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TRISTEZA the Worldwide Threat from Destructive Isolates of Citrus

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TRISTEZA the Worldwide Threat from Destructive Isolates of Citrus TRISTEZA The Worldwide Threat from Destructive Isolates of Citrus Tristeza Virus-A Review C. N. Roistacher and P. Moreno ABSTRACT. This paper reviews the effects of extremely destructive forms of citrus tristeza virus (CTV) which poses serious threats to citrus industries worldwide. These include Capao Bonito CTV in Brazil, navel orange stem pitting CTV in Peru, stem pitting 12B CTV found in the university orchards in Southern California, severe grapefruit stem pitting CTV found in South Africa, recent forms of CTV responsible for decline of sweet orange on sour orange rootstock in Florida and Israel and other severe CTV isolatesreported from Spainand elsewhere. Many ofthesedestructive CTVisolates are transmitted by Toxoptera citricidus but most can be transmitted by Aphis gossypii at relatively high levels of efficiency. The impact of recent changes in aphid transmissibility and population dynamics, and the threat of movement of T. citricidus into new regions of the world are reviewed. The appearance and impact of new strains or mutants of CTV differing in pathogenic capacities or in aphid transmissibility are discussed. Methods for the identification of new or destructive isolates of CTV are also reviewed. Concepts for prevention which include quarantine, eradication and education are presented. The immediate need is to test for presence of CTV in those countries where sour orange is the predominant rootstock. Also, to test for and eliminate very destructive forms of CTV, to strengthen quarantine laws and regulations, and to educate scientists, nurseryman and growers to the dangers involved in budwood importation and virus or vector spread. Tristeza, caused by the citrus ravages of tristeza once it begins to tristeza virus (CTV) remains today as spread. Whenever a search is made in one of the most destructive of all dis- countries where citrus is successfully eases of citrus. It is an international grown on sour orange rootstock, CTV disease recognizing no borders. The is usually found. The amount of CTV potential for spread of severe forms of found is in direct proportion to the the virus, and the movement of its prin- energy put into the search. Unaware cipal vector, Toxoptera citricidus, into growers and nurserymen traveling a- new areas, remains a constant threat. broad continue to bring in foreign bud- The destruction of many tens of mil- wood. Some of these introduced cul- lions of trees by tristeza in the 1930's tivars contain severe CTV isolates through the 1950's was a major incen- which eventually can be transmitted tive for the formation of the Interna- efficiently by local aphids (6). In tional Organization of Citrus Viroloists California and Israel, a continuing bat- (IOCV). Publications on tristeza are in tle is being fought to keep certain citrus the thousands and predominate in each growing areas free of tristeza by inten- of the eleven proceedings of the IOCV, sive tristeza eradication programs. In signifying its importance and the ex- many countries where tristeza is now tent of worldwide interest and re- endemic, exotic and destructive CTV search. isolates are a constant threat and some There are still many citrus produc- of these can debilitate and evendestroy ing countries in the Mediterranean re- the citrus industry regardless of the gion, in Central America, in Mexico or rootstock used. Also, many countries in the Caribbean Islands where the where T. citricidus is not present face predominant rootstock is sour orange. the real threat of its possible introduc- The citrus industries in these countries tion. face a real threat from the potential Eleventh ZOCV Conference The objective of this review is to old and the industry has had to gear report the serious threat posed by new itself to this phenomenon by the regu- and severe isolates of CTV; to review lar removal of declining trees and the changes in transmissibility of the virus replanting of entire orchards after they and to review some of the new tech- reach 16 to 20 years in age". One puzzl- niques for detection of CTV. Also, to ing development was the bringing of re-emphasize the importance for trees of a registered Redblush grape- strengthening quarantine regulations fruit into one area in Natal where they and to encourage campaigns to educate developed severe stem pitting. Trees and inform the public; especially those from the same parent propagated in in the citrus industry on this most seri- other areas showed few symptoms. ous threat. Climatic differences may be a factor in this differential symptom develop- NEW DESTRUCTIVE ISOLATES ment. OF CTV. At the variety collection in Several severe and potentially de- Kuchinotsu, Japan, several trees of structive isolates of CTV have been virus-free Marsh grapefruit imported reported. Some attack the scions di- from California were grown at the ex- rectly and are not dependent on sensi- periment station. During the first few tive rootstooks. Some isolates may be years, the fruit was normal and large new to a region and can severely affect sized. However, afterthe third fruiting the bud unions of citrus on CTV-sensi- year, the fruit was considerably smal- tive rootstocks. Examples are: ler. Many fruit were not larger than small lemons and most were very small -- Grapefruit stem pitting isolates in (Fig. 1). Apparently, local severe iso- South Africa (15, 40), Australia lates of CTV were transmitted by T. (14), and Japan (42). citricidus to these virus-free grape- -- The Capao Bonito isolate attacking fruit, inducing the smaller sized fruit. sweet orange in Brazil (12,45,46). In the extensive variety collection -- The severe 12-B sweet orange stem at the Citrus Research Station at pitting isolate reported from Riverside, California, yearly surveys California (11, 57). between 1970 and 1977 showed -- The sweet orange stem pitting iso- dieback, decline and death of many late destroying navel orange pro- trees. Many of these were seedling duction in Peru (59). trees. The number of declining trees -- Recently appearing destructive increased exponentially each year and isolates of CTV affecting sweet indexing showed presence of CTV-SY orange on sour orange rootstock in isolates in many trees showing branch Florida (10). dieback. This suggested that seedling -- A recent development of a new yellows tristeza was spreading in the CTV isolate destroying trees in Is- variety collection (54). At this same rael (8, 9). time a very severe form of CTV was -- A new stem pittingand destructive discovered spreading in field 12B isolate affecting navel oranges in near the variety collection (11). This Australia. (This outbreak is very '12B' CTVisolate was found to severely recent and occurred in 1990, after stem pit and stunt all inoculated sweet presentation of this review). orange varieties. Transmission studies In South Africa, grapefruit is diffi- with A. gossypii showed the 12B iso- cult to grow free of stem pitting, small late was 100% transmissible (55, 56). fruit size and tree decline. Da Graca Also, when used as challenge inoculum (15) reported that the problem of de- in a cross protection experiment to cline of grapefruit associated with stem sweet orange, there was no protection pitting is widespread. "Because of this by 18 of 20 California field isolates of decline there are few productive grape- CTV (61). Because ofthe serious poten- fruit trees in South Africaover20 years tial for destruction of this and possibly other seedling yellows or stem pitting The symptoms of stem-pitting and isolates, an extensive indexing pro- small fruit size found in Peru appeared gram was undertaken to test for and similar to the symptoms of Capao Boni- destroy all severe forms of CTV in the to CTV-infected trees seen in Brazil. Citrus Research Station orchards. Also, the easy breakage due to exces- In Capao Bonito County, San Paulo sive stem pitting and the puffed and State, Brazil. a severe CTV isolate was cheesy bark were the same as that ob- discovered by Muller et al. (45, 46). served in sweet orange seedlings in- "This particular strain induced stunt- oculated with the 12B isolate found in ing, stem pitting and poor and some- California. Indexing of the Peru isolate times misshapened fruits on practically at the USDA facilities at Beltsville, all sweet orange scions budded on Maryland showed exceptionally severe sweet orange or Rangpur lime root- reactions on indicator plants (26). stocks. Most sweet orange citrus types Reactions were comparable to those of previously had been considered toler- the Brazil Capao Bonito and the ant to the commonly occurring CTV California 12B isolates. strains in Brazil". It is believed that In Florida, Brlansky et al. (10) re- the Capao Bonito strain was intro- ported the decline of sweet orange duced from Japan or East Asia (60). trees on sour orange rootstock in a 100 In Peru, navel oranges grown sq. mi. area west of Ft. Pierce and in primarily on rough lemon rootstock a 200 sq. mi. area south of Labelle. were aprofitable crop domestically and Losses greater than 50% occurred in for export. Navel orange trees have some plantings. They reported that se- been declining since the 1960s. In a vere isolates of CTV causing quick de- consultancy visit to the citrus produc- cline and stunting were becoming more ing regions of Peru in August 1988 abundant in Florida. In the LaBelle under the sponsorship of concerned cit- area, 90% of the young trees were rus growers and nurserymen, the found infected with a severe stunting senior author visited the major citrus isolate. It is apparent from this report growingregions in the country. Declin- and others that new isolates of tristeza ing trees of navel orange showing small are posing a serious threat to citrus crops of very small fruit were observed plantings on sour orange rootstock in in all coastal citrus growing areas and many areas in Florida.
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