Hanu R. Pappu

Global status of tospovirus epidemics in diverse cropping systems: Successes achieved and challenges ahead

The diseases caused by thrips-transmitted tospoviruses (genus Tospovirus, family Bunyaviridae) are a major constraint to production of important vegetable, legume and ornamental crops in different parts of the world. Tospoviruses are characterized by having tripartite RNA genomes and utilizing both negative and ambisense genome expression strategies. Their often wide and overlapping host ranges, emergence of resistance-breaking strains, circulative and propagative relationship with polyphagous thrips vectors, and difficulties in predicting their outbreaks pose challenges to development and implementation of effective management programmes. Despite these challenges, for a few tospoviruses, considerable progress has been made in successful development and deployment of practical and effective integrated disease management programmes. This has been due to increased understanding of their molecular biology, plant-virus and virus-vector interactions and epidemiology, and to identification of risk factors that contribute to increased disease incidence and of tactics to mitigate those risk factors. However, challenges remain as resistance-breaking or other new strains of known tospoviruses and completely new tospovirus species continue to be described from various parts of the world and have the potential to cause damaging epidemics. To protect crops from the losses caused by severe tospovirus outbreaks, continued vigilance is required to identify and characterize these emerging tospoviruses, determine their impact on crop production, understand their epidemiologies and develop, evaluate and implement control measures to reduce their impact on crop production.

Iris yellow spot virus: An Emerging Threat to Onion Bulb and Seed Production

This paper reviews the history of Iris yellow spot virus in the USA and the world, epidemiology of Iris yellow spot and the onion thrips vector, and management of Iris yellow spot and thrips. Future prospects and outlook are also discussed.

Biological and Molecular Analyses of the Acibenzolar-S-Methyl-Induced Systemic Acquired Resistance in Flue-Cured Tobacco Against Tomato spotted wilt virus

Tomato spotted wilt virus (TSWV) is an economically important virus of flue-cured tobacco. Activation of systemic acquired resistance (SAR) by acibenzolar-S-methyl (ASM) in flue-cured tobacco was studied under greenhouse conditions by challenge inoculation with a severe isolate of TSWV. ASM restricted virus replication and movement, and as a result reduced systemic infection. Activation of resistance was observed within 2 days after treatment with ASM and a high level of resistance was observed at 5 days onward. Expression of the pathogenesis-related (PR) protein gene, PR-3, and different classes of PR proteins such as PR-1, PR-3, and PR-5 were detected at 2 days post-ASM treatment which inversely correlated with the reduction in the number of local lesions caused by TSWV. Tobacco plants treated with increased quantities of ASM (0.25, 0.5, 1.0, 2.0, and 4.0 g a.i./7,000 plants) showed increased levels of SAR as indicated by the reduction of both local and systemic infections by TSWV. The highest level of resistance was at 4 g a.i., but this rate of ASM also caused phytotoxicity resulting in temporary foliar spotting and stunting of plants. An inverse correlation between the TSWV reduction and phytotoxicity was observed with the increase of ASM concentration. ASM at the rate of 1 to 2 g a.i./7,000 plants activated a high level of resistance and minimized the phytotoxicity. Use of gibberellic acid in combination with ASM reduced the stunting caused by ASM. Present findings together with previous field experiments demonstrate that ASM is a potential option for management of TSWV in flue-cured tobacco.

Tactics for Management of Thrips (Thysanoptera: Thripidae) and Tomato Spotted Wilt Virus in Tomato

Abstract Four studies were conducted in Georgia during spring 1999, 2000, 2001, and 2002 to evaluate various management tactics for reducing thrips and thrips-vectored tomato spotted wilt virus (TSWV) in tomato and their interactions relative to fruit yield. Populations of thrips vectors of TSWV, Frankliniella occidentalis (Pergande) and Frankliniella fusca (Hinds), were determined using flower and sticky trap samples. The management practices evaluated were host plant resistance, insecticide treatments, and silver or metallic reflective mulch. Averaged over all tests, the TSWV-resistant tomato ‘BHN444′ on silver mulch treatment had the largest effect in terms of reducing thrips and spotted wilt and increasing marketable yield. Of the insecticide treatments tested, the imidacloprid soil treatment followed by early applications of a thrips-effective foliar insecticide treatment provided significant increase in yield over other treatments. Tomato yield was negatively correlated with the number of F. fusca and percentage of TSWV incidence. F. occidentalis per blossom was positively correlated with percentage of TSWV incidence, but not with yield. No significant interactions were observed between cultivar reflective mulch main plot treatments and insecticide subplot treatments; thus, treatment seemed to be additive in reducing the economic impact of thrips-vectored TSWV. Control tactics that manage thrips early in the growing season significantly increased tomato yield in years when the incidence of TSWV was high (>17%).

Phenotypic and Etiological Differences Between Psyllid Yellows and Zebra Chip Diseases of Potato

Both potato psyllid yellows and zebra chip (ZC) potato diseases are associated with the potato psyllid, Bactericera cockerelli (Sulc). Aboveground plant symptoms of both diseases are similar but there is a difference in symptoms in potato tubers. ZC has recently been associated with a new species of the bacterium liberibacter, ‘Candidatus Liberibacter solanacearum’, also known as ‘Ca. Liberibacter psyllaurous’. Mechanisms by which the potato psyllid might cause either ZC or potato psyllid yellows symptoms are not understood. Insect transmission studies were conducted to demonstrate psyllid vectoring of both diseases and to compare symptoms and development of the two diseases. Potato plants were exposed to both liberibacter-free and liberibacter-carrying potato psyllids and later evaluated for plant and tuber symptoms. These plants and tubers were then tested for liberibacter by polymerase chain reaction (PCR). In addition, potato plants exhibiting severe psyllid yellows/ZC-like symptoms were collected from a commercial potato field heavily infested with the potato psyllid and tested for liberibacter. PCR detected ‘Ca. Liberibacter solanacearum’ in ZC symptomatic plants and tubers resulting from exposure to liberibacter-carrying psyllids. Despite development of foliar symptoms that resemble those of ZC in plants exposed to liberibacter-free psyllids, no liberibacter was detected in these plants with psyllid yellows. Moreover, tubers from these plants with psyllid yellows did not exhibit any symptoms of ZC infection and tested negative for the bacterium. No liberibacter was detected in plants or tubers collected from the psyllid-infested potato field, suggesting that the observed symptoms were due to psyllid yellows. Furthermore, potato plants that were infected with liberibacter died sooner than plants that were infected with psyllid yellows. Although an association between liberibacter and ZC has been established, no pathogen is yet associated with potato psyllid yellows and mechanisms by which psyllid yellows symptoms are induced by the potato psyllid remain unclear.ResumenLas dos enfermedades de la papa, el amarillamiento de la papa por psílidos y zebra chip (ZC), están asociadas con el psílido de la papa Bactericera cockerelli (Sulc). Los síntomas aéreos de la planta por ambas enfermedades son similares, pero hay una diferencia en los síntomas del tubérculo. La ZC se ha asociado recientemente con una nueva especie de bacteria liberibacter, “Candidatus Liberibacter solanacearum” , también conocida como “Ca. Liberibacter psyllaurous”. No se han entendido los mecanismos por los cuales el psílido de la papa puede causar los síntomas, ya sea de la ZC o el amarillamiento de la papa por psílidos. Se condujeron estudios de la transmisión por insectos para demostrar la transmisión de ambas enfermedades por psílidos y para comparar los síntomas y el desarrollo de ambas enfermedades. Se expusieron plantas de papa a psílidos de la papa, tanto libres como con liberibacter, y se evaluaron posteriormente los síntomas de la planta y del tubérculo. Estas plantas y tubérculos se probaron después para liberibacter con la reacción en cadena de la polimerasa (PCR). Además, se colectaron plantas de papa que exhibieron síntomas severos de amarillamiento por psílidos/ZC, de un campo comercial de papa severamente infestado con el psílido de la papa, y se probaron para liberibacter. La PCR detectó “Ca. Liberibacter solanacearum” en plantas y tubérculos con síntomas de ZC, como resultado de su exposición a los psílidos con liberibacter. A pesar del desarrollo de síntomas foliares parecidos a los de ZC en plantas expuestas a los psílidos libres de liberibacter, no se detectó liberibacter en estas plantas con amarillamiento por psílidos. Aún mas, los tubérculos de estas plantas con amarillamiento por psílidos no exhibieron síntomas de infección por ZC y resultaron negativas a la bacteria. No se detectó liberibacter en plantas o tubérculos colectados del campo de papa infestado por psílidos, lo cual sugiere que los síntomas observados fueron debidos al amarillamiento por psílidos. Incluso, las plantas infectadas con liberibacter murieron más pronto que las infectadas con amarillamiento por psílidos. Aún cuando se ha establecido una asociación entre liberibacter y ZC, no se ha asociado a algún patógeno con el amarillamiento de la papa por psílidos y permanecen si aclararse los mecanismos por los cuales se inducen los síntomas del amarillamiento por el psílido de la papa.

Maize Lethal Necrosis (MLN), an Emerging Threat to Maize-Based Food Security in Sub-Saharan Africa

In sub-Saharan Africa, maize is a staple food and key determinant of food security for smallholder farming communities. Pest and disease outbreaks are key constraints to maize productivity. In September 2011, a serious disease outbreak, later diagnosed as maize lethal necrosis (MLN), was reported on maize in Kenya. The disease has since been confirmed in Rwanda and the Democratic Republic of Congo, and similar symptoms have been reported in Tanzania, Uganda, South Sudan, and Ethiopia. In 2012, yield losses of up to 90% resulted in an estimated grain loss of 126,000 metric tons valued at

Differential expression of tomato spotted wilt virus-derived viral small RNAs in infected commercial and experimental host plants

52 million in Kenya alone. In eastern Africa, MLN was found to result from coinfection of maize with Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV), although MCMV alone appears to cause significant crop losses. We summarize here the results of collaborative research undertaken to understand the biology and epidemiology of MLN in East Africa and to develop disease management strategies, including identification of MLN-tolerant maize germplasm. We discuss recent progress, identify major issues requiring further research, and discuss the possible next steps for effective management of MLN.

Sequence diversity of the nucleoprotein gene of iris yellow spot virus (genus Tospovirus, family Bunyaviridae) isolates from the western region of the United States.

Background Viral small RNAs (vsiRNAs) in the infected host can be generated from viral double-stranded RNA replicative intermediates, self-complementary regions of the viral genome or from the action of host RNA-dependent RNA polymerases on viral templates. The vsiRNA abundance and profile as well as the endogenous small RNA population can vary between different hosts infected by the same virus influencing viral pathogenicity and host response. There are no reports on the analysis of vsiRNAs of Tomato spotted wilt virus (TSWV), a segmented negative stranded RNA virus in the family Bunyaviridae, with two of its gene segments showing ambisense gene arrangement. The virus causes significant economic losses to numerous field and horticultural crops worldwide. Principal Findings Tomato spotted wilt virus (TSWV)-specific vsiRNAs were characterized by deep sequencing in virus-infected experimental host Nicotiana benthamiana and a commercial, susceptible host tomato. The total small (s) RNA reads in TSWV-infected tomato sample showed relatively equal distribution of 21, 22 and 24 nt, whereas N. benthamiana sample was dominated by 24 nt total sRNAs. The number of vsiRNA reads detected in tomato was many a magnitude (~350:1) higher than those found in N. benthamiana, however the profile of vsiRNAs in terms of relative abundance 21, 22 and 24 nt class size was similar in both the hosts. Maximum vsiRNA reads were obtained for the M RNA segment of TSWV while the largest L RNA segment had the least number of vsiRNAs in both tomato and N. benthamiana. Only the silencing suppressor, NSs, of TSWV recorded higher antisense vsiRNA with respect to the coding frame among all the genes of TSWV. Significance Details of the origin, distribution and abundance of TSWV vsiRNAs could be useful in designing efficient targets for exploiting RNA interference for virus resistance. It also has major implications toward our understanding of the differential processing of vsiRNAs in antiviral defense and viral pathogenicity.

A new and distinct species in the genus Caulimovirus exists as an endogenous plant pararetroviral sequence in its host, Dahlia variabilis.

Summary.Iris yellow spot virus (IYSV), a tentative virus species in the genus Tospovirus and family Bunyaviridae, is considered a rapidly emerging threat to onion production in the western United States (US). The present study was undertaken to determine the sequence diversity of IYSV isolates from infected onion plants grown in California, Colorado, Idaho, Oregon, Utah and Washington. Using primers derived from the small RNA of IYSV, the complete sequence of the nucleoprotein (NP) gene of each isolate was determined and the sequences compared. In addition, a shallot isolate of IYSV from Washington was included in the study. The US isolates of IYSV shared a high degree of sequence identity (95 to 99%) with one another and to previously reported isolates. Phylogenetic analyses showed that with the exception of one isolate from central Oregon and one isolate from California, all the onion and shallot isolates from the western US clustered together. This cluster also included onion and lisianthus isolates from Japan. A second distinct cluster consisted of isolates from Australia (onion), Brazil (onion), Israel (lisianthus), Japan (alstroemeria), the Netherlands (iris) and Slovenia (leek). The IYSV isolates evaluated in this study appear to represent two distinct groups, one of which largely represents isolates from the western US. Understanding of the population structure of IYSV would potentially provide insights into the molecular epidemiology of this virus.

Transmission of Iris Yellow Spot Virus by Frankliniella fusca and Thrips tabaci (Thysanoptera: Thripidae)

Viruses in certain genera in family Caulimoviridae were shown to integrate their genomic sequences into their host genomes and exist as endogenous pararetroviral sequences (EPRV). However, members of the genus Caulimovirus remained to be the exception and are known to exist only as episomal elements in the infected cell. We present evidence that the DNA genome of a new and distinct Caulimovirus species, associated with dahlia mosaic, is integrated into its host genome, dahlia (Dahlia variabilis). Using cloned viral genes as probes, Southern blot hybridization of total plant DNA from dahlia seedlings showed the presence of viral DNA in the host DNA. Fluorescent in situ hybridization using labeled DNA probes from the D10 genome localized the viral sequences in dahlia chromosomes. The natural integration of a Caulimovirus genome into its host and its existence as an EPRV suggests the co-evolution of this plant-virus pathosystem.