Immunological Detection of a GarV-Type Virus in Argentine Garlic Cultivars

Viruses of the genus Allium in the Mediterranean region

Allium species are economically important crops in the Mediterranean basin. Viruses are among the most important pathogens affecting their yield and especially those belonging to the genera Potyvirus, Carlavirus, and Allexivirus. Members of the genus Potyvirus are usually the most abundant and cause most of the damage induced. Nevertheless, coinfections with different viruses are not scarce, especially in garlic, and can have synergistic effects that lead to even greater crop losses. Vegetative propagation of alliums and the transmission of most of their viruses by arthropod vectors have significantly contributed to their wide dissemination in the Mediterranean region and elsewhere in the world. Here, we review the general biological and molecular features, the epidemiology, incidence, and methods of diagnosis of the most widespread allium viruses in the basin. Control measures are proposed depending on the mode of propagation of the various alliums, the epidemiology of their viruses and the cultivation procedures adapted by the Mediterranean farmers. The importance of the production and use of virus-free propagative material in order to combat viral diseases of allium crops is especially highlighted. A final discussion focuses on the main shortages identified in the research area of allium viruses, and proposals are made for putative future developments.

Recombinant expression of Garlic virus C (GARV-C) capsid protein in insect cells and its potential for the production of specific antibodies

Garlic cultivars in Brazil are infected by a complex of viruses and for some virus species, such as the allexivirus, purification of the virions is sometimes cumbersume. To overcome this problem, recombinant expression of viral proteins in heterologous systems is an alternative method for producing antibodies. The capsid gene from Garlic virus C (GarV-C), an Allexivirus, was inserted into the genome of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) generating the recombinant virus vSynGarV-C. The recombinant protein expression was confirmed by SDS-PAGE and western-blot of extracts from recombinant virus infected insect cells, where a protein band of approximately 32KDa was observed only in extracts from recombinant infected cells. This protein corresponded to the predicted size of the capsid protein of the GarV-C. A rabbit polyclonal antibody was raised against this protein, shown to be specific for the GarV-C protein in western-blot and dot-Elisa, however with a low titer.

Yield Loss in Garlic Caused by Leek yellow stripe virus Argentinean Isolate

Garlic plants (Allium sativum) are naturally infected by a complex of viruses in the genera Potyvirus, Carlavirus, and Allexivirus. The yield of virus-free garlic plants (noninoculated control) was compared with that of plants infected with an Argentinean isolate of Leek yellow stripe virus (LYSV; L treatment) and garlic plants infected with the virus complex (VC). Evaluations were conducted in the field and in anti-aphid cages during two crop cycles after planting three sizes of cloves (categories). The percent plant emergence in the noninoculated control and in the L treatments (between 80 and 100%) did not differ statistically, but the percent emergence for these two treatments was double that for the VC treatment (25 to 62%). Plant height and leaf number in the L treatment were lower than in the noninoculated control during the first evaluation (year 1), but they did not differ during the second evaluation (year 2). However, both treatments produced taller plants with more leaves than those of VC in both years. The L treatment decreased bulb weight up to 28% and perimeter up to 9% when compared with those in the noninoculated control maintained in the anti-aphid cages until the end of the experiment. However, differences between these treatments were higher in the field experiments where plants were exposed to infection by other viruses (up to 36% in bulb weight and 13% in perimeter). Bulbs of the VC-infected plant treatment were reduced up to 74% in weight and 37% in perimeter. In field evaluations, a high percentage of plants were infected with Onion yellow dwarf virus (58 to 100%), whereas fewer were infected with LYSV (15 to 68%). Garlic virus A infection was high in plants previously infected with LYSV (96 and 97%), but lower in the noninoculated control (12 and 68%). These results show the high impact of the virus complex on garlic yield and the effect of LYSV as a component of the garlic virus complex.

Changes in the Concentration of an Allexivirus During the Crop Cycle of Two Garlic Cultivars

Garlic can be infected by a number of viruses, including allexiviruses. The coat protein sequence of an Allexivirus was detected in Argentina and deposited in the EMBL database as Garlic mite-borne filamentous virus (accession number X98991); it has high homology with Garlic virus A (GarV-A). For reliable virus detection, plants should be sampled when virus titer is high to reduce the risk of identifying infected plants as healthy. The objective of this study was to describe fluctuations in the concentration of this Argentine isolate of GarV-A in two garlic cultivars, Morado-INTA and Nieve-INTA, throughout the crop cycle using the double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA). Over a 2-year period, for both cultivars, virus concentration was assessed in samples from the tips section of the youngest leaves of GarV-A-infected plants, and from basal sections of both dormant and devernalized cloves of stored bulbs of Morado-INTA. The concentration of GarV-A varied during the crop cycle, but peaked at the beginning and again at the end of the crop cycle. Virus concentration was slightly higher in devernalized cloves compared with dormant cloves of Morado-INTA. No correlation between virus concentration and mean air temperature was observed. The results of this study recommend sampling times at the beginning of the crop cycle at 64 to 81 days after planting, and towards the end of the crop cycle to evaluate for the presence of GarV-A by DAS-ELISA.

Effect of Two Allexivirus Isolates on Garlic Yield

Garlic (Allium sativum) is infected by numerous viruses forming a viral-complex, which is widely distributed in the garlic production regions of Argentina. This work is the first report of the effect of two Allexivirus isolates, Garlic virus A (GarV-A) and Garlic virus C (GarV-C), on garlic yield. Garlic cvs. Morado-INTA and Blanco-IFFIVE were used in the experiments, and four treatments were evaluated: plants inoculated with GarV-A only, GarV-C only, virus-free plants (negative control), and plants infected with the virus-complex. Assays were performed in anti-aphid cages and in the field during 2002 and 2003. GarV-A caused significant reductions in bulb weight (14 to 32%) and diameter (6 to 11%) compared with the negative control in the two cultivars under both assay conditions. GarV-C caused less damage than GarV-A (15% in weight and 5% in diameter) with respect to the negative control in cv. Blanco-IFFIVE, and did not produce significant yield losses in cv. Morado-INTA in either year or under either assay condition.

Yield Losses Associated with Virus-Infected Garlic Plants During Five Successive Years

Virus-free garlic plants, when planted in the field, are quickly infected by viruses, but it is not known to what extent this affects the yield over successive crop cycles. The yield loss curve was studied for these plants during 5 years of tests in the field. Highly significant differences were detected in the weight and perimeter of bulbs in relation to the years of exposure to virus infection. An increase was observed in yield compared with chronically diseased plants of between 66 and 216% in weight and 13 and 37% in perimeter of bulbs in the first crop cycle and 49% in weight and 16% in perimeter in the fifth year. These results showed a gradual loss in yield until the third year, and subsequently the production values remained steady for the fourth and fifth years of testing but were still higher than those reached by chronically diseased plants.

Variations of Leek yellow stripe virus Concentration in Garlic and Its Incidence in Argentina

The purpose of this work was to determine variations in titer of Leek yellow stripe virus (LYSV) throughout the crop cycle and bulb storage, and to evaluate the incidence of infected plants in the main garlic-production regions of Argentina. One hundred plants with LYSV from each of five cultivars were analyzed by double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) in six different vegetative stages in White- and Red-type garlic cultivars and seven stages in cv. Rosado Paraguayo, throughout the year. In two White-type garlic cultivars, LYSV showed peaks of viral concentration in May, at the beginning of the crop cycle, and in November, just before harvest. In two Red-type garlic selections, an increase was detected in November (period of bulbing). The highest virus titers for these four garlic cultivars were detected in devernalized clove. In Rosado Paraguayo, the peak virus concentration occurred in September prior to harvesting. In a survey at 14 different localities in Argentina, 3,066 random samples were analyzed. LYSV was found in 80 to 98% of the plants from all regions, except in Santa Cruz, where 34% of plants were infected. The importance of this study is that it allows us to recommend the most suitable moment of the year to make the analysis with DAS-ELISA.

Garlic viral complex: identification of Potyviruses and Carlavirus in Central Brazil

Garlic viruses often occur in complex infections in nature. In this study, a garlic virus complex, collected in fields in Brazil, was purified. RT-PCR was performed using specific primers designed from the consensus regions of the coat protein genes of Onion yellow dwarf virus, a garlic strain (OYDV-G) and Leek yellow stripe virus (LYSV). cDNA of Garlic common latent virus (GCLV) was synthesized using oligo-dT and random primers. By these procedures individual garlic virus genomes were isolated and sequenced. The nucleotide sequence analysis associated with serological data reveals the presence of two Potyvirus OYDV-G and LYSV, and GCLV, a Carlavirus, simultaneously infecting garlic plants. Deduced amino acid sequences of the Brazilian isolates were compared with related viruses reported in different geographical regions of the world. The analysis showed closed relations considering the Brazilian isolates of OYDV-G and GCLV, and large divergence considering LYSV isolate. The detection of these virus species was confirmed by specific reactions observed when coat protein genes of the Brazilian isolates were used as probes in dot-blot and Southern blot hybridization assays. In field natural viral re-infection of virus-free garlic was evaluated.

Mixed Virus Infections of Garlic Determined by a Multivalent Polyclonal Antiserum and Virus Effects on Disease Symptoms

A garlic virus-specific polyclonal antiserum was developed against a mixture of flexuous rodshaped virus particles isolated from mosaic-diseased garlic plants (15). This antiserum was used in Western blot analysis against tissues from mosaic-diseased garlic plants, at least seven viral coat protein (CP) bands (from 38 to 32 kDa) were identified. Using Western blot analysis with Potyvirus-specific antibodies and reverse transcription-polymerase chain reaction (RT-PCR) analysis, we concluded that three of the seven bands corresponded to CPs of Leek yellow stripe virus (LYSV) (38 kDa) and two different Onion yellow dwarf virus (OYDV) strains (35.5 or 34 kDa). The 35 kDa band corresponded to the CP of GV1-Carlavirus, and the other four bands, 36, 35 (not GV1), 33, and 32 kDa, were identified as the CPs of four mite-borne viruses, based on RT-PCR analysis. Based on the molecular weights of CP, mixed infections of Potyvirus, Carlavirus, and mite-borne viruses were characterized. LYSV causes apparent disease symptoms in garlic plants, however, little reduction in bulb weights. Conversely, garlic plants infected with three different mite-borne viruses expressed weak symptoms and yield losses. Mixed infections of OYDV, the mite-borne viruses, and LYSV caused severe disease symptoms and considerable reduction of bulb weights.

Effects of Onion Yellow Dwarf and Leek Yellow Stripe Viruses on Symptomatology and Yield Loss of Three French Garlic Cultivars

This study was conducted to determine the effect of two potyviruses, onion yellow dwarf virus (OYDV) and leek yellow stripe virus (LYSV), on the symptoms, growth, and potential yield loss of garlic (Allium sativum). For 2 consecutive years, the impact on leaf length, pseudostem diameter, and bulb weight was evaluated after mechanical inoculation of cultivars Messidrome, Germidour, and Printanor, the three main garlic cultivars grown in France. The reduction in bulb weight due to OYDV ranged from 39% for Germidour to about 60% for the two other cultivars. For LYSV, the reduction in bulb weight was less on Messidrome (17%) and Germidour (26%) than on Printanor (54%). Coinfection with both viruses further reduced growth and bulb weight. When cloves originating from bulbs infected by each virus alone or a mixture of both viruses were planted, results indicated that such chronic infection induced further yield reduction. An assay designed to evaluate the role of LYSV inoculation date on yield revealed that yield losses were the lowest for late-season infections. However, yield loss was greater than 30% when the inoculation was performed at the end of April, the time when natural contamination generally occurs in southern France. A comparison of the impact of mixed infections of OYDV and LYSV from different origins suggested that the isolates did not differ significantly in their effects on yield loss.