Sixteenth IOCV Conference, 2005—Citrus Tristeza Virus The Response of Star Ruby Grapefruit to Different Citrus tristeza virus Isolates S. P. van Vuuren1 and B. Q. Manicom2 1Citrus Research International, P.O. Box 28, Nelspruit 1200, South Africa; 2ARC–Institute for Tropical and Subtropical Crops, Private Bag X11208, Nelspruit 1200, South Africa ABSTRACT. The failure of GFMS 12 (Nartia) as a cross-protecting Citrus tristeza virus (CTV) isolate for Star Ruby grapefruit in South Africa necessitated the use of GFMS 35 as a substitute for the interim. Seven new CTV isolates derived from Star Ruby and Rosé grapefruit trees were evalu- ated and compared to GFMS 12, GFMS 35, to two isolates from mother trees at the Citrus Founda- tion Block that were pre-immunized with GFMS 12, two severe isolates (GFSS 1and GFSS 5), and to trees that were planted virus-free. Trees pre-immunized with isolates GFMS 35 and GFMS 78 had the best production over a 5-yr period. These trees produced significantly better than trees that were planted virus-free, trees with mild isolates GFMS 12, GFMS 67, and those with the two severe isolates. The crop value (fruit size coupled with market prices) of trees with GFMS 35 was 5% better than that of trees with GFMS 78. Trees with GFMS 12a were third best and were 16% lower than those with GFMS 35. The latter isolate was collected from a good parent tree at the Cit- rus Foundation Block that was pre-immunized with GFMS 12. The results show that CTV isolate GFMS 35, which is the present pre-immunizing isolate for red grapefruit, together with isolate GFMS 78, are superior to the other isolates. However, it will be beneficial to see if the superiority will be maintained and to what extent tree life and economic production will be increased. Shoot tip grafting is a technique decline and production of small fruit. to eliminate all graft-transmissible With the initiation of the southern agents from citrus bud-wood sources African Citrus Improvement Pro- in South Africa (5). However, the gram (CIP), all grapefruit selections benefit of optimum growth and pro- are pre-immunized with the GFMS duction of virus-free trees can not be 12 CTV isolate (14). This isolate orig- utilized because of the abundance of inated from a 50-year-old Nartia Citrus tristeza virus (CTV) and its (Marsh type) grapefruit tree in the aphid vector, Toxoptera citricida Western Cape Province. Budwood (Kirkaldy) (10). Virus-free plantings source trees at the Citrus Founda- become infected with various strains tion Block (CFB) at Uitenhage (East- of CTV within a few years after ern Cape Province), which is the only planting. Many strains of CTV exist, source for propagating certified trees, and they usually occur as mixtures are evaluated visually each year for in a host (7). Factors such as virus decline and stem pitting symptoms. strain, host plant and environment In addition, each tree of every selec- will determine dominance of a spe- tion is biologically indexed on an cific strain and this may change if annual basis to establish the CTV any of these factors viz. co-infection severity. When there are indications of a new strain or extreme tempera- of severe CTV, such a tree is termi- tures, are changed (4, 9). The only nated as a bud-wood source. known method to overcome this In 1993 6-yr-old Star Ruby bud- problem is by cross-protection, a wood source trees were found with deliberate infection of virus-free various degrees of stem pitting and material with a known CTV isolate variable fruit sizes (unpublished (8). Of the commercial citrus culti- data). Biological indexing indicated vars grown in southern Africa, that severe strains were dominant grapefruit is the most sensitive to in four of the five budwood source stem pitting, which causes tree trees. At first it was thought that 112 Sixteenth IOCV Conference, 2005—Citrus Tristeza Virus 113 GFMS 12 did not protect against co- 1. GFMS 12 (derived from Nartia infection of severe strains. However, grapefruit A, the standard at the subsequent research showed the time); presence of a severe strain in the 2. GFMS 12a (derived from Star original isolate and that segrega- Ruby mother tree at CFB pre- tion of the strains, where the severe immunized with GFMS 12; strain became dominant, may have showing mild stem pitting); occurred (13). The unsuitability of 3. GFMS 12b (derived from GFMS GFMS 12 as a protecting isolate for 12 pre-immunized mother tree at Star Ruby was also confirmed in a CFB; displaying severe stem pit- field trial (12). Consequently, GFMS ting and small fruit); 35 (derived from a Rosé grapefruit 4. GFMS 35 (derived from Rosé tree) has been approved for the pre- grapefruit at Komatipoort; Marsh immunization of all red grapefruit grapefruit trees pre-immunized as an interim protector (6). with this isolate performed better The first step in searching for than trees with GFMS 12 over a mild isolates for cross protection 12-year period (11); this is the purposes is to look for old trees that present pre-immunizing isolate are healthy and produce good qual- for red grapefruit); ity fruit (8). The Star Ruby grape- 5. GFMS 65 (derived from a Star fruit industry in South Africa Ruby tree at Tambankulu started in the late 1970s and there- Estates, Swaziland); fore no trees older than 15 yr 6. GFMS 67 (similar to 5); existed at the time. To overcome this 7. GFMS 71 (derived from old bud- problem, the best producers in the wood source Star Ruby, Esselen oldest plantings at Malelane, Mpu- Nursery, Malelane); malanga Province, and Swaziland 8. GFMS 73 (similar to 7); were selected. Isolates from these 9. GFMS 77 (similar to 7); trees were evaluated in glasshouse 10.GFMS 78 (derived from 10-yr-old tests and those with the best poten- planting, F. Esselen, Malelane); tial were also evaluated in the field. 11. GFSS 1 (derived from 5-yr-old This report is on the field evalua- Marsh grapefruit tree with severe tion of the best isolates identified, stem pitting, Nkwaleni Valley); and the objective of the study was to 12.GFSS 5 (derived from 5-yr-old find superior CTV isolates for pre- Star Ruby grapefruit with severe immunization that will maximize stem pitting, F. Esselen, Male- the profitability (productive life and lane). quality) of Star Ruby grapefruit. 13.Control (trees planted as virus- free). MATERIALS AND METHODS After DAS-ELISA (double anti- body sandwich enzyme-linked Plant material and virus iso- immuno-assay) using polyclonal lates. Virus-free Star Ruby trees on antiserum (1) confirmed infection, a Swingle citrumelo rootstock were field trial was planted at Nelspruit grown under aphid-free conditions in a randomized block. Tree size was using standard nursery practices. measured and calculated as a cylin- When the scions developed to der and half sphere according to approximately 5 mm in diameter, Burger et al. (3). Fruit were har- they were bud-inoculated with differ- vested, graded in export sizes (2), ent CTV isolates. These isolates were and weighed. Tree health was moni- selected from healthy-looking trees tored by evaluating stem pitting and and they showed potential as cross- decline. protecting isolates in glasshouse The approximate monetary value tests. The following treatments were of the fruit associated with each CTV applied in replicates of five: isolate was calculated, and a projec- 114 Sixteenth IOCV Conference, 2005—Citrus Tristeza Virus tion of income was made for a hectare when compared to trees pre-immu- planting (6 × 3 spacing). The average nized with GFMS 12a, GFMS 12b, production over three years of such a GFMS 35, GFMS 73, GFMS 76, as planting was determined and the well as those that were planted value calculated according to fruit virus-free. Trees pre-immunized size distribution of each treatment. with GFMS 35 produced the largest The value of the crop in relation to crop and were significantly larger fruit size was determined by calculat- than trees pre-immunized with ing the average value per export box GFMS 12, GFMS 67, GFMS 71, (15 kg) for 10 yr. The highest price GFMS 73, as well as the trees that equalled a value of ten while the were planted virus-free and those other values were calculated accord- with the two severe isolates. The ingly. The value of the crop per hect- highest yield efficiency (kg/canopy are for each treatment was volume) was achieved by trees pre- determined by multiplying the pro- immunized with GFMS 65 (the duction of a specific fruit size (export smallest trees with a mild isolate) boxes) by the value for that size. but was not significantly better than those of trees pre-immunized with RESULTS AND DISCUSSION isolates GFMS 35, GFMS 67, GFMS 77 and GFMS 78. The highest cumu- Growth, production and disease lative yield over four seasons was rating of the 7-yr-old Star Ruby produced by trees pre-immunized by trees on Swingle citrumelo rootstock GFMS 35, but this was not signifi- are given in Table 1. The canopy vol- cantly better than those of trees pre- umes of the trees of all the treat- immunized with GFMS 12a, GFMS ments were equivalent, except for 12b, GFMS 65, GFMS 76, GFMS 77, trees pre-immunized with GFMS 65 and GFMS 78. High yields can and the two severe isolates, GFSS 1 reduce fruit size and therefore the and GFSS 5. Canopy sizes of these value of the crop. However, trees trees were significantly smaller with GFMS 35 also had the best TABLE 1 THE AVERAGE TREE SIZE, PRODUCTION, YIELD EFFICIENCY AND STEM PITTING RAT- ING OF 7-YR-OLD STAR RUBY TREES PRE-IMMUNIZED WITH DIFFERENT CTV ISOLATES.