complex pathosystem

El cultivo de chile para secado, ajo y cebolla son económicamente importantes para el estado de Zacatecas, México. Sin embargo, epidemias virales han aparecido desde la década de 1990; el Campo Experimental Zacatecas, dependiente del INIFAP ha llevado a cabo investigación fragmentada sobre estas enfermedades, por lo que el objetivo del presente ensayo fue el de revisar y compartir la información sobre la interacción entre los cultivos de chile para secado, ajo y cebolla con algunos patógenos virales, vectores y maleza así como apuntar las áreas con falta de conocimiento donde se requiere nueva o mayor información. Se dado especial importancia a la identificación de virus y sus vectores como un primer paso para el establecimiento de una estrategia de manejo integrado: en pocos años la presencia de Curtovirus, Tospovirus, Carlavirus y Potyvirus, entre otros, ha sido reportada infectando esos cultivos en Zacatecas, México.

The vegetable production is the most important agricultural activity from the economic point of view in Zacatecas; however, the production process is affected by a number of fungal pathogens, bacterial, nematologic and viral (Velásquez et al. 2002;Velásquez and Medina, 2004;Velásquez and Medina, 2005;Velásquez et al., 2010a).Since the 1990s there has been an emergence of ha registrado la emergencia de síntomas asociados con enfermedades provocadas por virus en hortalizas; estos brotes epidémicos han sido, al menos parcialmente, objeto de investigación por parte del Campo Experimental Zacatecas-INIFAP.Sin embargo, en éste proceso de investigación poco se ha considerado en forma integral el patosistema formado por varias hortalizas que comparten las áreas de producción, traslapan sus ciclos de cultivo y son afectadas por los mismos patógenos virales y sus vectores.
Además, durante el ciclo primavera verano es posible encontrar superficies variables de otras solanáceas como papa (Solanum tuberosum L.) y tomatillo o tomate de hoja (Physalis spp.) que pudieran compartir algunos patógenos virales como los virus del mosaico del pepino (CMV), mosaico del tabaco symptoms associated with diseases caused by viruses in vegetables; these outbreaks have been, at least partially, under investigation by the INIFAP Experimental Field Zacatecas.However, little research on this process has been considered in a comprehensive manner regarding the pathosystem consisting of several vegetables that share production areas, crop cycle's overlaps and affected by the same viral pathogens and vectors.
In recent years, in Zacatecas, developments were generated primarily in the area of identification of viruses and their vectors; however, this constitute only a fraction of the knowledge required to lead to successful management of these diseases.However, it should be recognized that the identification of pathogens is an essential step in the establishment of an integrated management strategy.Therefore, the aim of this contribution is to review and share information about the interaction between vegetable crops such as chili pepper (Capsicum annuum L.), garlic (Allium sativum L.) and onion (A.cepa L.) with some viral pathogens, vectors and weeds generated in the INIFAP.

The hosts
The seasonality production of these vegetables in the State leads to a first glimpse of the complexity of the pathosystem; chili is a spring-summer crop, although traditional seedlings (open pit with minimal use of seedling production technology) is established since January, mainly transplanted in April, occupying about 39 000 hectares annually (Bravo et al., 2010) and harvested mostly from August but often lasts until October or even November, this represents a growing cycle of approximately eight months and the hosts provide the time necessary for the development of virus epidemics.On the other hand, rainfed in the summer brings the optimal conditions for its development, and also weed that eventually serves as harbor vectors and host of viruses that infect the crop.
Moreover, during the spring-summer cycle is possible to find other solanaceous variable surfaces such as potato (Solanum tuberosum L.) and husk tomato or tomatillo (Physalis spp.) that could share some viral pathogens such as cucumber mosaic virus (CMV), mosaic virus (TMV) and tomato spotted wilt virus (TSWV) (De la Torre et al., 2002) or vectors with chili pepper cultivation as in the case of Circulifer tenellus Baker acting as a vector of the agent of the purple tip potato in the regions of Coahuila and Nuevo Leon (Almeyda et al., 2008), which transmits a Curtovirus to the chili plant or, in another case, thrips such as Frankliniella occidentalis (Pergande), vector of the spotted wilt virus (Johnson et al., 1995).
On the other hand, Zacatecas is the largest producer of garlic in the country with about 2000 acres that remain in the open-field from mid-September to mid-June (Reveles et al., 2009), this period is an even longer growing season, about 10 months, with the epidemiological consequences just described.
The onion production is economically important in Zacatecas; the area occupied by this crop has grown from 776 hectares in 1980 to 4110 hectares in 2010 (Velásquez et al., 2010c;Reveles et al., 2012) and involves establishing open seedbeds for seedling production from October, the crop remains in the field, thanks to several dates of transplanting and cycle time, until September.
Occasionally, Zacatecas producers move onion seedlings (scallions) from other States such as Chihuahua and Morelos which could facilitate the movement of different viral pathogens as happened in Colorado, USA.UU (Gent et al., 2006).Often, due to the low probability of successful commercialization, onion growers abandon the plots allowing increased thrips vectors and inoculum of different viruses.
The presence of both crops, garlic and onion, cut open during autumn-winter and with them a certain amount of weed, could be an example of "green bridge" mentioned by Gent et al. (2006), for some viral pathogens and their vectors that eventually could reach the summer crops, such is the case of chili peppers, with higher insect populations and inoculum level.
Although the occurrence of virus-like diseases in these crops globally and nationally have been fully recognized (Davis, 1995;Murphy and Warren, 2005;Pérez-Moreno et al., 2007), only recently has begun to identify some of the viral pathogens present in vegetables and their vectors in Zacatecas (Velásquez-Valle et al., 2008;Velásquez et al., 2009;Velásquez-Valle et al., 2010b).In here we review the information generated on the pathosystem vegetables (garlic, onion and chili)-virus in Zacatecas, Mexico.
En ciclos de cultivo posteriores se comprobó la dispersión de la enfermedad en los estados de Aguascalientes y Zacatecas donde afectaba los principales tipos de chile para secado (Velásquez-Valle et al., 2006).A la fecha no se cuenta con información acerca de la susceptibilidad bajo condiciones controladas del germoplasma local de chile para secado al agente causal de amarillamiento.Aunque en este periodo se generó información acerca de la incidencia, sintomatología y distribución de la enfermedad no se obtuvo ningún avance en cuanto a la identificación puntual del agente causal.Es hasta el ciclo de cultivo 2007 que se envían muestras de plantas de chile con síntomas de la enfermedad al laboratorio de Fitopatología de la Universidad estatal de Nuevo México, EE.UU, a cargo de la Dra.Rebecca Creamer, donde se identifica por primera ocasión en México a un Curtovirus denominado Beet mild curly top virus (BMCTV) infectando plantas de chile para secado colectadas en Zacatecas.

Chili pepper
Even though, since the beginning of the 1990s the presence of a disease affecting chili plants whose main features include stunting, chlorosis and loss of reproductive structures and, initially, chili producers called "dog pee" was mentioned, in 2003 began to report the presence in the region of advanced lines in Mirasol and Ancho chili peppers originated in Zacatecas, a possible viral disease that, according to the symptoms manifested infecting plants associated with the Beet curly top virus and was called "Chili yellowing" (Velásquez-Valle et al., 2003) (Table 1).
In subsequent crop cycles, we showed the spread of the disease in the States of Aguascalientes and Zacatecas which affected the main types of dried chili (Velásquez-Valle et al., 2006).Up to this date, there is no information about the susceptibility under controlled conditions of local germplasm for dried chili causal agent of yellowing.
Although, during this period we generated information about the incidence, symptomatology and distribution of the disease though it hasn´t got any progress in terms of the timely identification of the causative agent.It is up to the 2007 growing season samples with symptoms of the disease at the Pathology Laboratory of New Mexico State University, USA.In charge by Dr. Rebecca Creamer, which identifies for the first time in Mexico a Curtovirus called Beet mild curly top virus (BMCTV) infecting dried chili pepper plants collected in Zacatecas.
Simultaneously, confirming the presence of its vector, the leafhopper C. tenellus Baker in the chili producing areas for drying in Zacatecas, San Luis Potosí and Aguascalientes (Velásquez-Valle et al., 2008), although the incidence of this insect in Aguascalientes and other northern areas of the country have been mentioned since 1954 (Young and Frazier, 1954), it is not until after this paper that is given the importance  Simultáneamente se confirma la presencia de su vector, la chicharrita C. tenellus Baker en las áreas productoras de chile para secado de Zacatecas, San Luis Potosí y Aguascalientes (Velásquez-Valle et al., 2008); aunque la incidencia de este insecto en Aguascalientes y otras áreas del norte del país ya había sido mencionada desde 1954 (Young y Frazier, 1954), no es hasta después de este reporte que se le concede importancia como vector de ese curtovirus.Para 2009 se menciona la presencia de BMCTV en plantas de frijol Aluvori que mostraban amarillamiento, enanismo y follaje coriaceo y que fueron colectadas en Zacatecas (Velásquez-Valle et al., 2009;Velásquez-Valle et al., 2012b); es necesario comprobar que los aislamientos de BMCTV de frijol son capaces de infectar a chile y viceversa, lo cual expandiría considerablemente este patosistema toda vez que en el estado se cultivan alrededor de 600 000 hectáreas anualmente con esta leguminosa (Rincón et al., 2004).
Aunque se conoce la existencia de diversas cepas del patógeno a nivel mundial (Chen et al., 2010), poco se ha avanzado en la detección de otras razas en las plantaciones de as a vector of that Curtovirus.For 2009, the presence of BMCTV in Aluvori bean plants showing yellowing, stunting and leathery foliage, collected in Zacatecas (Velásquez-Valle et al., 2009;Velásquez-Valle et al., 2012b), it is necessary to probe that BMCTV bean isolates are able to infect chili and vice versa, which greatly expand this pathosystem since in the State are grown about 600 000 hectares annually with this legume (Rincón et al., 2004).
The development of this legume during the spring-summer crop overlaps with garlic and onion and completely with chili.
The control of pests in beans, especially in rainfed conditions is poor so, this legume may act as a refuge for C. tenellus, the role of beans in the vegetable-virus pathosystem remains undefined so far, but may acquire considerable importance in vector breeding and BMCTV during the summer.
It has been reported that, leafhopper populations in California, USA, are able to migrate from cultivated areas in the summer to shrubby areas in the winter (Creamer et al., 1996), in Aguascalientes and Zacatecas, the presence of a pathogen during the winter season was recorded (January-March) in patches of weeds and alfalfa plots located within the growing area for drying chili (Velásquez-Valle et al., 2011a) (Table 2), suggesting then that at least a part of the vector population remains in the chili growing area during the winter.It was confirmed that in Zacatecas some weeds such as jaramao (Eruca sativa Mill) Dog quelite (Chenopodium spp.), and bad mother (Solanum rostratum L.), can harbor the BMCTV during the winter (Velásquez-Valle et al., 2012) although it is necessary to determine the presence of the pathogen in other weeds and in pastoral areas.chile y otros cultivos susceptibles en Zacatecas a pesar de que esta actividad es de crucial importancia en el establecimiento de programas de manejo de la enfermedad.
En las hortalizas cultivadas en Zacatecas se ha reportado (Velásquez et al., 2009) sólo la presencia de dos especies de trips (Frankliniella occidentalis Pergande y Thrips tabaci Although, the existence of different strains of the pathogen is quite aware worldwide (Chen et al., 2010), little progress has been made in the detection of other races in chili pepper plantations and other crops susceptible in Zacatecas, though this activity is of crucial importance in the establishment of programs for disease control.
The symptoms of yellowing observed for dried chili plants is complex and includes the manifestation of symptoms associated with BMCTV and phytoplasma infection, such as the so-called Chinese lanterns and small leaf (Velásquez et al., 2011b) (Velásquez-Valle et al., 2011b), which is common in this global culture (Murphy and Bowen, 2006;Kim et al., 2010;Renteria-Canett et al. 2011); in Zacatecas the presence of some RNA viruses have been detected in weeds such as in quelites (Amaranthus spp.) jaramao (E.sativa) and rodadora (Salsola kali L.) hosts for TMV, CMV, PVY, PepMOV and TEV.
The presence of this group, mixed pathogens (BMCTV, phytoplasmas and viruses RNA) in one chili pepper plant prevents one or more of the symptoms observed that can be assigned to a specific pathogen and therefore the term "chili yellowing "could be describe not only as a disease, but the symptoms observed corresponding to different diseases so it is essential to perform transmission tests of these pathogens in controlled conditions.In 2007, indicating the lack of information about the identity and epidemiological information of the chili yellowing (Velásquez-Valle and Amador-Ramírez, 2007), up to this date found partial information on these issues, but it is still incomplete.
The virus of tomato spotted wilt virus (TSWV: Tomato spotted wilt virus) has been mentioned in Zacatecas (Velásquez et al., 2009) under field and greenhouse conditions infecting chili pepper and tomato plants.
Lindeman), both species can be detected in flowers of various vegetables, legumes, and wild plants common in the north central Mexico (Table 3), but it is necessary not only to determine if there are other species of the pest in the region, but it is required to clarify their role as potential vectors of TSWV.
A recent study (Beltrán et al., 2011) revealed that different types of flowers for drying chili collected in Zacatecas appear to be the predominant species for F. occidentalis with sporadic presence of T. tabaci (Table 4), the first one has been mentioned as the most common vector responsible for the emergence of TSWV and other tospoviruses all over the world (Kikkert et al., 1998).
In Zacatecas there is little information on the specific composition of the populations of thrips in horticultural crops in both summer and winter and is virtually nonexistent on the potential transmission of viruses such as TSWV and other tospoviruses.

Garlic
The most important diseases for garlic and onion crops are due to fungus and white rot (Sclerotium cepivorum Berk.) though, especially in the main varieties of garlic in Zacatecas, symptoms mentioned and the presence of agents viral RNA and onion yellowing dwarf virus (OYDV: Onion yellow dwarf virus), of Leek yellow stripe (LYSV: Leek yellow stripe virus), etch (TEV: Tobacco etch virus) Potyvirus genus; shallot latent (SLV: Shallot latent virus) and Garlic common latent (GarCLV: Garlic common latent virus) belong to the genus Carlavirus (Velásquez et al., 2010a;Velásquez-Valle et al., 2010b).(Table 5).It is important to highlight that it´s required generating even more information about the presence and effect of VTE in garlic plants as it is the first mentioned associated with this crop.
Although the effect of viral infection on agronomic and quality characteristics of garlic have been quantified under the conditions of Guanajuato, Mexico (Pérez-Moreno et al., 2008), there is no information on the impact of these pathogens in the development or yield of garlic varieties grown in Zacatecas.The presence of vectors, especially aphids, of these viruses in garlic is well known globally; however, in Zacatecas there is no detailed information about the species, population dynamics and their importance as vectors of these viruses.Consequently, there is no integrated management scheme of these insects as a pest of economic importance as virus vectors.

Onion
The presence of the Iris yellow spot virus (Iris yellow spot virus: IYSV) belonging to the tospovirus was detected in May, 2010; affecting commercial plots in several municipalities in the south and center of Zacatecas (Velásquez-Valle and Reveles-Hernández, 2011).This virus has been reported worldwide mainly affecting the seed production of onion (Gent et al., 2006).There are reports originated in North Africa, not fully confirmed about the nature of IYSV pathogenicity on potato plants, tomato and pepper (Gent et al., 2006); however, it is important to generate information about the possible behavior of local genetic material for chili for this disease, since it has been confirmed that the main types of regional chili could harbor vector populations (Beltrán et al., 2011).
Transmission tests conducted by Kritzman et al. (2001) showed that only IYSV was transmitted only by T. tabaci, although we must emphasize that two biotypes of F. occidentalis failed the transmission of the pathogen; however, in Zacatecas there are populations of F. occidentalis in crops and weeds throughout the year that can also become vectors of this virus, it is necessary to generate local information on the biotypes of T. tabaci and F. occidentalis and eventual IYSV interaction.
It has been reported that, the infection of this virus on weed species such as Sonchus asper (L.) Hill, Amaranthus retroflexus L. and Portulaca oleracea L. (Gent et al., 2006;Nischwitz et al., 2007) are present on specific or genus level in the growing areas of garlic and onion in Zacatecas; a sampling and weed serological analysis performed around onion plots revealed IYSV also the presence in pigweed plants (Amaranthus spp.) aceitilla (Bidens odorata Cav.), beaded (Brassica campestris L.), pigweed dog (Chenopodium spp.) jaramao (Eruca sativa Mill), mallow (Malva parviflora L. ), alfalfa (Medicago sativa L.), sisimbrio (Sisimbrio spp.) and sow thistle (Sonchus oleraceus L.); however, these results need to be confirmed by RT-PCR to determine subsequently the value of these weeds in the epidemiology of IYSV.
Within the host range of IYSV confirmed, globally, is garlic (Gent et al., 2006), but continuous inspection of commercial plots of this vegetable crop cycles in 2009-2010 and 2010-2011 did not reveal the presence of plants with symptoms of the disease in Zacatecas.However, it is appropriated to la presencia de plantas con síntomas de la enfermedad en Zacatecas.No obstante, es oportuno realizar los estudios necesarios en condiciones controladas que permitan conocer la susceptibilidad o tolerancia de las variedades de ajo utilizadas en la región a este virus.

Cuadro 2. Presencia de Circulifer tenellus Baker en manchones de maleza en Zacatecas y Aguascalientes, México (Velásquez-Valle et al., 2011). Table 2. Presence of Circulifer tenellus Baker in patches of weeds in Zacatecas and Aguascalientes, Mexico (Velásquez-Valle et al., 2011).
. Furthermore, in Zacatecas and the neighboring areas in the States of Aguascalientes and San Luis Potosí has been detected in plants with RNA viruses, chili from different viral families but remarkably Potyvirus, Cucumovirus and Tobamovirus such as the mosaics (TMV: Tobacco mosaic virus), cucumber mosaic (CMV: Cucumber mosaic virus), and potato (PVY: Potato Virus Y), pepper mottle (PepMoV: Pepper mottle virus) and etch (TEV: Tobacco etch virus) in individual infections but more commonly in mixed infections