V. Muniyappa

Host-plant viral infection effects on arthropod-vector population growth, development and behaviour: management and epidemiological implications

Publisher Summary This chapter reviews the evidence for positive, negative and neutral effects on the population growth of several arthropod vector species when feeding on virus-infected compared with uninfected host plants. The emphasis is on whiteflies and aphids as these have received the greatest attention. Two important examples are the B. tabaci -borne cassava mosaic disease (CMD) pandemic that continues to devastate cassava production in large areas of sub-Saharan Africa (SSA); and the tomato leaf curl disease epidemics in the Indian subcontinent that have had equally serious implications for tomato production in this region. These problems have been considered in detail and experimental data have been included to show, for both pathosystems, that the B. tabaci colonizing virus-infected host plants have significantly higher-population growth rates compared to those colonizing virus-free hosts. This effect has been discussed in the context of the significantly higher densities of B. tabaci present on symptomatic cassava and the behavioral changes associated with this effect. Data has also been presented to show that the concentrations of four amino acids are significantly higher in the phloem sap of CMD-infected cassava plants. These interacting effects are considered in relation to probable mechanisms contributing to the rapid spread of these epidemics.

Diversity and distribution of begomoviruses infecting tomato in India

Summary.Leaf curl begomoviruses cause serious yield losses to Indian tomato crops. Total DNAs were extracted from leaves of 69 tomato plants and 34 weeds or neighbouring crops collected from all the major tomato producing areas of India. Eighty-one of the 103 samples were positive by PCRs using begomovirus genus-specific primers. Coat protein (CP) genes from 29 samples were PCR amplified, cloned and sequenced. Phylogenetic analyses of the CP sequences revealed five different tomato leaf curl begomovirus (TLCB) clusters each <88% identity to the others. Four clusters represented known Indian TLCBs, whereas one cluster contained sequences originating from Haryana State with most identity (89%) to the provisional Begomovirus species Croton yellow vein mosaic virus.Sixty-five begomovirus positive samples were characterised further by PCR with DNA-β, DNA-B, four Indian TLCB species, PALIc1960/PARIv722 (universal begomovirus primers), and by sequencing. The majority of samples represented monopartite TLCBs associated with DNA-β components. All four known TLCBs appeared to be present throughout India. TLCBs were also present in chilli, cowpea, okra and tobacco crops, as well as in some common weeds. Papaya leaf curl virus and Pepper leaf curl Bangladesh virus sequences were detected in tomato. Mixed begomovirus infections, a prerequisite for recombination, were evident in 13 samples.

Occurrence of three genotypic clusters of Bemisia tabaci and the rapid spread of the B biotype in south India

The whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), is generally considered to have originated from the Indian subcontinent, although little information has so far been collected on the molecular diversity of populations present in this region. The genetic diversity of B. tabaci populations from Karnataka State, south India was analysed using the random amplified polymorphic DNA‐polymerase chain reaction (RAPD‐PCR) technique and partial mitochondrial cytochrome oxidase I (mtCOI) gene sequences (689 bases) of 22 selected samples. A total of 108 whitefly samples analysed by RAPD‐PCR produced 89 polymorphic bands, and cluster analyses grouped them according to their geographic origin into ‘north’ and ‘south’ Karnataka. Phylogenetic analysis of mtCOI gene sequences with reference B. tabaci sequences from other Asian countries divided them into three genotypic clusters. Each cluster was supported with high bootstrap values (82–100%) and the individuals belonging to each cluster shared high nucleotide identities (up to 100%). This indicated at least three distinct genotypes, apparently indigenous to India, which are also present in China, Malaysia, Nepal, Pakistan, and Thailand. These coexist with the B biotype, which was first reported in India in 1999, and has since spread rapidly to other states in south India. The B biotype was more common than the indigenous B. tabaci, in locations where it had been present for more than 2 years. This is reminiscent of the situation in the Americas during the early 1990s, where the B biotype replaced existing biotypes and caused unprecedented losses to agriculture.

First report of the Bemisia tabaci B biotype in India and an associated tomato leaf curl virus disease epidemic

In May 1999, in the Kolar district of Karnataka State, Bemisia tabaci numbers on tomato increased by approximately 1,000-fold that observed previously (3). This was associated with an epidemic of severe tomato leaf curl disease that caused complete crop failure. DNAs extracted from 35 symptomatic tomato leaf samples collected within the epidemic region all gave the expected 500 to 600 bp amplicon with begomovirus-specific primers A/B (1). These primers amplify from the conserved nonanucleotide TAATATTAC in the common region of DNA-A to the conserved amino acid sequence CEGPCKYG within the coat protein gene. AluI and TaqI restriction patterns of all 35 polymerase chain reaction (PCR) products were identical. One PCR product from an epidemic (GenBank no. AF321929) and a non-epidemic (AF321930) site (Bangalore) were cloned and sequenced. The two 531-bp inserts showed 96% nucleotide identity to each other and 94% nucleotide identity to the equivalent region of Tomato leaf curl Bangalore virus (ToLCBV-Ban-4) (AF165098), suggesting that the epidemic was caused by an indigenous ToLCBV strain. Adult B. tabaci were collected from tomato plants at nine sites within the epidemic. DNA was extracted from 9 to 13 individuals per site and analyzed by RAPD-PCR using primers OpB20 and OpB11. Eighty to 100% of individuals per site had identical patterns to those of B biotype individuals from Israel and Florida, which were different to the patterns produced by the indigenous Indian B. tabaci. Adult B. tabaci from the epidemic and nonepidemic (Bangalore) regions were cultured separately on zucchini plants (n = 20) vars. Fordhook and Ambassador. Distinct silverleaf symptoms appeared in all plants fed on by the epidemic B. tabaci, but not on those fed on by the nonepidemic whiteflies. Irregular ripening of tomatoes was also a widespread problem in the epidemic area. Cytochrome oxidase I (COI) (720 bp) gene sequences were obtained for epidemic (AF321927) and nonepidemic (AF321928) B. tabaci, which had only 80% nucleotide identity to each other. A GenBank BLAST search showed that the former were most similar to B biotype whitefly from Israel (AF164667; 97%) and Texas (AF164675; 99%). The B biotype transmits Indian ToLCBV (2) and its introduction into India is of great concern as it is already associated with a devastating plant-disease epidemic. References: (1) D. Deng et al. Ann. App. Biol. 125:327, 1994. (2) P. F. McGrath and B. D. Harrison. Ann. App. Biol. 126:307, 1995. (3) H. K. Ramappa et al. Ann. App. Biol. 133:187, 1998.

Transmission of Pigeon pea sterility mosaic virus by the Eriophyid Mite, Aceria cajani (Acari: Arthropoda)

The transmission characteristics of Pigeon pea sterility mosaic virus (PPSMV) to pigeon pea (Cajanus cajan) by its eriophyid mite vector, Aceria cajani, were studied. Nonviruliferous A. cajani colonies were established on detached healthy leaflets of a PPSMV-immune pigeon pea cultivar floating on water. The transmission efficiency of single A. cajani was up to 53% but was 100% when >5 mites per plant were used. A. cajani acquired PPSMV after a minimum acquisition access period (AAP) of 15 min and inoculated virus after a minimum inoculation access period (IAP) of 90 min. No latent period was observed. Starvation of A. cajani prior to, or following, PPSMV acquisition reduced the minimum AAP and IAP periods to 10 min and 60 min, respectively, and mites retained virus for up to 13 h. None of the mites that developed from eggs taken from PPSMV-infected leaves transmitted the virus, indicating that it is not transmitted transovarially. Taken together, these data suggest a semipersistent mode of transmission of PPSMV by A. cajani.

Reaction of Lycopersicon cultivars and wild accessions to tomato leaf curl virus

Summary1201 tomato cultivars, breeding lines and accessions of Lycopersicon species were screened for tomato leaf curl virus (TLCV) under field and laboratory conditions during summer seasons of 1986 to 1989. Two lines of L. hirsutum (PI 390658 and PI 390659) and 2 lines of L. peruvianum (PI 127830 and PI 127831) were resistant to TLCV infection. These accessions did not produce any leaf curl symptoms either in field or after inoculation by whitefly Bemisia tabaci with TLCV. Adult whiteflies died within 3 days after releasing on resistant accessions (PI 390658, PI 390659, PI 127830), whereas the whiteflies survived upto 25 days on susceptible tomato cultivars. Under field conditions 0–4 and 5–25 adult whiteflies were observed on resistant and susceptible cultivars respectively. Hybridization was effected using the popular tomato cultivars Arka Sourabh, and Arka Vikas, as the female parents and the resistant Lycopersicon wild species as the pollen parents, to incorporate the resistant gene(s) into the edible tomatoes.

Comparison of resistance to Tomato leaf curl virus (India) and Tomato yellow leaf curl virus (Israel) among Lycopersicon wild species, breeding lines and hybrids

The objective of this study was to screen wild and domesticated tomatoes for resistance to Tomato yellow leaf curl virus, Israel (TYLCV-Is) and Tomato leaf curl virus from Bangalore isolate 4, India (ToLCV-[Ban4]) to find sources of resistance to both viruses. A total of 34 tomato genotypes resistant/tolerant to TYLCV-Is were screened for resistance to ToLCV-[Ban4] under glasshouse and field conditions at the University of Agricultural Sciences, Bangalore, India. Resistance was assessed by criteria like disease incidence, symptom severity and squash-blot hybridization. All the tomato genotypes inoculated with ToLCV-[Ban4] by the whitefly vector Bemisia tabaci (Gennadius) produced disease symptoms. In some plants of the lines 902 and 910, however, the virus was not detected by hybridization. The tomato genotypes susceptible to ToLCV-[Ban4] by whitefly-mediated inoculation were also found susceptible to the virus under field conditions. However, there were substantial differences between genotypes in disease incidence, spread, symptom severity and crop yield. Despite early disease incidence, many genotypes produced substantially higher yields than the local hybrid, Avinash-2. Sixteen tomato genotypes from India resistant/tolerant to ToLCV-[Ban4] were also tested for TYLCV-Is resistance at the Hebrew University of Jerusalem, Rehovot, Israel. Accessions of wild species, Lycopersicon hirsutum LA 1777 and PI 390659 were the best sources of resistance to both viruses. Lines 902 and 910, which were, resistant to TYLCV-Is were only tolerant to ToLCV-[Ban4] and accession Lycopersicon peruvianum CMV Sel. INRA, resistant to ToLCV-[Ban4], was only tolerant to TYLCV-Is. Implications of using the resistant lines in breeding programme is discussed.

Sterility mosaic disease - the 'green plague' of pigeonpea: advances in understanding the etiology, Transmission and control of a major virus disease.

Sterility mosaic (SMD) is the most damaging disease of pigeonpea (Cajanus cajan (L.) Millsp.) in the Indian subcontinent. After seven decades of research, vital breakthroughs made on the identification, detection, and transmission of the causal agent of this major disease are enabling the development of broad-based durable resistant pigeonpea cultivars. These breakthroughs will contribute greatly to sustainable pigeonpea production and enhance the income and livelihood of poor farmers in the semiarid tropics of the Indian subcontinent. The Crop

Resistance of tomato and sweet-pepper genotypes to Tomato leaf curl Bangalore virus and its vector Bemisia tabaci

The primary objective of this study was to identify sources of resistance to Tomato leaf curl Bangalore virus—Bangalore strain 4 (ToLCBV-[Ban4]) and its whitefly vector Bemisia tabaci (Gennadius) in tomato and sweet-pepper genotypes, because resistant cultivars did not exist. A total of 25 wild and cultivated tomato and nine sweet-pepper genotypes obtained from the Asian Vegetable Research and Development Centre, Taiwan and local cultivars were screened at the University of Agricultural Sciences, Bangalore, India, both by whitefly-mediated inoculation of virus in the glasshouse and natural infection in the field. Resistance to virus was assessed by disease incidence, symptom severity and DNA hybridisation tests. Resistance to B. tabaci was measured by counting the number of eggs, nymphs and pupal cases on the tomato genotypes. The wild species Lycopersicon peruvianum INRA sel. and L. chilense LA 1969 were resistant to ToLCBV-[Ban4] but highly susceptible to whiteflies, whereas L. hirsutum LA 1777 was resistant to both the virus and the vector. Among the L. esculentum genotypes, H-24, FL 744-6-9, FL 699 and FL 699 sp + were tolerant to ToLCBV-[Ban4], but were susceptible to whiteflies. Of the nine pepper genotypes inoculated with the virus, only five became infected, at low incidence levels and none were suitable for the development of B. tabaci. Use of tomato lines with resistance to both the virus and the vector offer the best way of controlling these related problems, and the prospects for tomato leaf curl virus disease management in South India are discussed.

Socio-Economic and Scientific Impact Created by Whitefly-Transmitted, Plant-Virus Disease Resistant Tomato Varieties in Southern India

Research carried out to assess the impact of open-pollinated Tomato leaf curl virus (ToLCV)-resistant tomatoes and hybrids on the livelihoods of resource-poor farmers in Southern India is described and discussed. Three high-yielding ToLCV-resistant tomato varieties were developed initially using conventional breeding and screening techniques involving inoculation by ToLCV-viruliferous whitefly, Bemisia tabaci. In 2003 and 2004, respectively, these varieties were released officially by the Karnataka State Seed Committee and the Indian Ministry of Agriculture through notification in the Gazette of India. From 2003 to 2005, eleven seed companies bought breeder seed of the ToLCV-resistant varieties and used them to begin breeding F1 hybrids from them. Socio-economic studies carried out to assess the benefits obtained from growing the ToLCV-resistant varieties found that farmers could gain up to 10 times the profit by growing the ToLCV-resistant varieties compared to the pre-existing ToLCV-susceptible varieties. Adoption of ToLCV-resistant tomatoes was also associated with reduced pesticide use. Extra income from tomato sales was prioritised by farmers to pay for children’s education, better nutrition and medicines. In a joint effort with the commercial seed sector in India, a promotional field day was organised in 2007. As well as the three ToLCV-resistant varieties, 62 ToLCV-resistant hybrid tomatoes were exhibited during a farmer-field day by 17 commercial seed companies and several public institutes. Tomatoes with ToLCV-resistance are now grown widely in South India and seeds of the three open-pollinated varieties have been distributed to more than 12 countries. In 2007, a conservative estimate of the financial-benefit to cost of the research ratio was already more than 837:1.