ABSTRACTS of the
colloquium presentations
Influence of nature and civilization on plant virus emergence. S. A. TOLIN. Dept. of Plant Pathology, Physiology and Weed Science, Virginia Tech, Blacksburg, VA 24061. Publication no. P-1999- 0081-SSA.
As we mark the 100 years of virology since tobacco mosaic virus, one cannot help but reflect that the number of recognized plant viruses has increased in a seemingly exponential manner during this century. Many of today's viruses are among those first described in the first third of the century and may be re- emerging with increasing pathogenic diversity as crop plants with resistance genes are developed. Has there been genetic selection pressure for new strains? Have they invaded as invisible hitchhikers in global movement of germplasm and plant material? Or, have vector biotype or distribution played a role in their re- emergence? These questions can be asked for the early viruses as well as for the numerous entirely new viruses that have emerged in the last third of the century to cause serious new diseases. Are these truly new viruses, or have we just now developed the molecular tools to characterize them? Is their emergence a natural phenomena? Or, have perturbations brought about by civilization, increased global travel, and the cultivation of plants for food and fiber enhanced their emergence. Other speakers will explore a few case studies.
Emergence of geminiviruses of tomato in the Western Hemisphere: The tomato yellow leaf curl virus paradigm. J. E. POLSTON. University of Florida, Gulf Coast Research and Education Center, 5007 60th St. E., Bradenton, FL 34203. Publication no. P-1999-0082-SSA.
The number of whitefly-transmitted geminiviruses that infect tomato have significantly increased in the Western Hemisphere over the last 12 years. Their appearance follows the spread of the B biotype of Bemisia tabaci, a whitefly that readily feeds and reproduces on tomato, and which was introduced from the Eastern Hemisphere in the mid 1980's. Most of these viruses are causing new tomato diseases, but are not necessarily new viruses. The geographic ranges of several tomato geminiviruses have been increased through the movement of transplants. In addition, one of these viruses, tomato yellow leaf curl virus (TYLCV), is the result of an introduction from the eastern Mediterranean. This virus is spreading through the eastern Caribbean and by 1997 had been found in Florida. TYLCV is highly virulent on tomato, and infects at least four other vegetable and ornamental crops grown in Florida and the eastern Caribbean. TYLCV is a classic example of how serious pathogens can be disseminated long distances by human activity and the subsequent devastating effects that such introductions can cause.
Tospoviruses: New species, new hosts and a continuing problem. J. W. MOYER. Dept of Plant Pathology, North Carolina State University, Raleigh, NC 27695-7616. Publication no. P-1999-0083-SSA.
Tomato spotted wilt virus (TSWV) was little more than an intellectual curiosity outside of a few isolated areas until the mid 1980's. Presumably changes in pesticide useage patterns resulted in shifts in the vector species which favored spread of TSWV and what we now recognize as related viruses. By the mid 1980's spotted wilt-like symptoms were becoming increasingly prevalent in floral crops such as impatiens, gloxinia, cyclamen and many others as well as in field crops. Significant economic losses were occurring in all of these crops. Armed with increasing support an intensive research effort was initiated in various research laboratories around the world. As of 1990, there was still little agreement as to the molecular organization of the virus and the basis for diversity. TSWV was still considered a unique virus in a monotypic taxon of plant viruses. Since 1990, a new genus has been established which contains at least twelve distinct species. The genome organization has been determined and proof of replication in its thrips vector has been obtained. However, they remain a significant economic problem.
High plains virus - A new twist to an old story. S. G. JENSEN. USDA-ARS, Dept. Plant Pathology, Univ. Nebraska, Lincoln, NE 68583. Publication no. P-1999-0084-SSA.
The high plains virus (HPV) was identified in 1993 after an epidemic year in corn and wheat. It resembles wheat spot mosaic virus described by Slykhuis in 1956 and wheat spot chlorosis reported by Nault et al. in 1969. Since its characterization in 1993, HPV has been found in 11 states west of the Missouri River, Florida, Brazil, Chile, Israel and possibly China and Australia. It has reappeared yearly in many areas and the losses may be catastrophic. Its appearance and incidence are unpredictable. Sources of genetic resistance are known for corn but not for wheat where cultural control is needed. HPV is obligately transmitted by the eriophyid mite, Aceria tosichella and also by seed. Purified, infectious HPV contains five species of dsRNA and a 32 kDa nucleoprotein coded for by RNA-3. It resembles a tenuivirus in the EM. Infected plant cells have viroplasmas and 200nm in diameter, double membrane bound bodies (DMBs). Gold colloid labeling with antiserum to the 32 kDa nucleoprotein located the 32 kDa nucleoprotein in the viroplasm and within the lumen of the DMBs. Fig mosaic, rose rosette, thistle mosaic and redbud yellow ringspot pathogens are similar in transmission and cytopathology.
Plum pox potyvirus: A monster in the wings. L. LEVY and A. Tschanz. USDA-APHIS, PPQ, NPGQC, Beltsville, MD 20705. Publication no. P-1999-0085-SSA.
Plum pox potyvirus (PPV) is not a new disease, yet, it is the most damaging emerging disease of Prunus. Dissemination within a region results from graft and non-persistent aphid transmission. Fast epidemics are induced through natural spread of PPV by winged-viruliferous aphids not to adjacent trees but several trees away from the acquisition site. For example, in Europe, following establishment, PPV disseminated rapidly in a four-year period resulting in 60-90% of orchard trees with symptoms. Long distance movement to a new region, country or continent, is through the movement of propagative materials. PPV was limited to the European continent and the middle East until 1992, when it was discovered at an experiment station near Santiago, Chile, marking the first appearance of PPV in the western hemisphere. In 1994, PPV was documented in the region surrounding Santiago, and since has been reported to be widespread. The dynamics of the new infection in Chile will be examined. Several aspects of PPV biology, molecular and serological detection, and control by eradication and resistance will be discussed, as well as the role of Federal and State agencies in prevention and control of PPV and similar diseases.
Why are we having new citrus virus problems? S. M. GARNSEY (1), R. H. Brlansky (2), and S. Korkmaz (3). (1) USDA, ARS, Orlando, FL 32803; (2) University of Florida, IFAS, Lake Alfred, FL 33850; (3) Univ. of Cukurova, Adana, Turkey. Publication no. P-1999-0086-SSA.
Virus and viruslike diseases of citrus are a dynamic system and new problems continue to arise. Dispersal of existing pathogens to new areas, changes in the properties of existing viruses or vectors, and introduction of new cultivars are all factors. New problems may involve ingress of a pathogen from other hosts, or changes in vector dynamics associated with other crops. The close relationships of badna-, capillo-, and ilarviruses of citrus to similar viruses in other crops indicate a probable common origin or some inter- crop movement of these pathogens. Citrus is a long-lived perennial reservoir into which viruses from outside sources can be introduced and accumulated. The impact of these ingress events is determined by the potential for secondary spread, and the pathogen and vector reservoirs in other crops. Citrus chlorotic dwarf (CCD), a disease that appeared in citrus groves in Turkey after introduction of the bayberry whitefly (Parabemesia myricae), provides an illustration of a new citrus virus problem with a probable non- citrus origin, and a vector with citrus and non-citrus hosts.
The emergence of whitefly-transmitted RNA viruses: The criniviruses. BRYCE W. FALK, Tongyan Tian, Luis Rubio, and Hsin-Hung Yeh. Dept. of Plant Pathology, University of California, Davis, CA 95616. Publication no. P-1999-0087-SSA.
Since 1975 a new type of plant virus that is transmitted by specific whiteflies has been recognized. Now at least seven similar viruses are known. All induce yellowing symptoms in their plant hosts, are generally phloem-limited, non-mechanically transmissible, and have large ss-RNA genomes characteristic of viruses in the family Closteroviridae. These viruses affect herbaceous plants and since the early 1980's several have rapidly emerged as the causal agents for important diseases within temperate, tropical and sub- tropical regions of the world. Characterization of one of these viruses, lettuce infectious yellows virus (LIYV), showed that the LIYV genome was composed to two ss-RNAs of 8.1 and 7.2 kb. RNA 1 encodes for proteins associated with RNA replication and RNA 2 encodes the hallmark closterovirus gene array. LIYV is now recognized as the type member of the genus Crinivirus within the family Closteroviridae. The geographic incidence, molecular variation and whitefly-transmission characteristics of the emerging Criniviruses will be discussed.
Man's role in emerging and re-emerging plant virus diseases. R. R. MARTIN. USDA-ARS, 3420 NW Orchard Ave., Corvallis, OR 97330. Publication no. P-1999-0088-SSA.
Many of the virus outbreaks over the past decade have resulted either directly or indirectly from man's activities. Overuse or misuse of pesticides has resulted in the development of resistance in virus vector populations. A misplaced confidence in our ability to control vectors with pesticides has resulted in multiple chemical resistances in much the same manner as misuse of antibiotics has led to the development of bacteria with multiple resistances. Rapid increases in the international movement of plant material increases the opportunity for hitchhiking viruses to move between countries and continents. Germplasm collecting expeditions that move wild relatives of crop plants which have coevolved with their own viruses offer the opportunity for unknown viruses to be introduced into new areas. How will the use of genetically engineered crop plants containing viral genes impact the development of new and re-emerging viruses?
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