mation on ELB, the goals of the pro- Invisible invaders . . . gram and specifics on the treatments in their area. STEP also made presen- tations to neighborhood associations and displayed an information table at Insect-t ransmitted viruses a popular outdoor market throughout the summer. Through community education and outreach, the coopera- threaten agriculture tion of city agencies, good monitoring and the development of new strategies Robert L. Gilbertson a Diane E. Ullman, first authors to control elm leaf beetles in ”hot Raquel Salati 3 Douglas P. Maxwell spots,” the IPM program for Sacra- mento is being developed and should Elizabeth E. Grafton-Cardwell o MaryLou Polek be fully implemented within several years. The vast movement of people and Despite technological advances lead- D.L. Dahlsten is Professor and Associate agricultural products between dis- ing to tremendous yield increases for Dean, D.L. Rowney is Biostatistician and tant geographical regions has cre- many crops, modern agricultural pro- A.B. Lazuson is Graduate Student Re- ated unprecedented opportunities duction continues to face pest threats, searcher, Center for Biological Control, for introducing plant viruses and among them insects, plant pathogens, Division of Insect Biology, College of the insects that carry them (vec- and weeds. Often growers are faced Natural Resources, UC Berkeley. tors) to new areas. Outbreaks of with multiple pests, which exacerbates The authors thank the many individu- new viruses may be favored in crop damage and complicates manage- als and agencies that have assisted in the these agroecosystems by crop ment strategies. A dramatic example development of this program over the of how two types of pests “team up” susceptibility, the presence of par- years, and particularly Steve Dreistadt, to cause major problems for California UC Davis, UC/IPM Project; Beverly ticular weeds and certain agricul- agricultural production is the case of Gingg, San Luis Obispo; Anne Fenkner tural practices. In some cases, plant viruses and their insect vectors. and Barbara Gemmill, Sacramento Tree conditions in these ecosystems Plant viruses are foreign genetic el- Foundation, Sacramento; and Martin may be ideal for the emergence of ements that take over a plant’s cellular Fitch and Bob Hughes, Department of sltered plant viruses and new machinery and use it to produce their Neighborhood Services, Sacramento. virus/vector relationships. This own genetic material and proteins. They thank the UC/IPM Project, UC may result in the appearance of This has dire consequences for the Davis and the Slosson Foundation for insect-transmitted plant viruses in plant, altering many normal plant funding earlier aspects of the study, and crops and regions where they functions such as cell division and the California Department of Pesticide have not been seen before. Be- photosynthesis, thus resulting in dis- Regulation Grant #95-0254for funding Zause plant viruses and their in- ease. While symptoms of virus infec- the current program in Sacramento. sect vectors are intimately linked, tion vary depending on the particular rhe status of both must be consid- virus and plant, they generally involve ?red in formulating strategies to stunted and distorted growth, changes References wevent or slow their introduction, in leaf coloration and shape, and poor 3s well as to manage any inva- flower and/or fruit yield and quality. Dreistadt SH, Dahlsten DL. 1990. Distribu- Most plant viruses are not stable out- tion and abundance of Eurnniopsis antennata sions. To illustrate these points we [Dipt: Tachinidae] and Tetrastichus 9ighlight two situations that could side of living plant cells and so need a brevisfigma [Hyrn.: Eulophidae], two intro- way to spread from infected plants to Weaten California agriculture. First, duced elm leaf beetle parasitoids in northern uninfected plants. Plant-feeding in- California. Entomphaga 35(4):527-36. 9 devastating plant virus, tomato sects are ideal agents for spreading Dreistadt SH, Dahlsteh DL, Rowney DL, et iellow leaf curl geminivirus, is not al. 1991. Treatment of destructive elm leaf plant viruses because of their high beetle should be timed by~. temperature. Cat wesent in California, but an insect rates of reproduction, dispersal abili- Ag 45(2):23-5. ‘the silverleaf whitefly) that trans- ties, and obligate use of particular Luck RF. Scriven GT. 1979. The elm leaf nits it is present. Second, the beetle, Pyrrhalfa luteola, in southern Califor- plants as food. Indeed, intricate rela- nia: Its host preference and host impact. En- brown citrus aphid is not present in tionships have coevolved among in- vironmental Entomology 8:307-13. Zalifornia, but a citrus virus (citrus sects, viruses and the plant hosts they Wu 2, Jarnieson S, Kielbaso J. 1991. Ur- ban forest pest management. J Arboriculture ‘risteza closterovirus) that this in- share, resulting in remarkably efficient 17(6). sect efficiently spreads, is present. spread of viruses from plant to plant.

CALIFORNIA AGRICULTURE, MARCH-APRIL 1998 23 emerging virus. duction. In the first case, tomato yel- There are vari- low leaf curl, the virus is not present ous possible in California, but the vector insect, reasons under- silverleaf whitefly, is well established. lying the emer- In the second case, the opposite is true: gence of dis- citrus tristeza closterovirus is present eases caused by in California but an efficient vector in- plant viruses: sect (the brown citrus aphid, Toxopteva ecological con- citvicida) is not. ditions may have favored Tomato yellow leaf curl virus the evolution of Tomato yellow leaf curl new viruses or geminivirus (TYLCV) may be the most new vector- damaging virus to tomatoes because virus relation- plants infected at an early stage of ships (such growth fail to produce fruit. While Symptoms of tomato yellow leaf curl as geminivirus in leaves of an infected to- the recent appearance of tomato TYLCV-infected tomato plants pro- mato plant. Below, plants infected at a mottle geminivirus in Florida associ- duce abundant flowers, the virus young age are severely stunted and new ated with the introduction of silverleaf causes flowers to fall off (abscise) long shoots grow straight up, resulting in small, compact plants with bushy tops whitefly, Bemisia argentifolii); there before setting fruit. In the field, yield that are commonly referred to as ‘broccoli’ may have been an expansion of the losses can reach 100%.TYLCV gets its or ‘bonsai’ plants. geographic range of existing plant vi- name froin the fact that infected leaves ruses, insect vectors and susceptible are yellow except for the veins, show crop plant species; or the emerging vi- strong upward curling of the outer ruses may have been either unknown leaf margins and are small and or relatively unimportant until chang- crumpled. Plants infected at a young ing agricultural practices or altered age are severely stunted and new vector biology (for example, changes shoots grow straight up, resulting in in host preferences or population dy- small, compact plants with bushy tops namics) favored their dramatic in- that are commonly referred to as crease. The unprecedented movement ’broccoli’ or ’bonsai’ plants. In general, of people, agricultural products and the younger the plant at time of infec- propagules (such as seeds, bulbs, tu- tion, the more severe the stunting. The bers and transplants) also provides relative age at which a plant was in- tremendous opportunities for plant vi- fected can be determined because only ruses and the insects that spread them the new growth will show symptoms. (vectors) to move between widely TYLCV is a geminivirus separated geographic regions. Clearly, (Geminiviridae family), a group of an awareness of these threats and the plant viruses characterized by twinned actions needed to avoid introducing icosahedral virus particles, which look these exotic pests is critical to all in- like two identical-sized soccer balls volved in California agriculture. stuck together and from which the fam- The completion of the proposed ily name is derived (the Latin word contained research facility on the UC gemini means twin). Geminiviruses can California farmers are already faced Davis campus would provide aca- be carried by either leafhoppers or with a plethora of insect-transmitted demic, government and private indus- whiteflies, and whitefly-transmitted viruses that can limit crop production, try researchers the opportunity to con- geminiviruses are one of the major and for which there are few effective duct important research on these emerging groups of plant viruses world- management options. While the viral exotic pests that could lead to new wide. For instance, whiteflies spread diseases California growers currently management strategies. various tomato-infecting geminiviruses face can be severe, they pale in com- In this article, we will review the in Central and South America, Asia, parison to some that have emerged in current status of two diseases caused India and the Middle East. While other areas of the world. by insect-transmitted plant viruses TYLCV and other whitefly-transmitted When a virus, either ”new” or pre- that have wreaked havoc in many geminiviruses that infect tomato are viously existing, causes sudden epi- parts of the world, and could someday not present in California, a whitefly demics in a region, it is often called an pose a threat to California crop pro- that transmits these viruses is present.

24 CALIFORNIA AGRICULTURE, VOLUME 52, NUMBER 2 (The vector is silverleaf whitefly. See re- sphere tomato-infecting geminiviruses lated article, p. 29). such as tomato mottle geminivirus in In contrast to many diseases caused Florida and tomato leaf crumple by whitefly-transmitted gemini- geminivirus in Mexico. viruses, tomato yellow leaf curl is not In January 1994, a team of gemini- a new problem. This disease was first virus researchers, R.L. Gilbertson (UC described in Israel around 1940, and Davis) and D.P. Maxwell (University was associated with outbreaks of the of Wisconsin-Madison), visited the sweetpotato whitefly (Cohen and Dominican Republic and noticed that Antignus 1994). Tomato yellow leaf the symptoms resembled those caused curl was subsequently described in by TYLCV. Using molecular genetic parts of the Middle East, Africa, techniques, the researchers confirmed Southern Europe, India and Asia. The that the mysterious tomato disease in causal agent of tomato yellow leaf curl the Dominican Republic was indeed The silverleaf whitefly is a vector of was identified as a geminivirus in 1988 caused by TYLCV. By 1997, the re- tomato yellow leaf curl in other countries. and it is now known that a number of searchers also showed that the DNA distinct geminiviruses cause tomato sequence (the genetic code) of the Do- yellow leaf curl-like symptoms in dif- minican Republic ferent parts of the world. virus was 98% Although TYLCV is not present in identical to that of A California, it has recently become es- an Eastern Mediter- tablished in the Caribbean. By the late ranean isolate of 1980s, the Dominican RepubIic had a TYLCV (Nakhla et flourishing processing tomato indus- al. 1994; Polston et try. Enough tomatoes were grown to al. 1994).Thisvirus supply the local canneries that pro- had previously duced tomato paste for the country’s been reported only nearly 7 million people. However, in in the Eastern the fall of 1992, unusual virus-like Hemisphere and symptoms were noticed on tomato had probably been plants in the northern production ar- inadvertentIy intro- eas, and during the 1992-93 growing duced into the Do- season entire seed beds and fields minican Republic R were lost. One year later, in 1993, these through infected viral symptoms were observed in plants or through southern production areas as well. By whiteflies carrying the 1993-94 growing season, losses the virus on plant had reached millions of dollars, and material (such as the tomato acreage in the Dominican tomato cuttings or Republic had fallen by more than 75%. transplants). Canneries stayed in production by im- Once the iden- porting tomato paste but small farm- tity and distribu- ers who depended on tomato produc- tion of the virus in- tion for much of their living were fecting the devastated and the tomato production Detection of tomato yellow leaf curl geminivirus (TYLCV) in tomatoes was whiteflies shows that a host-free period in the Dominican areas became economically depressed. known, the Do- Republic dramatically reduced virus incidence. TYLCV was Based on the association of the minican Republic detected in whiteflies at different times throughout the year problem with high populations of Ministry of Agri- with a polymerase chain reaction (PCR) test (the presence of the virus is revealed by a fluorescent DNA fragment vi- whiteflies and the severity of the culture and the sualized with agarose gel electrophoresis, as shown in symptoms, the disease was suspected processing tomato panels A and B). During the growing season, whiteflies to be caused by a whitefly-transmitted industry formu- from all tomato fields sampled carried TYLCV (lanes 1-14 in geminivirus. The predominant disease panel A; each lane represents a field), whereas during the lated a strategy to host-free period, whiteflies from native vegetation carried symptoms were stunted growth and revive the once- almost no TYLCV (lanes 1-16 in panel B). Lane 17 repre- leaf yellowing and upcurling, which flourishing indus- sents a negative control (water only), whereas lanes 15 and differed greatly from the symptoms try. The strategy’s 16 in panel A and 1 and 19 in panel B are positive controls (TYLCV-infected tomato tissue). associated with known Western Hemi- key component was

CALIFORNIA AGRICULTURE, MARCH-APRIL 1998 25 Other suc- diversity of crops grown and the wide cessful compo- plant host range of the silverleaf white- nents of the fly. These factors would make it diffi- strategy in- cult to implement solutions that have cluded planting helped elsewhere, such as the host-free TYLCV-resistant period in the Dominican Republic. tomato varieties There are at least three possible and hybrids and ways new whitefly-transmitted using insecti- geminiviruses could enter/appear in cides (primarily California: (1) introduction via in- the systemic in- fected plant material or viruliferous secticide imida- whiteflies on plant material; (2) intro- cloprid) to man- duction via flights of viruliferous age whitefly whiteflies from areas with whitefly- populations on transmitted geminiviruses; and (3) Citrus tristeza virus is not a serious tomatoes. As a evolution of new geminiviruses from problem in San Joaquin Valley citrus, but result, the Do- native forms. However, the most likely could become so if the brown citrus aphid minican Republic’s processing tomato way TYLCV could enter California is is introduced. industry has rebounded dramatically, via infected plant material. Thus, mea- and the 1996-97 growing season pro- sures should be taken to prevent vided record yields that kept canneries movement of plant material, especially from importing tomato paste for the tomato transplants, into California first time in 6 years. from areas known to have TYLCV or However, it is unlikely that TYLCV other whitefly-transmitted will ever be eradicated from the Do- geminiviruses. minican Republic. Moreover, TYLCV It is also important to be able to has spread from the Dominican Re- rapidly detect and characterize TYLCV public, possibly via long-distance and other whitefly-transmitted movement of viruliferous whiteflies geminiviruses. We have developed a (those carrying the virus), to neighbor- number of DNA-based rapid detection ing Caribbean countries including tests for TYLCV and other gemini- Haiti, Jamaica and Cuba. The virus viruses. Anyone seeing tomatoes with was also found for the first time in TYLCV-like symptoms should send Florida in 1997, where it was initially samples to R.L. Gilbertson at the De- detected in tomato transplants sold in partment of Plant Pathology, UC retail stores for home gardens (as Davis, for testing; results can be ob- seedlings in pony-packs). This raises tained in 24 to 48 hours. the disturbing possibility that TYLCV If the virus did become established has been inadvertently spread in California, it would be imperative throughout the state via infected to- to develop TYLCV-resistant tomato Symptoms of CTV can be severe as shown here, where a ‘Valencia’ scion on a sour mato transplants. However, whether varieties. This is being approached us- orange rootstock has been infected. the virus will become established in ing conventional breeding as well as Florida remains to be seen. biotechnology methods. TYLCV is clearly a potential threat prohibiting whitefly host crops (such to California’s tomato industries, par- Citrus tristeza virus as tomatoes, beans, melons and pep- ticularly because one of its vectors (the The most economically important pers) from being grown between the silverleaf whitefly) is well established virus disease of citrus in the world is end of June (which is when the pro- in Southern California and seems to be tristeza, which means sadness in Portu- cessing tomato s&on ends) and the expanding northward. Thus, introduc- guese. Tristeza is caused by citrus end of September. Whitefly popula- ing TYLCV into an area with tomato tristeza virus (CTV), which belongs to tions and TYLCV were further re- production and high whitefly numbers the Closteroviridae, a family of plant duced by plowing under old tomato could result in the rapid spread and viruses that have extremely large ge- fields. During this host-free period, establishment of the virus in Califor- nomes (amounts of genetic material). whitefly populations and the inci- nia. Once established, TYLCV would Their large genomes are thought to dence of TYLCV in whiteflies de- probably be difficult if not impossible contribute to the tremendous variabil- creased dramatically. to eradicate from California due to the ity of CTV strains. Plant responses to

26 CALIFORNIA AGRICULTURE, VOLUME 52, NUMBER 2 CCPP: preventive medicine for citrus

CTV vary widely: some have such mild symptoms that they are not readily discernible while others un- dergo quick decline and death. CTV may have a severe impact on citrus when susceptible rootstock/scion combinations are infected. Because the severity of the symptoms depends so greatly upon the citrus cultivar and the rootstock/scion combination (Garnsey et al. 19871, the relationship between symptom severity and CTV strain diversity remains unclear. The virus can be spread by grafting of in- fected budwood, but CTV is most com- monly dispersed by aphids. Several potential vectors, including Aphis gossypii, Toxoptera aurantii and Aphis spireacola colonize citrus in California. Aphis gossypii has been implicated as the principal vector of CTV in Califor- nia (Dickson 1957), but is thought to be relatively inefficient in spreading the virus (Dickson et al. 1951, 1956; Grafton-Cardwell, Polek and Ullman, unpublished). Variation in CTV may alter how aphids transmit the virus, but the relationship is not fully known (Raccah et al. 1978). CTV has been present in Southern California since 1939, killing more than 3 million trees. Incidence of CTV re- mains high in Southern California and many areas are under quarantine, pro- hibiting movement of any citrus plant- ing material beyond quarantine borders. CTV was first detected in the San ved as a model for Joaquin Valley (SJV)in the early 1960s but most citrus trees there have mild or nondiscernible symptoms, even though 15%to 20% of them are on the highly CTV-susceptible sour orange rootstock (Dodd and Gumpf 1987). The reason for this is still unknown. One possibility is that CTV strains transmitted by aphids in the SJV are virus-free nursery budwood from the cause: (1) this aphid is the most effi- mild and remain so over time. A more Citrus Clonal Protection Program. cient vector of CTV, transmitting the likely possibility is that the low inci- The SJV is one of the few citrus- virus at a rate 6 to 25 times greater dence of severe tristeza in the SJV is growing regions in the world that can than A. gossypii (California’s current largely due to the continuous removal grow citrus without intensively bat- principal CTV vector); (2) this aphid of CTV-infected trees there by the tling losses to CTV. However, this has been associated with rapid spread Central California Tristeza Eradication could be changed by the introduction of CTV strains causing severe symp- Agency, as well as regulatory efforts of the brown citrus aphid (Toxoptera toms in many citrus cultivars; and (3) preventing movement of citrus plant- cituicida), a CTV vector. The introduc- the honeydew this aphid excretes ing and propagative materials from tion of the brown citrus aphid into during feeding causes serious dam- areas under quarantine, and use of California would be problematic be- age to citrus due to sooty mold for-

CALIFORNIA AGRICULTURE, MARCH-APRIL 1998 27 mation (reviewed in Yokomi et al. spread of CTV isolates through Depnrtiizeiit of Entomology, UC Riverside, 1994). grafting. mid M.L. Polek is Program Manager/ The brown citrus aphid, native to Current methods for managing Plant Pathologist, Central California China, is widely distributed through- CTV in areas where the brown citrus Tris teza Erad ica tion Agency, C DFA. out the citrus-growing regions of Asia, aphid is established include using Tke nutliors acknowledge the funding India, Australia, New Zealand, the Pa- chemicals and biological control receiued from tke Binational Agricultural cific Islands including Hawaii, sub- agents to suppress the aphids, and us- Research and Development Program, Saharan Africa and South America. ing mild CTV strains to cross-protect Grant #96-34339-3511, the US.Depart- The introduction of the brown citrus citrus trees against infection by severe ment of Agriculture National Researck aphid to South America was blamed strains. However, both approaches Init in t izle, Grant #95OO83 8, t ke Cal ifori I in for the rapid spread of CTV isolates have limitations. Efforts to suppress Citrus Resenrck Board and the California that killed tens of millions of trees dur- the brown citrus aphid may be inad- Tristeza Resenrck Conlition; tlie support ing the epidemics of the 1930s and equate because even very small num- and nssistance of Ins. Mipel Sanckez, 1940s (reviewed in Yokomi et al. 1994). bers of aphids carrying the virus can Trausagricola S.A,, Sant iago, Dom in icniz In recent years, the brown citrus aphid cause severe epidemics. The drawback Republic, in conducting TYLCV research has also been introduced into Central of cross-protection is that it works and Mnrv Kinsey, Sandra Kelley and Greg America and the Caribbean, where it only against closely related isolates Montez for their technical assistance in has spread rapidly. In 1995, it was de- and/or strains, and CTV is so geneti- conducting CTVlaphid research. tected in Florida and is now found on cally diverse that cross-protection citrus throughout the state. could fail to protect against the many References Given the rapid geographic expan- unrelated severe strains. Also, severity sion of the brown citrus aphid, the Cohen S, Antignus Y. Tomato yellow leaf is a relative measure of damage, and curl virus (TYLCV), a whitefly-borne presence of this insect in Central under some conditions the cross-pro- gerninivirus of tomatoes. 1994. In: Advances America and Florida brings it danger- tecting strains may be more severe in Disease Vector Research. Vol. 10. ously close to the borders of Califor- than is acceptable to growers. Springer-Verlag, New York. p 259-288 Dickson CR, Flock RA, Johnson M McD. nia. While the brown citrus aphid has One hope for combating CTV and 1951. Insect transmission of citrus quick de- not been detected in California crops, the brown citrus aphid may lie in in- cline virus. Econ Entornol 44:172-176. it has been intercepted at ports of en- troducing genes into citrus trees that Dickson CR, Johnson MMcD, Flock RA, try by the California Department of will make them resistant to these pests. Laird EF. 1956. Flying aphid populations in Southern California citrus groves and their re- Food and Agriculture (CDFA). For example, introducing the gene for lation to the transmission of the tristeza virus. There are several current regulatory the CTV coat protein could protect cit- Phytopathology 46:204-210. and educational efforts aimed at pre- rus trees against infection by the virus. Dodds JA, Gumpf DJ. 1991. Citrus tristeza virus in Central California. Citrograph 76: 4-1 1. venting introduction of the brown cit- Other viral genes that might protect Garnsey SM, Gumpf DJ, Roistacher CN, rus aphid and new CTV strains, to citrus trees include those that would Civerolo EL, Lee RF, Yokomi RK, Bar-Joseph prevent spread of severe CTV strains interfere with the normal replication of M. 1987. Toward a standardized evaluation of within California, and to eradicate the virus, or produce compounds that the biological properties of citrus tristeza vi- rus. Phytophylactica 19: 151-1 57. CTV in those regions where virus inci- interfere with the aphid’s transmission Polston JE, Bois D, Serra C-A, dence is relatively low. For instance, or acquisition of CTV to and from Concepcion S. 1994. First report of a tomato the CDFA recently increased surveil- plants. Another possible strategy would yellow leaf curl-like geminivirus in the West- lance for the brown citrus aphid on ag- ern Hemisphere. Plant Disease 78: 831. be to identify genes that give citrus trees Nakhla MK, Maxwell DP, Martinez RT, ricultural products entering the state. resistance to the brown citrus aphid, by Carvalho MG, Gilbertson RL. 1994. Wide- To help ensure the success of this ef- preventing feeding and establishment, spread occurrence of the Eastern Mediterra- fort, citrus personnel have been for instance. These genes could be trans- nean strain of tomato yellow leaf curl geminivirus in tomatoes in the Dominican Re- trained to identify the brown citrus ferred into citrus by traditional breeding public. Plant Disease 78:926. aphid in educational workshops con- or through biotechnology. Raccah B, Bar-Joseph M, Loebenstein G. ducted by Drs. Raymond Yokomi 1978. The role of aphid vectors and variation (USDA/ARS Fresno) and Elizabeth R.L. Gilbertson is Associate Professor, in virus isolates in the epidemiology of frisfeza disease. In Scott, P.R. and Grafton-Cardwell (Cooperative Ex- Department of Plant Pathology, and D.E. Bainbridge, A. (eds.) Plant Disease Epiderni- tension entomologist, UC Riverside). Ullman is Professor, Department of Ento- ology, Blackwell Scientific Publications, New Another effort to limit CTV in Cali- mology, UC Davis; R. Salati is Graduate York. Yokomi RK, Lastra R, Stoetzel MB, fornia is the Citris Clonal Protection Research Assistant, Department of Plant Darnsteegt VD, Lee RF, Garnsey SM, Program, which supplies virus-free Pathology, UC Davis; D.P. Maxwell is Gottwald TR, Rocha-Pena MA, Niblett CL. nursery budwood and is under the Professor, Department of Plant Pathology, 1994. Establishment of the brown citrus aphid leadership of David Gumpf, UC University of Wisconsin, Madison; E.E. (Homoptera: Aphididae) in Central America and the Caribbean Basin and transmission of Riverside plant pathologist. This Grafton-Cardwell is Associate Extension citrus tristeza virus. J Econ Entornol program has been critical to reducing lntegvated Pest Management Specialist, 87:1078-1085.

28 CALIFORNIA AGRICULTURE, VOLUME 52, NUMBER 2