To Tomato Spotted Wilt Virus Research Articles

Row Pattern and Seeding Rate Effects on Agronomic, Disease, and Economic Factors in Large-Seeded Runner Peanut

ABSTRACT Recent peanut cultivar releases are trending to a larger seed size, but have great resistance to tomato spotted wilt virus (TSWV). Larger-seeded cultivars cost more to plant than smaller at an equivalent population. Reduced seeding rates could save growers on seed costs and impede the spread of southern stem rot, but can reduce plant stands which can lower yields and increase TSWV incidence. Therefore, the objectives of this experiment were to compare seven peanut cultivars (Georgia Green, Georgia-06G, AT 3085RO, Florida-07, Tifguard, AP-3, and Georgia-03L) in single and twin row patterns at three seeding rates (17, 20, and 23 seed/m) on a sandy loam soil at Plains, GA for disease incidence, agronomic, and economic performance. Measured variables included yield and grade, plant height and stand, TSWV and southern stem rot incidence, and adjusted net revenue in 2008 and 2009. Twin rows outperformed single rows whenever differences occurred. The only factors consistently affected by reducing seeding rate were plant height and stand, both decreased at the lowest seeding rate. There was a trend toward lower yields (approximately 6% reduction) at the 17 seed/m rate in twin row pattern, although net returns were not diminished compared to the higher seeding rates since lower seed costs offset yield reductions. The cultivars Georgia-06G and Florida-07 had the highest yield and adjusted net revenue among the seven cultivars in both years. Tifguard and Georgia Green had lowest overall yields and would not be preferred cultivars in sandy loam soils. This study demonstrates that twin rows have higher yield, plant stands, and net revenue, plus reduced TSWV incidence than single row pattern, and a reduction in seeding rate to 17 seed/m can be made without serious risk of lost revenue. However, benefits of reducing seeding rate in twin rows were not as pronounced as they were for single rows, and exhibited a greater potential for lower yield. A grower planting in single rows would likely have the most to gain from planting fewer seed, especially under heavy southern stem rot pressure, but planting in twin rows would still be a preferred option over single rows.

Influence of Paraquat on Yield and Tomato Spotted Wilt Virus for Georgia-02C and Georgia-03L Peanut

Abstract Paraquat is a common herbicide used in peanut production; however, visible injury and reduced yield have been observed in some instances. Most research regarding paraquat injury on peanut has taken place on cultivars that are no longer available and were more susceptible to tomato spotted wilt virus (TSWV) than current cultivars. Field experiments were conducted over three growing seasons to determine the effect of paraquat on yield and TSWV incidence in two moderately TSWV-resistant cultivars (Georgia-02C and Georgia-03L). Paraquat and paraquat plus bentazon were evaluated against a non-treated control at four application timings [7, 14, 21, and 28 d after ground cracking (DAGC)]. There were no yield differences among herbicide treatments or application timings for Georgia-02C peanut, but there was a treatment interaction with Georgia-03L for yield. The majority of interaction comparisons showed no yield differences, but the non-treated control had higher yields than the herbicide treatments when significance did occur. Yields were similar for the 7 DAGC timing in all comparisons. In all instances when differences occurred for both cultivars, TSWV was higher in non-treated plots than where herbicides were applied. This data supports the use of paraquat in Georgia-02C and Georgia-03L peanut since there is minimal chance of yield reduction and may also reduce TSWV incidence; however, additional studies are required.

‘Tribeca’ Hybrid Tomato; Fla. 8124C and Fla. 8249 Breeding Lines

‘Tribeca’ is a fresh market hybrid tomato (Solanum lycopersicum L.) that has resistance to Tomato spotted wilt virus (TSWV) and heat-tolerant fruit setting (HT). ‘Tribeca’ has performed well on the peninsula of Florida where its ability to set fruit under high temperatures makes it an attractive cultivar choice even when TSWV is not a major production issue. It appears well adapted to the southeastern United States where TSWV has been problematic, and it has done well in trials in Turkey and some other eastern European countries. Inbred Fla. 8124C is the TSWV-resistant parent and has moderate HT. Inbred Fla. 8249, the other parent, has good H-7 fruit setting ability.

Tomato breeding for resistance to Tomato spotted wilt virus (TSWV): an overview of conventional and molecular approaches

The disease caused by Tomato spotted wilt virus (TSWV) belongs to the most destructive diseases of tomato all over the world. Therefore, tomato has been subjected to many breeding efforts, including the incorporation of resistance to the virus. Recently emerged approaches, ideas and technologies could affect the future direction of the virus resistance breeding. In particular molecular techniques have provided opportunities in the form of linked molecular markers to speed up and simplify the selection of host resistance genes. Transformation of an inbred tomato line with the TSWV nucleoprotein gene cassette resulted in high levels of resistance to TSWV that have been retained in the hybrids derived from the parental tomato line. These and other techniques offer great opportunities for improving the virus resistance and, therefore, it is time to reconsider the future direction of resistance breeding in tomato. The effort has been made to review available sources of resistance, conventional breeding methods, marker-assisted selection, pathogen-derived resistance and transgenic resistance approaches in this paper.

Response of New Field-Resistant Peanut Cultivars to Twin-Row Pattern or In-Furrow Applications of Phorate for Management of Spotted Wilt.

Field experiments were conducted at Marianna, FL in 2006 and Tifton, GA in 2006 and 2007 to compare new peanut (Arachis hypogaea) cultivars to the moderately resistant cv. Georgia Green and the highly resistant cv. AP-3 for field resistance to Tomato spotted wilt virus (TSWV), genus Tospovirus, and to determine the effects of in-furrow application of phorate insecticide and use of twin-row versus single-row patterns on incidence of spotted wilt in these cultivars. Cvs. Georgia Green, AP-3, Georgia-03L, Georgia-01R, Florida-07, McCloud, and York were evaluated in all five experiments, and Tifguard was added in experiments at Tifton. All cultivars except McCloud had lower incidence of spotted wilt than Georgia Green in all experiments. McCloud was intermediate in resistance to TSWV and had lower incidence of spotted wilt than Georgia Green in four of five experiments. Use of the twin-row pattern also reduced incidence of spotted wilt in McCloud in both years. On Georgia Green, phorate reduced incidence of spotted wilt in 2007 and twin-row pattern reduced incidence in both years. Phorate had no effect on spotted wilt in AP-3, Georgia-03L, McCloud, Georgia-01R, or Tifguard in either year. Twin-row pattern reduced either final incidence or area under the disease progress curve in all cultivars in at least 1 year of the study. All of these new cultivars should reduce the risk of losses to spotted wilt compared with Georgia Green. In highly resistant cultivars, especially AP-3, York, and Tifguard, use of phorate insecticide or twin-row pattern may not be necessary, and may not provide noticeable benefit in reduction of spotted wilt or increased yield.

Registration of ‘Georgia‐07W’ Peanut

‘Georgia‐07W’ (Reg. No. CV‐102, PI 652441) is a high‐yielding, large‐seeded, runner‐type peanut (Arachis hypogaea L. subsp. hypogaea var. hypogaea) cultivar with resistance to tomato spotted wilt virus (TSWV) and white mold (Sclerotium rolfsii Sacc.). It was developed at the University of Georgia, Coastal Plain Experiment Stations at Tifton, GA, and was released by the Georgia Agricultural Experiment Stations in 2007. Georgia‐07W originated from a cross made between ‘C‐99R’ × ‘Georgia Green’. Pedigree selection was practiced within the early‐segregating generations. Performance testing began in the F4:6 generation with the advanced pure‐line selection, GA 011514, the experimental designation of Georgia‐07W. Averaged over 14 tests conducted at three locations in Georgia during three years (2004–2006), Georgia‐07W had significantly less midseason TSWV incidence and late‐season total disease (TSWV and soilborne disease) incidence, and significantly higher yield, grade, and dollar value return per hectare compared with the two parental check cultivars, C‐99R and Georgia Green, respectively. Georgia‐07W has also shown significantly less white mold and significantly higher pod yield compared with the resistant female parent, C‐99R, when grown in soil with high disease pressure from S. rolfsii

Evidence That the Nonstructural Protein ofTomato spotted wilt virusIs the Avirulence Determinant in the Interaction with Resistant Pepper Carrying theTswGene

All known pepper cultivars resistant to Tomato spotted wilt virus (TSWV) possess a single dominant resistance gene, Tsw. Recently, naturally occurring resistance-breaking (RB) TSWV strains have been identified, causing major concerns. We used a collection of such strains to identify the specific genetic determinant that allows the virus to overcome the Tsw gene in Capsicum spp. A reverse genetic approach is still not feasible for TSWV; therefore, we analyzed reassortants between wild-type (WT) and RB strains. Our results confirmed that the S RNA, which encodes both the nucleocapsid protein (N) and a nonstructural protein (NSs), carries the genetic determinant responsible for Tsw resistance breakdown. We then used full-length S RNA segments or the proteins they encode to compare the sequences of WT and related RB strains, and obtained indirect evidence that the NSs protein is the avirulence factor in question. Transient expression of NSs protein from WT and RB strains showed that they both can equally suppress post-transcriptional gene silencing (PTGS). Moreover, biological characterization of two RB strains carrying deletions in the NSs protein showed that NSs is important in maintaining TSWV infection in newly emerging leaves over time, preventing recovery. Analysis of another RB strain phenotype allowed us to conclude that local necrotic response is not sufficient for resistance in Capsicum spp. carrying the Tsw gene.

Reaction of Peanut Cultivars to Tomato Spotted Wilt Virus (TSWV) Under Field Conditions and Their Response to Mechanical Inoculation by TSWV Under Greenhouse Conditions

Spotted wilt virus disease caused by Tomato spotted wilt tospovirus (TSWV) has become a limiting factor in peanut (Arachis hypogaea L.) production in the Southwest region of the U.S. Selected peanut cultivars (Okrun, Southwest Runner, Tamrun-98, Tamrun-96, Georgia Green, Tamrun OL-02, Tamrun OL-01, and Georgia Hi Oleic) were evaluated for reaction to TSWV in field plots in 2001, 2002, and 2003. Field plots were located near Pearsall, TX, where TSWV is endemic. In 2001, at 72 days post planting (DPP), incidence of TSWV was significantly (P , 0.05) higher in Southwest runner, Okrun, and Tamrun98 than the other cultivars except Tamrun OL-02 in the test. In 2002, disease incidence of TSWV was low and thus only one reading was taken at 135 DPP where the cultivars Georgia Green and Georgia Hi Oleic exhibited the least incidence of spotted wilt. In 2003, at 58 DPP, Georgia Green, and Georgia Hi Oleic showed the lowest incidence of spotted wilt compared with other cultivars. The response of the eight peanut cultivars to TSWV following mechanical inoculation with TSWV at three plant ages was evaluated under greenhouse conditions. Disease severity index (DSI) and plant growth parameters such as root length, root volume, plant height, and fresh weight were determined. ELISA values (as indicative of the relative presence of TSWV), and relative water content were also determined to show the impact of TSWV infections in inoculated plants. Reductions in most plant growth parameters were observed when plants were inoculated with TSWV at five days post-planting, than twelve and nineteen days post-planting inoculation. Positive values of correlation coefficient (CC) between DSI and root volume or root length in the cultivars Georgia Hi Oleic, Georgia Green, Tamrun-96 and Tamrun OL-01 indicate resistance or tolerance to mechanical inoculation with TSWV. Negative values of CC between DSI and root length or root volume in the cultivars Tamrun-98, Tamrun OL-02, Southwest Runner, and Okrun provide evidence for their susceptibility to TSWV. Comparison of results obtained from the field with those from greenhouse evaluations suggest that Georgia Hi Oleic, Georgia Green, Tamrun-96 and Tamrun OL-01 cultivars are useful as potential tools for management of TSWV in peanut for the Southwestern United States.

Evaluation of peanut genotypes for resistance to Tomato spotted wilt virus by mechanical and thrips inoculation

The objective of this work was to evaluate the reactions of three peanut breeding lines (IC-10, IC-34, and ICGV 86388) to Tomato spotted wilt virus (TSWV) by mechanical and thrips inoculation, under greenhouse conditions, and compare them to the reactions of cultivars SunOleic, Georgia Green, and the breeding line C11-2-39. TSWV infection by mechanical inoculation was visually assessed using an index ranging from 0 (no symptoms) to 4 (apical death). Enzyme-linked immunosorbent assay was used to confirm TSWV infection from both mechanical and thrips inoculations. IC-10, IC-34, ICGV 86388, and C11-2-39 were more resistant than the cultivars SunOleic and Georgia Green based on mechanical inoculation. Upon thrips inoculation only IC-34 and ICGV-86388 were infected by TSWV, as demonstrated by reverse transcription polymerase chain reaction (RT-PCR), although no symptoms of infection were observed. The peanut breeding lines IC-10, IC-34, and ICGV 86388 show higher level of resistance to TSWV than cultivar Georgia Green considered a standard for TSWV resistance.

First report in Italy of a resistance‐breaking strain of Tomato spotted wilt virus infecting tomato cultivars carrying the Sw5 resistance gene

In 2004, leaf samples of a processing tomato variety carrying the Sw5 resistance gene to Tomato spotted wilt virus (TSWV) were collected from field-grown plants in Mesagne (BR), Apulia, southern Italy. Leaf extracts were tested by lateral flow and/or ELISA (Roggero et al., 2002) for TSWV, Impatiens necrotic spot virus (INSV), Cucumber mosaic virus (CMV), Tobacco mosaic virus (TMV) and Potato virus Y (PVY). Leaf dips were also observed with a transmission electron microscope, and two of these samples were inoculated mechanically on to a set of test plants. Only TSWV was detected in all field samples tested. One of the TSWV field isolates, T992, was investigated for the ability to overcome the resistance gene Sw5. T992 was mechanically inoculated onto 20 plants of each F1 hybrid tomato cultivar carrying the Sw5 gene (cvs Donald, York, Rovente, Valiente, Hermes, UGX 9233, Diaz, ISI 19343, Es 5302, Scipio and Herdon); cv. Marmande was used as a susceptible control. Another set of F1 hybrids was mechanically inoculated with strain p105, a wild-type strain of TSWV (Roggero et al., 2002). Tomato plantlets were inoculated at the four- to five-true-leaf stage and systemic infection was tested 20 days postinoculation using ELISA. All hybrids carrying the Sw5 gene were uninfected systemically by strain p105, with the exception of four plants of F1 UGX 9233. In contrast, T992 systemically infected all F1 hybrids tested. Marmande was infected systemically by both p105 and T992. These results showed that strain T992 can overcome Sw5 gene resistance. Portions of the S and M genome of T992 were cloned and sequenced, and the data deposited in GenBank (Accession Nos AY848922 and AY848921, respectively). Using a 560 bp fragment corresponding to part of the nonstructural protein of the middle segment (NSm), the closest identity was found to the SAN-1 isolate (AY124966), previously described from Apulia (Finetti-Sialer et al., 2002). Using a 780 bp N fragment, the closest identity was to the LE98-527 strain from Bulgaria (99·6% identity at the nucleotide level; Heinze et al., 2001). T992 was classified as an A-type isolate according to the MaeI restriction pattern used previously (Finetti-Sialer et al., 2002). This is the first confirmed report of RB strains of TSWV in tomato in Italy. Previously, RB strains of TSWV on tomato in Europe had been identified only in Spain (Aramburu & Martí, 2003).

Occurrence of Tospoviruses in Ornamental and Weed Species in Markazi and Tehran Provinces in Iran.

Damage to agricultural crops by tospoviruses has occurred sporadically in Iran in the past; however, since 2000, outbreaks of tospoviruses have been recorded every year. The most affected ornamental crops were surveyed in two main cultivation areas in provinces of Markazi (Mahallat) and Tehran in 2000-01 and 2001-02. A few weed species also were collected. In all, 513 samples (with or without any conspicuous virus symptoms) were collected and analyzed by double- and triple-antibody sandwich enzyme-linked immunosorbent assay (ELISA) with polyclonal antibodies to Tomato spotted wilt virus (TSWV), Impatiens necrotic spot virus (INSV), and Tomato Varamin virus (ToVV), a new Tospovirus sp. from Iran. These viruses frequently were detected in samples of many different ornamentals and often in mixed infections, whereas Iris yellow spot virus (IYSV) was detected in only four samples. ToVV also was found in weeds growing in Chrysanthemum fields and in a Cuscuta sp. Applying double-antibody sandwich ELISA, no positive reactions were found with Tomato chlorotic spot virus (TCSV). Of the total of 513 samples tested, 345 samples did not react with any Tospovirus antisera. In Tehran, INSV was identified in 21 samples (10%), IYSV in 4 samples (2%), TSWV in 16 samples (8%), and ToVV in 22 samples (11%). In Markazi province, INSV was identified in 24 samples (8%), IYSV in 1 sample (0.5%), TSWV in 40 samples (13%), and ToVV in 36 samples (12%). ToVV was found to prevail in Tehran province and TSWV in Markazi. Thrips spp. present at the plant sampling sites also were collected and identified.

Matrix attachment regions increase the efficiency and stability of RNA-mediated resistance to Tomato Spotted Wilt Virus in transgenic tobacco

Matrix attachment regions (MARs) are DNA elements that can increase and stabilize transgene expression. We investigated the effect of the RB7 MAR on transgenic virus resistance. Constructs for resistance to tomato spotted wilt virus (TSWV) with and without flanking RB7 MARs were used to transform tobacco and produce homozygous lines. The population with the MAR construct had a significantly higher percentage of TSWV resistant plants in the R1 generation than the nonMAR population. Each resistant line was advanced to the R4 generation, and significantly fewer MAR lines lost resistance over generations compared to the nonMAR population. Lines with TSWV resistance in growth chamber tests were also resistant in field trials. Two lines that were resistant in the R1 generation and susceptible in the R4 were examined in more detail in order to determine if transcriptional silencing of the transgene was occurring in the later generation. Short interfering 21-25 nt RNAs from the transgene that are characteristic of post-transcriptional gene silencing (PTGS) were present in the resistant R1 plants, but not the susceptible R4 plants, indicating that virus resistance was associated with PTGS of the transgene. Loss of resistance was accompanied by an increase in promoter methylation in both lines. In line M41, the transgene was fully silenced at the transcriptional level in the R4 as shown by nuclear run-on assays. In line NM13, transgene transcription and RNA accumulation was still present in the R4 generation, but the level of transcription was not sufficient to trigger PTGS, suggesting that this line may have partial transcriptional silencing. These results are consistent with the concept that MARs may prevent transcriptional silencing.

The development of an improved PCR-based marker system for Sw-5, an important TSWV resistance gene of tomato

Sw-5 is an important disease resistance gene of tomato, providing broad resistance to Tomato spotted wilt virus (TSWV). A cleaved amplified polymorphic sequence (CAPS) marker, closely linked to the gene, has been reported. Although the Sw-5 locus has been characterised, a gene-specific marker has not been developed. This paper presents a PCR-based marker-system that consists of the co-amplification of a dominant marker representing the Sw-5 gene sequence, and the modified CAPS marker as a positive control and indicator of genotype.

Molecular marker assisted transfer of resistance to TSWV in tomato elite lines

SummaryA backcross breeding programme was performed to transfer the Sw-5 gene conferring resistance to Tomato Spotted Wilt Virus (TSWV) to a fresh market (Poly 39) and a processing (AD-17) line of tomato, previously selected for good agronomic traits. In both cases, F1BC5,F2BC5,F3BC5 were obtained from F1 crosses with the donor parent Stevens, an Sw-5 resistant variety. The backcross schemes were assisted by a CAPS (cleaved amplified polymorphic sequence) marker tightly linked to the resistant gene; the codominant nature of this marker was particularly useful to select resistant heterozygous/homozygous plants. Further, an inoculation test with a TSWV virus strain was also performed to confirm the successful introgression of resistance in both elite lines. The advantages of using MAS for disease resistance are also discussed.

Screening Capsicum germplasm for resistance to tomato spotted wilt virus (TSWV)

SummaryResistance conferred by the Tsw locus from Capsicum chinense against Tomato spotted wilt virus (TSWV) has been widely used in breeding programmes. Nevertheless, this resistance depends on inoculation conditions, and isolates able to overcome it have already been detected. In this work 29 accessions of several Capsicum species have been mechanically inoculated with TSWV to identify new sources of resistance. Five accessions showed variable percentages of resistant plants, two of which did not show local lesions on inoculated leaves, suggesting that the response was not mediated through hypersensitivity. Two of these accessions also had a remarkable reduced viral accumulation compared to susceptible control. ECU‐973., a C. chinense accession, showed the best performance against TSWV, with 100% resistant plants. This response was confirmed after mechanical inoculation with three different TSWV isolates. The resistance was maintained when the accession was inoculated with TSWV using a high pressure of viruliferous thrips. These results open new possibilities in the development of a durable resistance to TSWV in pepper.

Tomato spotted wilt virus resistance among high and normal O/L ratio peanut cultivars with and without irrigation

In recent years, several high oleic (O) and low linoleic (L) fatty acid ratio peanut ( Arachis hypogaea L.) cultivars with varying levels of disease resistance have been developed and released for commercial production. Improved disease resistance has become of even greater importance, particularly resistance to tomato spotted wilt virus (TSWV) which causes spotted wilt disease in peanut. The objective of this study was to compare these high and normal O/L ratio peanut cultivars with and without irrigation for yield and TSWV resistance. For three consecutive years (1997–1999) the normal O/L ratio cultivar ‘Georgia Green’ and the high O/L ratio cultivar ‘Georgia Hi-O/L’ were found to have the highest pod yield, and a comparably high level of resistance to TSWV when compared to 11 other cultivars with and without irrigation. Georgia Hi-O/L was also found to have a significantly higher O/L ratio than the other high and normal O/L ratio cultivars for improved peanut oil quality both with and without irrigation. In general, irrigation appeared to increase expression of plant symptoms for TSWV disease incidence and had little effect upon O/L ratios among the 13 peanut cultivars evaluated.

Differential Response of Selected Peanut (Arachis hypogaea) Genotypes to Mechanical Inoculation by Tomato spotted wilt virus.

Screening of peanut germ plasm for resistance to Tomato spotted wilt virus (TSWV) has been largely inefficient due to the lack of a screening technique based on mechanical transmission of the virus under controlled environmental conditions. We have studied the reaction of three peanut cultivars (Georgia Green, Georgia Runner, C-99R) and one breeding line (C11-2-39) using a highly efficient mechanical inoculation procedure. The disease response was studied at two temperature regimes, 25 to 30°C (low temperature) and 30 to 37°C (high temperature). Based on percent transmission, symptomatology, distribution of TSWV, and relative levels of TSWV nucleocapsid (N) protein, Georgia Runner and Georgia Green were found to be susceptible, whereas C-99R and C11-2-39 were resistant. Of the four genotypes tested, C11-2-39 had the highest level of resistance to TSWV. The results correlated with the field performance of the genotypes except in the case of Georgia Green, which could not be distinguished from TSWV-susceptible Georgia Runner. Exposure of the inoculated plants to higher temperature (30 to 37°C) resulted in a better resistant response as reflected by reduced systemic infection, localized symptom expression, restricted viral movement, and reduced levels of TSWV antigen. To our knowledge, this is the first report of differential response of peanut genotypes to TSWV using mechanical inoculation. The four peanut genotypes should be useful as reference standards for the initial screening and identification of sources of TSWV resistance in peanut germ plasm.

Movement and distribution of tomato spotted wilt virus in resistant and susceptible accessions of Capsicum spp.

Resistant Capsicum chinense accessions PI-152225 and PI-159236 and the susceptible cultivar ‘Negral’ of Capsicum annuum were used in three experiments to characterize the resistance to tomato spotted wilt virus (TSWV) in these materials. In the first experiment viral movement in the whole plant was studied at two growth stages (2- and 4-leaf stage). In the second experiment the movement within the inoculated leaf was analysed at three growth stages (2-, 3-, and 4-leaf stage). Two techniques were used in this assay: double antibody sandwich – enzyme-linked immunosorbent assay (DAS–ELISA) and direct tissue blotting (DTB). In the latter experiment viral accumulation in different types of samples was evaluated by DAS–ELISA. The DTB technique showed that viral movement within the inoculated leaf is restricted in the resistant plants. The inoculated area was not totally infected in resistant accessions and slower viral movement within the inoculated area was observed. Detection of weak ELISA positives in inoculated and adjacent areas in resistant plants might not be caused by a lower viral replication, but may result from a low foliar area affected by the virus, as viral movement was restricted in certain areas around the necrotic lesions. However, ELISA absorbance values of the necrotic lesions were similar to the values of infected tissue of ‘Negral’. These observations point to a restricted viral cell-to-cell movement in the resistant accessions.

Tomato Production under Mesh Reduces Crop Loss to Tomato Spotted Wilt Virus in Some Cultivars

Seedlings of three tomato (Lycopersicon esculentum Mill.) cultivars [`RDD', carrier of the Sw5 gene, which confers resistance to tomato spotted wilt virus (TSWV); `Pitihué', tolerant to the virus; and the susceptible cultivar Rutgers] were placed at the four- to five-leaf stage in cages containing a population of viruliferous thrips (Frankliniella occidentalis Perg.), and remained there for 0, 7, or 15 days. Plants were subsequently transplanted either into the open field or in tunnels protected with a mesh of 14 × 10 threads/cm. Systemic symptoms and number of dead plants were recorded and enzymelinked immunosorbent assays (ELISA) were performed. `Rutgers' exhibited severe systemic symptoms regardless of treatment and a high number of plants died. The level of infected plants remained low when protective measures were applied to seedlings of `Pitihué' and acceptable yields were obtained. In open air cultivation, where seedling infection was severe, <20% of `RDD' plants became infected and high yields were obtained; protected cultivation did not reduce yield. Although the percentage of infected plants was higher when cultivated under mesh, the yield of all three cultivars was greater than in the open field. The environment created under mesh stimulated growth, neutralizing the effect of the infection.

トマト黄化えそウイルス（ＴＳＷＶ）によるトルコギキョウ（Ｅｕｓｔｏｍａ ｇｒａｎｄｉｆｌｏｒｕｍ （Ｒａｆ．） Ｓｈｉｎｎ．）黄化壊そ病

In November 1997, prairie gentian plants with necrotic yellow symptom were found in Iwate Prefecture, Japan. A spherical virus with a diameter of ca. 80nm was isolated. The virus infected 22 plant species of 11 families after mechanical inoculation. By serological tests, the virus was closely related to tomato spotted wilt virus (TSWV). In RT-PCR with TSWV specific primers, a fragment of about 550bp was amplified. The fragment had a high degree of sequence homology to TSWV-RNA. Based on these results, the virus isolated from prairie gentian was identified as TSWV. This is the first report of TSWV on prairie gentian in Japan.