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Charles A. Powell and Youjian Lin

One hundred single brown citrus aphid (BCA) (Toxoptera citricida Kirkaldy) transmission attempts were made from each of 16 different citrus trees [8 grapefruit (Citrus paradisi Macf.) and 8 sweet orange (C. sinensis (L.) Osbeck)] previously inoculated with decline-inducing (T36-CTV), non-decline-inducing (T30-CTV), a mixture of the two Citrus tristeza virus isolate types, or no CTV. Successful CTV transmission occurred in 1.5% of attempts from grapefruit trees that had been bark-chip-inoculated with T36-CTV, 3% of attempts from orange trees inoculated with T36-CTV, 3% of attempts from grapefruit trees inoculated with both T36- and T30-CTV, 4% of attempts from orange trees inoculated with both T36- and T30-CTV, 1.5% of attempts from grapefruit trees inoculated with T30-CTV, and 3.5% of attempts from orange trees inoculated with T30-CTV. Single BCA were able to recover T30-like-CTV from trees believed to be inoculated only with T36-CTV, and T36-like-CTV from trees believed to be inoculated only with T30-CTV, suggesting that these inoculum sources were also mixtures of T36-CTV and T30-CTV. The T36-CTV was not immunologically detectable in some of the trees from which it was transmitted indicating that single BrCA can recover T36-CTV from a T36-CTV/T30-CTV mixture in which the T36-CTV is an undetectable, minority component.

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Youjian Lin and Charles A. Powell

The distribution pattern of citrus tristeza virus (CTV) T-36 isolate in leaves of infected mexican lime [Citrus aurantifolia (Christm.) Swingle] plants was visualized using a whole-leaf-blot immunoassay (WLBIA) procedure in combination with a computer scanning imaging technique and CTV-specific monoclonal antibody 17G11 (CTV MAb 17G11). The distribution pattern of CTV T-36 in leaves varied with the age of the leaves and shoots of infected plants. In the young leaves, especially the about 5-day-old leaves and the completed expanded leaves, CTV T-36 was easily detected in most of the leaf veins, the main veins and the large and small primary veins. In the old leaves, CTV T-36 only was detected in the main veins, sometimes in a few of the large primary veins with weak signals, and seldom in the small primary veins. The distribution density and immunoassay reaction signals of CTV T-36 reacted to CTV MAb 17G11 in leaves from new shoots were much higher than that in leaves from old shoots. ELISA test results using leaves with different ages from different shoots of the same mexican lime plants infected with CTV T-36 supported the visualized-test results obtained by the WLBIA in combination with computer scanning imaging technique. This is the first reported visual analysis of the distribution pattern of CTV in leaves of infected citrus plants. The results indicate that the WLBIA in combination with computer scanning imaging technique is a useful tool for studying the distribution of plant viruses in leaves of virus-infected plants.

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Ed Stover, Youjian Lin, Xiaoe Yang and Tripti Vashisth

Bloom in individual citrus (Citrus) trees often continues for more than 1 month in south Florida, with even greater bloom duration within most orchard blocks because of variation in bloom timing between trees. Prolonged bloom contributes to variable fruit maturity as harvest approaches and increases severity of postbloom fruit drop (PFD) disease (caused by Colletotrichum acutatum). Hydrogen cyanamide (cyanamide) has been effective in accelerating bloom in various deciduous fruits, and its potential use in citrus was investigated in this preliminary study. Cyanamide was applied at a range of concentrations, from 0% to 1.0% a.i., to potted trees of six citrus types reflecting fairly broad diversity in commercial citrus that was readily available as seed [alemow (Citrus macrophylla), ‘Duncan’ grapefruit (Citrus paradisi), sour orange (Citrus aurantium), ‘Smooth Flat Seville’ sour orange hybrid (C. aurantium hybrid), ‘Swingle’ citrumelo (C. paradisi × Poncirus trifoliata), and ‘Sun Chu Sha’ mandarin (Citrus reticulata)] in Dec. 1999 while trees were quiescent. Phytotoxicity increased with cyanamide rate, with some damage at 0.125% cyanamide on most tested plants, and large variation among citrus types. All cyanamide rates hastened flushing. Airblast application of cyanamide (0, 0.025%, 0.05%, and 0.10%) was made to mature trees of ‘Valencia’ and ‘Navel’ sweet orange (Citrus sinensis) in Ft. Pierce, FL, on 27 Jan. 2000. On 15 Feb. and 28 Feb. additional trees received cyanamide at 0.05%. There was considerable defoliation, which increased linearly with cyanamide rate. Flushing and flowering were unaffected by cyanamide compared with controls except in February where cyanamide applied at 0.05% increased flowers per tree in ‘Valencia’ sweet orange, and in contrast, 0.1% cyanamide on 27 Jan. reduced ‘Navel’ sweet orange flowering. Cyanamide application to ‘Valencia’ sweet orange on 28 Feb., after initial flowering but 16 days before peak bloom, significantly reduced yield per tree but there were no other effects on cropping. In these trials, cyanamide was not an effective agent for hastening bloom in south Florida citrus with applications late January through February. Further work is needed to determine whether December applications of cyanamide to trees in the field may be more effective in concentrating subsequent flush and bloom.

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Youjian Lin, R.H. Brlansky and Charles A. Powell

Six severe and six mild Florida isolates of citrus tristeza virus (CTV) were used to evaluate the transmission efficiency of the virus from grapefruit seedlings by single brown citrus aphids (Toxoptera citricida Kirkaldy) (BrCA) from colonies initiated by aphids obtained from citrus groves in Fort Pierce, Fla. The transmission rate to 2120 receptor plants [`Mexican' lime (Citrus aurantifolia)] from grapefruit by single BrCA was 1.5%. Single BrCA transmitted four of the six severe isolates and three of the six mild isolates of CTV. The average transmission rate of severe isolates was 1.8%, higher than that (0.9%) of mild isolates. Severe isolate Y-7 had the highest transmission rate among six severe isolates, 3.6%. Mild isolate Y-23 had the highest transmission rate among the mild isolates, 3.0%. The transmission rates of CTV by alatae, apterae, or nymphs of BrCA were 1.5%, 1.5%, and 1.0%, respectively. The results suggested that BrCA is an inefficient vector of CTV when the source plant is grapefruit.