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EMERGING NEW POLEROVIRUSES ANDTOSPOVIRUSES AFFECTING VEGETABLESIN ASIA AND BREEDING FOR RESISTANCE

C. A. Relevante1, S. Cheewachaiwit2, J. Chuapong3, M. Stratongjun4,V. E. Salutan1, D. Peters5, C.H. Balatero1 and S. J. de Hoop2

1East-West Seed Company, Inc.,Hortanova Research Center,

Purok 3, Brgy. Pagolingin Bata,Lipa City, Batangas, the Philippines

2Hortigenetics Research (S.E. Asia) Limited,Station 7 Moo 9, Tambol Maefack Mai Sansai,

Chiang Mai 50290 Thailand;

3Hortigenetics Research (S.E.Asia) Limited,195 M.2 T. Pongphare, A.Maelao, Chiang Rai 57000 Thailand

4Hortigenetics Research (S.E.Asia) Limited,Station 33 Moo 4, Tambol Nongbur, Songphinong,

Suphanburi 72190 Thailand

5Laboratory of Virology, Department of Plant Sciences,Wageningen University,

P.O. Box 629, 6700 AP Wageningen, the NetherlandsE-mail: cherry.relevante@eastwestseed.com

ABSTRACT

The diseases caused by aphid-borne poleroviruses (genus Polerovirus, family Luteoviridae) and thrips-borne tospoviruses (genus Tospovirus, family Bunyaviridae) are emerging threats to the production of economically important vegetable and fruit crops in tropical and sub-tropical Asia. To date, at least 13 different polerovirus species have been characterized. In Asia, the reported poleroviruses include Cucurbit aphid-borne yellows virus (CABYV), Melon aphid-borne yellows virus (MABYV) and Suakwa aphid-borne yellows virus (SABYV). Tospoviruses are even more diverse in Asia, infecting a broad range of crops. About 70% of the globally described tospovirus species have been reported causing significant yield losses in several countries in South and Southeast Asia. The development and implementation of effective management strategies to reduce the damage due to these insect-transmitted viruses is a major challenge to the vegetable industry. Host plant resistance breeding against polerovirus and tospovirus is the most effective and environmentally-friendly approach to address the long-term management of these viruses. The identification and characterization of new polerovirus and tospovirus species in the Philippines and Thailand are discussed and efforts to identify sources of resistance to these viruses and breeding for resistance are presented.

Keywords: poleroviruses, tospoviruses, CABYV, MABYV, SABYV, resistance breeding

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INTRODUCTION

Current status of cucurbit-infecting poleroviruses in Asia

Cucurbit-infecting poleroviruses (genus Polerovirus, family Luteoviridae) are emerging as a major constraint in cucurbit production worldwide. The first polerovirus described to infect cultivated cucurbits naturally was the Cucurbit aphid-borne yellows virus (CABYV), which caused significant yield losses in melon and cucumber in France (Lecoq et al. 2002). Since then, the virus has been found worldwide (Lemaire et al. 1993; Abou-Jawdah and Fayyad 1997; Juarez 2004; Papayiannis et al. 2005; Bananej et al. 2006; Tomassoli and Meneghini 2007; Yardιmcι and Özgönen 2007; Mnari-Hattab et al. 2009; Omar and Bagdady 2012). CABYV is limited in the phloem and induces yellowing symptoms and thickening of older leaves in melon and cucumber. Early infection and significant reduction in the number of fruits produced by the affected plants resulted to 40-50% yield losses (Lecoq et al. 1992). Poleroviruses are efficiently transmitted in a persistent and circulative manner by two aphid species, Myzus persicae and Aphis gossypii (Lecoq et al. 1992). Virus particles are isometric, approximately 25 nm in diameter encapsidating the genome, which consists of a single-stranded (ss) positive sense RNA molecule of 5.7 kb (Mayo and D’Arcy 1999). Currently, there are 13 formally accepted virus species in the genus Polerovirus (Carstens and Ball 2009; Carstens 2010) as well as several other tentative species. At least three cucurbit-infecting polerovirus species have been reported in Asia: CABYV, Melon aphid-borne yellows virus (MABYV) and the recently proposed Suakwa aphid-borne yellows virus (SABYV) (Lecoq et al. 1992; Xiang et al. 2008; Shang et al. 2009). CABYV has been reported associated with yellowing symptoms of cucurbit crops in Iran (Bananej et al. 2006), China (Xiang et al. 2008b) and Taiwan (Knierim et al. 2010). The virus has also been found associated with the ‘Namamarako’ or NMK symptoms in bittergourd in the Philippines (Koster et al. 2006) and ‘Mara Ba’ symptoms in Thailand (Relevante et al. 2008). Recently, a newly proposed recombinant strain of CABYV (CABYV-R) has been identified in Taiwan (Knierem et al. 2010). MABYV and SABYV have been reported recently in China and Taiwan (Xiang et al. 2008; Shang et. al. 2009; Knierem

et al. 2010), although it is speculated that these two polerovirus species and the newly described CABYV strain in Taiwan are not newly emerging virus species but were probably misidentified as they were found widespread in central and southern Taiwan in 2008 and 2009 (Knierem et al. 2010). MABYV has also recently been found associated with ‘Mara Ba’ symptoms in bittergourd in Thailand (Cheewachaiwit et al. 2011, personal communication). So far, three polerovirus species have been detected in the Philippines: CABYV, SABYV and a recombinant strain of CABYV (Kenyon 2012, personal communication). Full understanding of the biology, host range and genetic diversity including interactions between species or strains of cucurbit-infecting poleroviruses is critical in the development of effective and long-lasting management strategies for these viruses. Moreover, reliable detection protocols that can differentiate species and strains, as described by Xiang et al. (2008) and Knierem et al. (2010), are needed. Recently, the full-length genome sequences of the three known polerovirus species infecting cucurbits in Taiwan (CABYV, MABYV, SABYV), including the recently identified recombinant strain of CABYV (Knierem et al. 2010) and the recently proposed SABYV (Shang et al. 2009) have been determined using a modified RT-PCR procedure that amplifies the polerovirus genome in a general approach directly from total RNA extracted from field samples (Knierem et al. 2012). This method will facilitate detailed study of the taxonomy and evolution of cucurbit-infecting poleroviruses including possible recombination events (Knierem et al. 2012).

Characterization and identification of CABYV isolate associated with ‘Namamarako’ or NMK disorder in bittergourd in the Philippines

Bittergourd is one of the most popular vegetable crops for commercial production in the Philippines because of its high yield and profitability. However, a disorder locally known as ‘Namamarako’ or NMK (maleness tendency) has brought serious economic losses in bittergourd production in the Philippines since 1996. Symptoms on bittergourd infected with NMK are characterized by interveinal chlorosis, green vein banding, wrinkling and thickening of younger leaves while older leaves show chlorotic patches (Fig. 1). Plants infected at the early stage show severe stunting and hardly bear fruit, hence the term ‘namamarako’.

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Through a collaborative research between East-West Seed Company Philippines and Wageningen University in the Netherlands, the cause of NMK has been successfully characterized based on aphid transmission, particle morphology, serology and molecular characterization. Virus particles were extracted from infected bittergourd plants and gave a positive reaction when subjected to ELISA with CABYV antiserum provided by Lecoq (INRA, Montpellier, France). Results of transmission tests indicated that the virus is preferentially transmitted by the aphid species Aphis gossypii in a persistent manner and by grafting. Molecular analysis using immuno-capture RT-PCR and sequence analysis of the coat protein revealed 96% homology between the NMK virus and CABYV, indicating that both viruses are strains of each other (Koster et al. 2006). In a separate study, NMK isolates collected from bittergourd fields in Central Luzon in 2008 were characterized by RT-PCR. The cloned CP gene revealed 95% homology with CABYV in France based on amino acid sequence (Relevante et al. 2008). Preliminary data on host range study revealed other cucurbits and weed species as alternate hosts of NMK or CABYV-PH under controlled experimental conditions (Koster et al. 2006). A specific polyclonal antiserum raised against CABYV-PH had been developed in Virology Laboratory, Wageningen University, the Netherlands and is now utilized in field surveys and detection of CABYV in the Philippines. CABYV had not been reported in the Philippines until recently, although CABYV-like

symptoms have been observed in bittergourd fields in Central Luzon as early as 1996 and in some parts of Visayas and Mindanao in succeeding years. It is suspected that the virus has probably been present in the country for a long time but remained undetected because the symptoms were attributed to other causes like nutrient deficiencies and genetic disorder. Similarly, the detection of CABYV has also been overlooked for several years as symptoms were thought to be caused by nutrient deficiencies, aging or infection of the plants (Lecoq et al., 1992). The recent detection of CABYV and SABYV including a recombinant strain of CABYV in cucurbits in the Philippines (Kenyon 2012, personal communication) indicating the need for more detailed analysis of the host range and genetic diversity of polerovirures present in the country for designing resistance breeding strategies of important cucurbit crops.

Characterization and identification of CABYV and MABYV isolates associated with ‘Mara Ba’ disorder in bittergourd in Thailand

Since 2005, a disorder in bittergourd locally known as ‘Mara Ba’ (crazy bittergourd or CBG), which has a similar symptom of ‘Namamarako’ in the Philippines, has been observed in bittergourd fields in the central part of Thailand. However, ELISA tests using antibodies raised against NMK or CABYV-PH failed to detect CABYV from symptomatic plants. Initial attempts to transmit

Fig. 1. Typical symptoms of ‘Namamarako’ in bittergourd caused by CABYV under natural field conditions: A) Chlorotic patches on older leaves and B) interveinal yellowing, green vein banding, wrinkling and thickening of the younger leaves

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the virus using the melon aphid, Aphis gossypii and by grafting have been successful, indicating that the causal agent of ‘Mara Ba’ could be a new polerovirus species (Relevante et al 2008). In a recent survey and molecular characterization of ‘Mara Ba’ isolates collected from Chiangmai (CM) and Suphanburi (SP) in 2011 and including ‘Mara Ba’ isolate collected in 2007 (CBG) revealed two distinct polerovirus species, i.e., CABYV and MABYV associated with ‘Mara Ba’ symptoms in bittergourd. Cloning and nucleotide sequence analysis of the CP gene of ‘Mara Ba’ isolates collected in 2007 and 2011 in Thailand indicated that CBG-2 (2007 isolate) is closely related to MABYV while CM1-3 and SP1-4 (2011 isolates) are closely related to CABYV (Cheewachaiwit et al. 2011, personal communication). The East-West Seed Company, in collaboration with Kasetsart University in Suphanburi, Thailand is now focusing on the development and production of specific antiserum for these new poleroviruses to enable reliable detection of these viruses in bittergourd and other cucurbits in Thailand. Further studies on host range and genetic diversity analysis of poleroviruses in Thailand are also underway.

Current status of tospoviruses infecting vegetables in Asia

Thrips-transmitted tospoviruses (genus Tospovirus, family Bunyaviridae) are among the most economically important plant pathogens in the world because of their ability to cause severe crop losses (Mumford et al. 1996; Pappu et al. 2009).

Diseases caused by tospoviruses are emerging as a major limiting factor in the production of important vegetable crops in South and Southeast Asia. Currently, the genus is comprised of at least 21 assigned and tentative species (Hassani-Mehraban et al. 2011) with Tomato spotted wilt virus (TSWV) as the type species, which occurs worldwide because of its wide host range (Pappu et al. 2009). Tospoviruses are exclusively transmitted by thrips (Thysanoptera: Thripidae) in a persistent and propagative manner. They have to be replicated in the thrips before they can be transmitted (Peters 2008). Of the 13 species of thrips that have been confirmed as vectors of one or more tospoviruses worldwide, 7 species are known vectors of tospoviruses in Asia (Table 1). Thrips palmi tends to be the predominant thrips vector species in tropical and sub-tropical Asia and transmits several different tospoviruses, thus appears to be the ‘F. occidentalis’ of tropical and sub-tropical Asia (Papu et al 2009). It is widely distributed in several countries including India, Indonesia, Japan, Thailand and the Philippines infesting several crops (Cannon et al. 2007). Tospoviruses have unique spherical and membrane-bound particles of about 8-20 nm in diameter covered with spike-like surface projections composed of two viral glycoproteins (G1 and G2). The core consists of infectious ribonucleoproteins composed of the genomic RNA tightly encapsidated by nucleocapsid (N) proteins and small amounts of the viral RNA-dependent RNA polymerase. The RNA genome is tripartite, of which one segment is of negative polarity, the

Table 1. Specific thrips vector of known tospoviruses in Asia

Thrips vector Tospovirus Occurrence

Cerathripoides claratris Capsicum chlorosis virus ThailandScirtothrips dorsalis Peanut yellow spot virus India

Peanut chlorotic fanspot virus Taiwan Groundnut bud necrosis virus IndiaFrankliniella occidentalis Tomato spotted wilt virus IsraelThrips tabaci Iris yellow spot virus Israel, IndiaFrankliniella cephalica Tomato spotted wilt virus JapanFrankliniella schultzei Groundnut bud necrosis virus IndiaThrips palmi Groundnut bud necrosis virus India Watermelon silver mottle virus Japan, Indonesia Melon yellow spot virus Japan

Reviewed by Pappu et al. (2009)

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large (L) RNA and the other two are ambisense, the medium (M) RNA and small (S) RNA (Adkins 2000; Hassani-Mehraban et al. 2011; Kormelink et al. 1992). The amino acid sequence of the N protein, which is coded by the small (S) RNA segment, is being used to differentiate species within the genus Tospovirus along with host range and thrips transmission (German et al. 1992; de Avila et al. 1993; Mumford et al. 1996; Goldbach and Kuo 1996).

Tospoviruses have been grouped into three based on the amino acid sequence of the N proteins: American, Eurasian and Asian (Hassani-Mehraban et al. 2011; Pappu et al. 2009). With the exception of TSWV, INSV and IYSV, this grouping indicates geographical distribution of tospoviruses (Fig. 2). Of the 21 tospovirus species characterized globally, 15 species have been reported present in Asia (Table 2) (Papu et al. 2009; Hassani-Mehraban et al. 2011; Chiemsombat et al. 2010; Seepiban et

Americantospovirus clade

0.1

Fig. 2. Phylogeny based on nucleocapsid (N) protein amino acid sequence of tospoviruses (Hassani-Mehraban et al. 2011). Tospoviruses fall in 3 groups (American, Eurasian and Asian) based on their geographical distribution.

Erasian tospovirusclade

Asian tospovirusclade

Table 2. Distribution of known tospoviruses in different countries in Asia

India Thailand Philippinesa Indonesia Taiwan China Korea Japan Middle East

GBNV CaCV WSMoV TSWV TSWV GBNV TSWV TSWV TSWVPYSV MYSV MYSV GBNV CaCV CaCV CNSV GBNVWBNV WSMoV IYSV WSMoV PCFV WSMoV TYRVIYSV TNRVb MYSV IYSVCaCV

Reviewed by Pappu et al. (2009)a East-West Seed Company (unpublished results)b Chiemsombat et al. (2010); Seepiban et al. (2011); Hassani-Mehraban et al. (2011)

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al. 2011), which include Capsicum chlorosis virus (CaCV), Calla lily chlorotic spot virus (CCSV); Chrysanthemum stem necrosis virus (CSNV); Groundnut ringspot virus (GRSV); Groundnut bud necrosis virus (GBNV) or Peanut bud necrosis virus (PBNV); Impatiens necrotic spot virus (INSV); Iris yellow spot virus (IYSV); Melon yellow spot virus (MYSV); Peanut yellow spot virus (PYSV); Tomato spotted wilt virus (TSWV); Tomato yellow ring virus (TYRV); Tomato zonate spot virus (TZSV); Tomato necrotic ring virus or Tomato necrotic ringspot virus (TNRV); Watermelon bud necrosis virus (WBNV) and Watermelon silver mottle virus (WSMoV). Among these tospoviruses, GBNV is considered the most economically important affecting several crops such as peanut, potato, tomato and soybean in parts of China, India, Iran, Nepal, Sri Lanka and Thailand, with estimated losses of more than US$89 million per year in Asia (Pappu et al. 2009). GBNV is most widespread in India causing serious damage in groundnut, tomato and potato (Mandal et al. 2012).

Emerging new tospoviruses in Thailand and the Philippines

In Thailand, the incidence of tospovirus infection in vegetables is increasing. In addition to the three tospovirus species (CaCV, WSMoV and MYSV) previously reported, TNRV has recently been described as a new tospovirus species affecting tomato and pepper in the northern and central regions of Thailand (Chiemsombat et al. 2010; Hassani-Mehraban et al. 2011; Seepiban et al. 2011). Tomato plants showing tospovirus-like

symptoms of distinct yellowing and necrotic rings on leaves and fruits were observed in the green houses in Chiangmai, Thailand in 2008 (Fig. 3). The identification and characterization of TNRV isolates from tomato and pepper plants collected from Chiangmai was done in collaboration with the Virology Laboratory, Wageningen University, the Netherlands (Hassani-Mehraban et al. 2011). The results support other reports on the occurrence of a new tospovirus species in Thailand, of which the name Tomato necrotic ringspot virus has been proposed (Chiemsombat et al. 2010; Seepiban et al. 2011). So far, no incidence of tospovirus infection has been reported in the Philippines although tospovirus-like symptoms had been observed in watermelon, melon, tomato and pepper based on field surveys conducted by the Plant Pathology Department of East-West Seed Company since 2003 (unpublished results). In a recent survey, at least three tospovirus species have been detected from plants showing tospovirus-like symptoms based on positive ELISA reaction using specific polyclonal antisera kindly provided by the Virology Laboratory, Wageningen University). These include WSMoV (watermelon), MYSV (watermelon, melon and cucumber) and IYSV (onion) (Table 3 and Fig. 4). These results, although preliminary, indicate that tospoviruses are emerging as new important plant viruses infecting vegetables in the Philippines just like Thailand and other Asian countries. The need for regular surveys to monitor and document tospovirus incidence and the availability of reliable and sensitive procedures for detection of these viruses is therefore imperative to enable effective management.

Fig. 3. Tomato (a) and pepper (b) plants naturally infected with TNRV showing typical necrotic ring symptoms

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Breeding for polerovirus resistance in cucurbits

The development of resistant cultivars is a promising and sustainable approach to manage emerging problems of poleroviruses in cucurbit crops. Currently, little is known about the genetic resistance of cucurbit-infecting poleroviruses. Dogimont et al. (1997) reported two complementary recessive genes, cab-1 and cab-2, conferring resistance to CABYV in an Indian melon line PI 124112. Earlier, several potential sources of CABYV resistance have been found in melon germplasm, mostly originating from India and some from Korea and Africa which included PI 124112 (Dogimont et al. 1996). Not much information is known about resistance

breeding for CABYV or other poleroviruses in Asia. In the Philippines, the East-West Seed Company initiated the screening of bittergourd germplasm for CABYV resistance in 2002. The discovery of CABYV-PH and its mode of transmission paved the way for the development of controlled screening protocols against this polerovirus species in bittergourd. These developments led to the systematic identification of sources of resistance to CABYV and its transfer to elite bittergourd lines. In 2002, screening of different bittergourd collections was intensified, resulting in the identification of accession EW 1696 as a reliable source of resistance to CABYV. This line forms the basis of the East-West Seed polerovirus resistance breeding programs. Line development was initiated in 2003 through series of backcross and pedigree selection

Table 3. Occurrence of tospoviruses in the Philippines based on positive ELISA reaction using specific polyclonal antisera (East-West Seed Company, unpublished results)

Tospovirus Crops Distribution

Watermelon silver mottle virus Watermelon Central Luzon, South Luzon, North Luzon

Melon yellow spot virus Watermelon, melon, Central Luzon, South Luzon, North Luzon cucumber

Iris yellow spot virus Onion Central Luzon

A

B C

Fig. 4. Symptoms on watermelon and onion naturally infected with tospoviruses: A) Narrowing of leaves with chlorotic and necrotic spots in watermelon caused by MYSV; B) Silver mottling in watermelon caused by WSMoV; and C) eye-like spots and green streak on onion leaves caused by IYSV.

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cycles that focused on CABYV resistance and good fruit qualities. It was demonstrated that the resistance is controlled by two partially dominant genes. First hybrids with CABYV resistance were then developed and tested in a series of replicated yield trials in different bittergourd production areas during both the dry and wet season. Introgression of CABYV resistance into Thai breeding lines was also initiated in 2007. In 2008, the East-West Seed Company Philippines released the first hybrid bittergourd with intermediate resistance to CABYV named ‘Galactica F1.’ This new hybrid performs well in areas where ‘Namamarako’ or CABYV is a serious problem. Breeding for resistance is underway to transfer resistance genes to elite lines in the various market segments in Thailand, Vietnam, India and Indonesia where CABYV has also been observed as emerging potential threats in bittergourd production.

Breeding for tospovirus resistance in vegetables

Host plant breeding for tospovirus resistance appears to be the most promising approach for long-term management of tospoviruses. Currently, only two single dominant resistant genes, i.e., Sw-5 and Tsw, are available for tospovirus resistance breeding in tomato and pepper, respectively. The hypersensitivity gene Sw-5, derived from S. peruvianum, has been used worldwide, providing high level of resistance to TSWV. However, resistance breaking strains of TSWV (e.g., TSWV6) have overcome the resistance imparted by this gene limiting its effectiveness (Latham and Jones 1998; Aramburu and Mati 2003; Margaria et al. 2004; Ciuffo et al. 2005). TSWV easily defeats resistance genes and can adapt rapidly to the resistant plants by genome re-assortment mechanism (Qiu and Moyer 1999). Another resistance gene, Sw-7, derived from S. chilense accession (LA1938) has been observed to provide acceptable levels of resistance to virulent strains of TSWV in the conditions of Hawaii, Florida, Georgia and South Africa (Stevens et al. 2006). The Sw-5 and Sw-7 genes, however, appear ineffective against South and Southeast Asian tospoviruses (Hanson et al. 2009). The resistance found in 60% of S. peruvianum accessions screened with Sw-5-breaking isolates of TSWV could be the best source of tospovirus resistance in tomato (Gordillo and Stevens 2008; Hanson et al. 2009). In pepper, resistance to TSWV but not to other tospoviruses, expressed as a hypersensitive response (HR) and controlled by the dominant

gene Tsw, has been found only in accessions of C. chinense ‘PI152225’ and ‘PI159236’ and deployed into several commercial sweet and hot pepper varieties. However, this HR mechanism is strongly influenced by both temperature and physiological conditions. Resistance is broken at high temperature and young plants are more susceptible. Moreover, virulent strains in the field can also overcome this gene (Roggero 2002). The identification of resistance genes conferring resistance to tospoviruses coupled with the increasing problem of tospovirus infection in vegetable crops in Asia stimulated more intensive studies aimed at finding sources of genetic resistance. In India where GBNV is most widespread, much effort had been directed towards the evaluation of germplasm and breeding lines which resulted in the identification of sources of resistance to GBNV in groundnut, mungbean, soybean, tomato and potato. Moreover, groundnut varieties (ICGS 44 and ICGS 11) with GBNV resistance have already been released (Mandal et al. 2012). In Thailand, field evaluation of PBNV resistance in peanut resulted in the identification of peanut lines suitable for use in PBNV resistance breeding programmes (Pensuk et al. 2002). In AVRDC tospovirus trials conducted in India and Southeast Asia in 2007-2008, 2 accessions of S. peruvianum (L06138 and L00671) were identified as potential sources for PBNV and CaCV resistance (Hanson et al. 2009). However, results are preliminary and have to be confirmed. Screening for CaCV and TSWV resistance in tomato and pepper using mechanical inoculation is also in progress at AVRDC, Taiwan (Kenyon 2012, personal communication). Limited information on tospovirus resistance in cucurbits has been recorded. Sugiyama et al. (2009) reported that cucumber accessions originating from South Asia, in particular Southeast Asia had moderate resistance to MYSV. Furthermore, in a separate study, they found out that resistance to MYSV in cucumber is temperature dependent. High temperature (25°C and 30°C) facilitated symptom expression and viral spread in the cucumber accessions with resistance (Sugiyama et al. 2009). The development of efficient and reliable methods for screening germplasm for tospovius resistance is essential in the production of improved varieties with high level of resistance. At East-West Seed Company, efforts to develop screening protocols for tospovirus resistance using thrips and mechanical inoculation have been initiated in 2008 in two research stations in Thailand (Chiangmai and Suphanburi). In 2011 screening consisting

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of 77 long cucumber inbred lines, one accession (18185) was identified resistant to MYSV using thrips inoculation. Within the same year, a total of 141 watermelon inbred lines were screened for resistance to WSMoV using thrips. Of which three lines (BL-15, 14778 and KK-359) showed high level of resistance, while three others (KK-289, KK-320 and KK-325) showed intermediate level of resistance. In 2012, a total of 159 watermelon accessions consisting of plant introductions and inbred lines were screened for resistance to WSMoV under controlled greenhouse conditions using thrips and 2 inbred lines (15633 and 15633) were identified with high level of resistance while 4 inbred lines (14801, 14774, 14781, 14810) showed intermediate level of resistance. Screening for resistance to TNRV in pepper and tomato, CaCV in pepper and MYSV in melon and cucumber are still in progress. In a preliminary screening of 13 pepper accessions under controlled greenhouse conditions using mechanical inoculation, 2 parental lines (PY6423 and PY6424) with high level of resistance to TNRV were identified.

CONCLUSION

Poleroviruses and tospoviruses are emerging as potential threats to vegetable production in tropical and subtropical Asia. The recent detection of new species or strains of cucurbit-infecting poleroviruses in countries like China, Taiwan, Thailand and the Philippines, the identification of new tospovirus species in Thailand and the detection of several tospoviruses in the Philippines indicate the need to do more intensive surveys and diversity analysis of these viruses in these countries to enable effective control. Like any other plant viruses, there is no effective chemical control for poleroviruses and tospoviruses and the only practical means of control is to minimize their spread, which can only be achieved by controlling the insect vector population in the field. Unfortunately, the occurrence of vector populations with insecticide resistance poses another challenge to the farmers who often rely on chemical pesticides for control. Host plant resistance has been widely utilized in developing vegetable varieties with acceptable level of resistance to tospoviruses. However, the development of resistance-breaking strains is a major setback of this management strategy. Similarly, the occurrence of different polerovirus species or strains in the field can complicate efforts being made to breed for durable resistance in cucurbit crops. Thus, good breeding strategies

must be developed and this requires comprehensive understanding of the virus-vector/virus-host interactions as well as diversity among these viruses. Resistance breeding, in combination with good cultural management practices, is the most sustainable solution to address emerging problems caused by poleroviruses and tospoviruses in Asia.

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