Rapid Detection of

Exocortis in citrus-an increasing problem for California growers-has emphasized the need for a rapid-indexing method for periodic testing of trees used as sources of budwood. Detection of this disease in symptomless citrus trees by field-indexing on sensitive indicator rootstocks has previ- ously required from one and one half to more than five years. The practical method of indexing exocortis in glasshouse plants reported here caused symptoms to de- velop within one to five months.

Methods of indexing and exocortis symptoms on citron: (1) propagation of citron (C) on a citrus seedling simultaneously inoculated with exocortis-infected buds (EX); (2) splice grafting of exocortis-infected shoot (EX) onto citron cutting (C) which is then rooted and forced to develop a shoot; (3) early (arrows) and advanced stages of cracking and scarring of underside of mid- vein; (4) control (left) and exocortis-infected shoots of citron seedling 60-13; (5) corky lesions (left) and cracking in stems of citron seedling 60-13; (6) severe epinasty and leaf curling on shoot (left) of seedling 60-13 from Etrog grafted on symptomless exocortis-infected shoot of seedling 60-7 from Etrog.

a CALIFORNIA AGRICULTURE, JANUARY, 1965 Exocortis in Citrus

E. C. CALAVAN E. F. FROLICH . J. B. CARPENTER

C. N. ROISTACHER * D. W. CHRISTIANSEN

IELD INDEXING of exocortis in citrus inedica L.) reacted severely to exocortis tion, Indio, were graft inoculated by Fplantings on sensitive indicator root- infection within 200 days. several methods with exocortis-infected stocks - principally trifoliate orange The present study showed that detec- citrus tissues. The old-clone tree carried (Poncirus trifoliata (L.) Raf.) , trifoliate tion of exocortis by inoculation of two vein enation virus (which causes out- orange hybrids, and Rangpur or Kusaie selected citron plants in the glasshouse growths) , but the seedling had no known lime (Citrus Zimonia Osbeck) -has pre- was possible within one to five months. infection. viously required from one and one half Infection caused cracking of the midvein Exocortis-infected buds and shoots of to more than five years. Although studies and epinasty (a bending-down with de- old-clone trees of Citron of Commerce and in 1961 indicated that inoculated shoots formity of leaves). Diamante citrons, Eureka lemon (C. and seedlings of Rangpur lime usually Zimon (L.) Burm. f.), and Washington dekeloped exocortis symptoms within four Experiments Navel orange (C. sinensis (L.) Osbeck) to twelve months, neither rootstocks nor Propagations of Etrog citron, PI were used as inocula for the first experi- inoculated seedlings were sufficiently reli- 109620, from a tree in the University of ment at Los Angeles. All were infected able indicators in California. A clue to California orchard at Los Angeles, and with vein enation and possibly other more rapid indexing was also reported in seedling 60-13 from Etrog, from the col- viruses. Some later experiments at River- 1961, when certain varieties of citron (C. lection at the IT. S. Date and Citrus Sta- side made use of inocula from two Eurcka

CALIFORNIA AGRICULTURE, JANUARY, 1965 9 lemon trees apparently infected only with may exist in the exocortis virus. Many exocortis. Indexing for exocortis has been citron seedlings, such as 60-7 from Etrog, completed for 244 exocortis-free and 125 were poor indicators of exocortis, photo 6. exocortis-infected trees, including many Plants of seedling 60-13 responded trees with symptoms on their rootstocks. with exocortis symptoms when inoculated Inocula from all trees with exocortis with buds infected with exocortis, and CALIFORNIA symptoms caused a reaction on the citron other viruses, such as cachexia, concave indicator plants. gum, psorosis-A, stubborn, tristeza, tris- Three effective and practicable meth- teza-seedling yellows complex, and vein and WORLD ods of indexing were: enation. Epinasty of the leaves occurred Citron on budded seedling-A cit- on plants inoculated from trees infected ron bud was grown into a shoot on an with a virus that caused stunting without s THE WORLD’S largest trading coun- exocortis-inoculated seedling of a suitable bark cracking of trifoliate orange root- A try, the United States has recog- variety such as rough lemon (C.jambhiri stocks. nized both the advantages and responsi- Lush.), Mexican lime (C. aurantifolia Plants of Etrog citron or seedling 60-13 bilities of international trade and devel- (Christm.) Swingle), or C.excelsa Wester were inoculated separately with cachexia, opment activities of the current decade. and observed for five months. The citron concave-gum, infectious-variegation, pso- Since the late President Kennedy signed bud and one or more buds from the in- rosis-A, stubborn, tatter-leaf, tristeza, the Trade Expansion Act of 1962, the fected tree were set close together or in tristeza-seedling yellows complex, vein United States has been preparing for the contact with each other, photo 1. Pre- enation, and yellow-vein viruses from “Kennedy Round” of tariff talks under inoculation or simultaneous inoculation rxocortis-free buds. Although the plants the auspices of the General Agreement and propagation favored early reaction, reacted to certain of these viruses, the on Tariffs and Trade (GATT). The tran- which sometimes occurred within one symptoms did not closely resemble those sition period has been characterized by : month in the developing citron shoot. of exocortis infection. All control plants (1) a new high level for US. exports This method made efficient use of citron remained symptomless during the index- with merchandise shipments totaling budwood and was successfully used on ing period, but several in one glasshouse nearly $22 billion for 1963, while mer- plants already budded for psorosis, tris- reacted later. Infestations of citrus red chandise imports have also risen to almost teza, and tatter-leaf indexing. mite, Metatetranychus citri (McG.), a $17 billion; (2) further growth in V.S. species of whitefly and citrus thrips, Splice graft on citron cutting- farm exports to a total of $5.6 billion in Scirtothrips citri (Moult.) were noted in 1963-a little over 27% of all U.S. ex- A diagonal splice was used to graft a this glasshouse. No natural spread of the two- to three-inch section of an exocortis- ports; (3) increased attention from the virus into control plants was apparent in U.S. as well as regional and international infected shoot onto a citron cutting, photo the insect-free quarantine glasshouse. The 2. The citron cutting was rooted and al- organizations to the trade and develop possibility of vector transmission is being ment needs of less developed countries. lowed to grow, but shoots on the inoculum investigated. piece were pinched back. To avoid confusing or erroneous results U.S. and trade expansion Budding on citron-Two or more in these tests, it was found necessary to United States merchandise exports (ex- buds of an exocortis-infected tree were maintain the citron budwood supply and cluding military sales) have increased grafted to a citron hudling or cutting. the indexing plants in separate pest-free in value from $12.3 billion in 1953 to which was then cut hack to force new compartments in order to avoid acciden- $21.9 billion in 1963, while imports for growth. tal spread of exocortis virus; to propagate consumption have increased from $11 Most plants of seedling 60-13 and control plants from each citron tree used; billion to $16.9 billion in the same period. Etrog citron PI 109620 reacted to exo- to maintain good growth in the index On the average, US. total exports since cortis infection within three months. The plants; to avoid infection of seedling 1960 (exclusive of military special ship- first symptoms were epinasty (bending stocks; and to use the most sensitive cit- ments) have been just over 3.5% of the downward) of the leaves, and cracking ron selections. gross national product and imports just of the lower side of midveins, photo 3. Rapid detection of exocortis virus has under 2.9%-the relationship remaining Some severely affected leaves curled already accelerated certain phases of almost stationary for 1962 and 1963. downward from the tip, while the blades research on citrus virus diseases and While increases have been character- twisted to varying degrees from the should soon permit the identification of istic of total US. trade, the regional des- normal plane, photo 4. Other symptoms sufficient numbers of exocortis-free trees tination of shares of U.S. total exports included dwarfing, stem epinasty, small to meet grower requirements for most has stayed remarkably stable. Canada. corky lesions or vertical cracking of the varieties. the major single-country recipient of US. stem, photo 5, and yellow blotching of exports, has a slightly smaller current the stem. Sometimes the petiole bases and E. C. Calavan is Professor, Department share (about 20%), with the main the lower sides of principal veins showed of Plant Pathology, University of Califor- growth in the “all Western Europe” share abnormal darkening. Mature leaves of nia, Riverside; E. F. Frolich is Labora- of US. exports and a steady decline in some plants dried up while firmly at- tory Technician IV, Department of Plant Latin America’s share (even excluding tached. Response of the indicator shoots Biochemistry, U. C., Los Angeles; J. B. Cuba). United States total imports have of both citron selections was about the Carpenter is Plant Pathologist, Crops had an even greater stability of division same with all methods of inoculation, but Research Division, ARS, USDA, Indio; among regions, with Canada, Europe, and variations in symptoms among plants of and C. N. Roistacher and D. W. Chris- Latin America as the major suppliers. the same clone inoculated with buds from tiansen are Laboratory Technicians, Dept. United States agricultural exports, different trees indicated that variations of Plant Pathology, U. C., Riverside. which amounted to $5.6 billion in 1963,

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