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Beet Curly Top Virus and Other Viruses of Concern

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Beet Curly Top Virus and Other Viruses of Concern Beet curly top virus and other viruses of concern Robert L. Gilbertson Department of Plant Pathology University of California Davis A diversity of viruses affect processing tomatoes in California • Beet curly top virus (BCTV) Leafhopper-transmitted • Alfalfa mosaic virus (AMV) • Cucumber mosaic virus (CMV) Aphid- • Tobacco etch virus (TEV) transmitted Potato virus Y (PVY) • Tomato mosaic virus (ToMV) Mechanically transmitted • Tomato spotted wilt virus (TSWV) Thrips-transmitted • Tomato necrotic spot virusE (ToNSV) Thrips-associated • Tomato yellow leaf curl virusE (TYLCV) Whitefly-transmitted • Tomato necrotic stunt virusRE (ToNSV) Whitefly-transmitted Viruses continue to be a threat to processing tomato production in California • Considerable progress has been made on identifying, understanding and managing these viruses • New tools for detection, prediction and management • Viruses change in terms of their importance and prevalence -Existing viruses -Beet curly top virus (BCTV) -Tomato spotted wilt virus (TSWV) -Alfalfa mosaic virus (AMV) -New and emerging viruses -Tomato necrotic spot virus (ToNSV) -Tomato yellow leaf curl virus (TYLCV) -Tomato necrotic dwarf virus (ToNDV) Tomato spotted wilt virus (TSWV) • Bronzing, necrosis, ringspots (fruits) • Symptoms vary depending on variety and plant age Prevalence of Virus Diseases in Processing Tomatoes of California Tomato spotted wilt virus (TSWV) Tomato spotted wilt virus (TSWV)-biology • Virus: homogenous in tomato in CA, resistance-breaking strains exist in other parts of the world; and new highly virulent tospoviruses have been introduced into Florida • Symptoms maybe confused with curly top early in the season • Detection: ELISA, PCR, immunostrips • Host range: wide, includes many weeds and crops • Insect vector: various species of thrips, F. occidentalis in CA • Mode of transmission: persistent propagative, must be acquired by larval stages • Sources of inoculum: -weeds/bridge crops -thrips • Means of survival: -perennial weeds/bridge crops -thrips (in soil) TRANSMISSION ACQUISITION BY LARVAE IS CRUCIAL Egg Tospovirus Transmission st Cycle 1 instar 2nd instar VIRUS PASSAGE VIRUS PASSAGE Only adults that acquire as larvae can transmit. Pupal Stages Do Not Feed Photos by J.K. Clark Tomato spotted wilt virus-management • Apply risk index to predict potential for TSW • Inoculum levels are low at the beginning of the season • Thrips and virus-free transplants • Resistant (Sw-5) varieties and field placement • Remove or rogue plants early in season (up to 30 d) • Monitor and manage thrips populations-spray early and according to a predictive model to delay the development of virus-carrying adults • Weed management (before, during and after the growing season) • Effective sanitation following harvest (all crops) • Minimize any overlap between bridge crops An IPM program has been developed for thrips and TSWV in processing tomatoes in California Tomato spotted wilt virus-current situation • Current TSWV issues -High incidences of TSWV in late-planted fields in Fresno County -Would risk index have predicted these outbreaks? -Increased incidence of TSW symptoms in varieties with the Sw-5 gene-possibility of the emergence of resistance-breaking TSWV strains or the new species (Tomato chlorotic spot virus and Groudnut ringspot virus) -Rapid molecular assays are available to detect resistance breaking TSWV strains and new species • Application of these tools could improve TSWV management Curly top disease is a potentially devastating disease of tomato • Plants are stunted and have up- or down curled leaves with dull green-yellow color and purpling of the veins • Plants infected at a young age often die • Plants infected later produce small fruits that ripen prematurely • No necrosis in leaves or fruits • Early in the season, curly top symptoms can be confused with those of tomato spotted wilt Prevalence of Virus Diseases in Processing Tomatoes of California Curly top disease Curly top is caused by Beet curly top virus (BCTV) • A geminivirus with a very wide host range (>300 species of plants) including crops and weeds • Some plants show severe symptoms when infected (e.g., tomatoes), whereas others show no symptoms Beet curly top virus (many species of weeds) • In nature, BCTV is only transmitted by the beet leafhopper, not by contact, other insects or via seed • Exists as a complex of strains, which can be differentiated by their genetic sequences Symptoms of BCTV and biological properties infection in tomato Classification of Beet curly top viruses G. Curtovirus G. Curtovirus G. Curtovirus BCTV strains Species BCTV strains BCTV BCTV-CA/Logan BCTV-Svr (BSCTV) Other species: BCTV-Wor (BMCTV) BCTV-Logan SpCTV BCTV-Mld (BMCTV) BCTV-CFH PeCTV BSCTV BCTV-PeCT BCTV-Worland PepYDV BCTV-SpCT BCTV-PeYD BCTV-CO** BCTV-LH71** BMCTV 2001 2014 Curly top disease cycle: Dependent on a migratory insect Spring: Viruliferous adult leafhoppers migrate to the valley floor and feed Winter/early spring: Adults overwinter and breed on annual and perennial plants Fall: adult leafhoppers Multiple generations on the migrate for overwintering valley floor in the foothills Beet curly top virus-management • Monitor for adult beet leafhopper populations and for levels of BCTV in leafhoppers and manage by targeted spraying (CTVCP) • No currently available resistant varieties • Apply systemic neonicotinoid insecticides at transplanting (sugar beets, tomatoes) • Field placement (avoid fields adjacent to the foothills) • Heavy plant populations • Weed management on the valley floor? Curly top outbreak of 2013 • Losses of ~1 million tons (~$100 million) • Associated with high beet leafhopper populations with high BCTV titers • Appearance of curly top in new Tomato field with 90% curly top in Fresno Co. locations, e.g., San Joaquin 1 2 3 4 5 6 7 8 9 10 11 12 13 14 + - County and in new crops, e.g., melons • Emergence of new strains High BCTV titers in most of the leafhopper samples of BCTV more virulent to tomato Rapid and Specific Detection of BCTV by the Polymerase Chain Reaction (PCR) Provides New Opportunities for Improved Disease Management • Detection in plants -rapid detection -ID viruses/strains -ID host plants • Detection in the beet leafhopper vector -predictor of curly top severity -target areas for spraying -ID prevalent viruses/strains PCR Detection of BCTV in beet leafhoppers Add 300ul STE Spin 5’ at Phenol/Chloroform buffer and grind 13,000 rpm extraction twice Monthly leafhopper collections from CDFA CTVCP personnel PCR reaction Precipitation of nucleic acid Curly top outbreaks are correlated with high populations of virus-carrying leafhoppers early in the growing season • Using the PCR detection method and leafhoppers provided by the Curly Top Virus Control Board (CTVCB), a study was conducted to ID factors associated with curly top outbreaks in tomato • The highest levels of curly top in tomato were correlated with high populations of virus-carrying leafhoppers early in the growing season (i.e., February-April) Monitoring BCTV in beet leafhoppers from October 2014 to July 2015 in the Central Valley of California (sweep net samples) PCR Positive Percentages of Sweep LH Samples 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Oct Nov Dec Jan Feb Feb Mar Apr May Jun Jul (IV) (IV) Strong % Medium % Weak % (IV=Imperial Valley) Large numbers of beet leafhoppers observed in winter and spring of 2015 High virus titer detected in these leafhoppers Disease incidence in 2015: Moderate* BCTV in reservoir hosts • Foothills samples: over 246 weeds tested and 79% were positive for BCTV • Weeds with the highest infection rates included Atriplex, filaree, goosefoot, London rocket, peppergrass, plantago, Russian thistle and shepherd’s purse • Most had no symptoms and low virus titers, but ~15% (29/246) had high virus titers • Similar results were obtained with weeds from Valley floor in the winter and during the growing season: >50% positive for BCTV (~400 total weeds tested), most with low titer but a low percentage with high titer • Alfalfa and safflower are poor hosts of the beet leafhopper and BCTV • Big question: Can beet leafhoppers acquire BCTV from low titer weed reservoir hosts? Appearance of New BCTV Strains in the Central Valley • Characterization of BCTV strains associated with the 2013 outbreak has revealed emergence of two new recombinant strains: BCTV-CO and BCTV-LH71 • In 2014 and 2015 , BCTV-CO and -LH71 were the predominant strains in leafhoppers, tomatoes and other crops, and weeds; the only exception was in sugar beets in Imperial Valley where BCTV-Srv (CFH) was predominant • BCTV-SpCT was detected in tomatoes and peppers for the first time • These results suggest a shift in predominant BCTV strains in the Central Valley and more diversity in the California BCTV population • These strains are highly virulent in tomato, though this alone is unlikely to be the main reason for the 2013 outbreak • These new strains are being used for screening for disease resistance New BCTV strains associated with the 2013 outbreak Mild phenotype strains Severe phenotype strains BCTV-Wor BCTV-CO** BCTV-Svr BCTV-LH71** Sugar beet Tomato N. benthamiana Agrobacterium-mediated inoculation for screening tomato tomato varieties and germplasm for resistance to BCTV BCTV 3rd Observe symptom development (up to 35 dpi) 2nd 48 hr old liquid culture of 1st Check for virus infection Agrobacterium tumefaciens Tomato (by PCR) (21-28 dpi) containing the viral DNA Ty genes confer resistance to BCTV • The Ty genes (Ty 1-6) are a series
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