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Host Effects on Natural Spread of Citrus Tristeza Virus in Florida

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Host Effects on Natural Spread of Citrus Tristeza Virus in Florida Host Effects on Natural Spread of Citrus Tristeza Virus in Florida R. K. Yokomi and S. M. Garnsey ABSTRACT. Aphid transmission and natural spread of citrus tristeza virus (CTV) in different citrus receptor hosts were studied under field and laboratory conditions. After 10 years of field exposure, natural infection of 12 citrus cultivars segregated into three groups: 1) highly susceptible varieties (89-100%) which included Alemow, Volkamer lemon, Ridge Pineapple, Orlando tangelo, and rough lemon; 2) varieties with low susceptibility (27-53%) which included Rangpur lime, Cleopatra mandarin, sour orange, and Palestine sweet lime; and 3) resistant varieties (0%) which included Swingle citrumelo, trifoliate orange, and Carrizo citrange. In greenhouse tests, Alemow and Mexican lime seedlings were the best receptor plants for transmission of the T-67 isolate of CTV by Aphis gossypii Glover with infection rates of 38 and 31%, respectively, whereas infection rates in Pineapple and Hamlin sweet oranges and Duncan grapefruit were 19, 17, and 11%, respectively. No transmis- sion was obtained to sour orange or Carrizo citrange. Citms hystrix DC, sweet orange varieties, Temple, and grapefruit were good hosts for nymph survival and production of A. gossypii and Aphis citricola van der Goot in laboratory tests with detached leaves. In comparison, Mexican lime was a poor host while Alemow was a nonhost. Carrizo citrange and Swingle citrumelo were also nonhosts of the aphids. These observations indicated that many varieties susceptible to CTV were good hosts for citrus aphids under test conditions, but vector transmission to different receptor plants was not necessarily correlated to aphid host suitability. Index words. Citrus rootstocks, aphid vectors, resistance, epidemiology, transmission, host range. Citrus varieties differ in suscepti- METHODS AND MATERIALS bility to citrus tristeza virus (CTV). The mechanism of these differences is Trees and plot. A 13-row citrus complex and not well understood. plot was planted in a commercial cit- Garnsey et al. (4) have described dif- rus grove in 1976 in Indiantown, ferences in virus titer of different Florida. Ten rows were planted with CTV isolates in different cultivars. Valencia orange trees infected with Some varieties such as trifoliate T-26, a mild CTV isolate, on different orange, Swingle citrumelo, and Car- rootstocks. Greenhouse-grown, 1- rizo citrange are highly resistant to year-old seedlings or rooted cuttings CTV infection, even by graft trans- of 12 cultivars (Table 1) were planted mission, whereas sweet orange vari- as tristeza-free trees in rows 1,8, and eties support high titers of virus. 13 in this plot. These trees were Natural spread of CTV is by aphid rootstock varieties and were planted vectors. The variety of the acquisition in a randomized complete block de- host and the variety of the receptor sign with six replications and three affected CTV transmission under ex- trees per replication. The entire plot perimental conditions (2, 7,9). Hence, area covered 1.4 ha with 80 trees per natural spread of CTV depends on a row and a tree spacing of 5.2 x 7.6 m. complex of factors which includes sus- The primary purpose of this plot test ceptibility to inoculation, virus repli- was to monitor citrus blight incidence cation, vector feeding behavior, vec- in grafted and nongrafted trees. Since tor reproduction, and vector move- other commercial plantings near this ment. The purpose of this study was plot were infected with CTV, a sur- to gain further insight into these in- vey was conducted of all rootstocks teractions by comparing field spread trees in the plot after 10 years of field of CTV into different cultivars with exposure. These trees provided an some laboratory tests of vector trans- opportunity to measure CTV spread mission, and to measure aphid repro- in the field under replicated condi- duction on different citrus varieties. tions. Tenth IOCV Conference CTV assay. Fully expanded cal analysis was conducted by SAS young leaves or twigs were harvested (Raleigh, NC). from two sides of each field tree in the spring of 1985 and 1986. Bark and/ RESULTS or leaf midribs were excised from Field spread. After 10 years of these samples and assayed for CTV field exposure to endemic and field infection by double antibody sandwich sources of CTV and natural popula- enzyme-linked immunosorbent assay tions of aphid vectors, sufficient field (1) using antisera to whole, unfixed spread had occurred to segregate the virus particles of CTV isolate T-4 (3). citrus varieties into three categories Vector transmission. Seedlings based on number or percentage of of nine citrus varieties were grown in trees infected (Table 1). The first the greenhouse. These plants were group was highly susceptible to CTV used as receptor plants when they with infection percentages ranging were ca. 10-20 cm tall. highly trans- from 89 to 100%. This group included missible CTV isolate, T-67, was prop- Ridge Pineapple (loo%), Alemow agated in Madam Vinous sweet (loo%), Volkamer lemon (loo%), Or- orange as the inoculum source for lando tangelo (94%), and rough lemon virus acquisition by Aphis gossypii (89%). The second group was rather Glover (9). Twenty aphids (late in- poorly susceptible to CTV with infec- stars and adults) were transferred to tion ranging from 27 to 53%. This each receptor plant. The virus acquis- group included Rangpur lime (53%), ition access and the inoculation Cleopatra mandarin (50%), sour periods were each 24 h. Donor and re- orange (52%), and Palestine sweet ceptor plants were maintained in an lime (27%). The last group, which in- air-cooled, partly shaded glasshouse. cluded Carrizo citrange, remained Daily diurnal temperatures fluctuated free of CTV. between 20-32 C. Aphid host range. Laboratory colonies of the melon aphid were TABLE 1 reared on okra, Hibiscus esculentum NATURAL INFECTION BY CITRUS L. var. Clemson spineless, while the TRISTEZA VIRUS OF CITRUS CUL- spirea aphid, Aphis citricola van der TIVARS ATER 10 YEARS IN A COMMER- Goot, was reared on C. hystrix DC. CIAL GROVE IN INDIANTOWN, Both aphids were reared in environ- FLORIDAz mental chambers at 25 C with a rela- No. tive humidity of 80 + 10% and a 16 h infectedY % photoperiod at 5000 foot candles. Two Cultivar no. tested Infected reproductively mature apterous Alemow 18/18 100 aphids were transferred from the col- Volkamer lemon 17/17 100 ony to detached leaves of each cul- Ridge Pineapple 212 100 tivar on 1% water agar, as described Orlando tangelo 17/18 94 by Yokomi and Gottwald (10). There Rough lemon 16/18 89 Rangpur lime 8/15 53 were 10 replications and the test was Cleopatra mandarin 9/18 50 repeated twice. Host suitability was Sour orange 9/17 52 evaluated by counting the number of Palestine sweet lime 4/15 27 live nymphs on each detached leaf at Swingle citrumelo 0118 0 Trifoliate orange 0118 0 day 7. This value was called the re- Canizo citrange 0117 0 productive rate. Because host precon- ditioning was not included prior to the "Trees were planted in November 1976, with a test, the early reproductive rate was row spacing of 5.2 x 7.6 m. often influenced by the colony host. YCTVinfection was determined by double anti- body sandwich enzyme-linked immunosorbent After 7 days, however, the number assay. Total treeslvariety was 18. Tree deaths of live nymphs was indicative of the due to freezes or Phytophthora foot rot ac- host suitability of the aphid. Statisti- counted for tree counts less than 18lvariety. Tristeza and Related Diseases Vector transmission tests. Influ- TABLE 3 ence of host receptor variety on A. REPRODUCTION OF APHIDS ON DE- TACHED LEAVES OF DIFFERENT CIT- gossypii transmission of T-67 was de- RUS CULTIVARSz termined under laboratory condi- tions. Infection rates in Mexican lime Avg. no. and Alemow were the highest at 31% nymphslaphidY and 38%, respectively (Table 2). Li- Aphis Aphis mited tests with pummel0 and rough Host gossypii citrieola lemon receptors resulted in transmis- Hystrix sion rates of 30 and 20%, respectively. Valencia Rates for Pineapple and Hamlin Temple sweet oranges were 19 and 17%, re- Marsh spectively. Infection in grapefruit Pineapple Navel was 11%, whereas no transmission to Mexican lime sour orange or Carrizo citrange was Orlando tangelo detected. Four hosts were common to Trifoliate orange the field test described above. Volkamer lemon Rangpur lime Aphid reproduction. Nymph pro- Cleopatra mandarin duction of A. gossypii under test con- Rough lemon ditions was generally lower than that Palestine sweet lime of A. citricola on comparable hosts. Carrizo citrange Sour orange C. hystrix was the best host for the Alemow melon aphid with 37.3 nymphs per Swingle citrumelo adult (P < .05), whereas Valencia and Temple were the next best hosts at "Leaves incubated on 1% water agar for 7 days 15.9 and 15.7 nymphs per adult, re- in environmental chamber set for 25 C and 16 h photoperiod at 5000 foot candles (10). spectively (Table 3). Some reproduc- Two reproductively mature adult aphids were tion (4-5 nymphs) occurred on Pineap- placed on each detached leaf on day 0. The data ple and navel oranges, and on Mexi- represent the averages of two separate tests can lime, but this group was not per aphid host after 7 days. Each test had 10 statistically different from nonhosts replications. "Means associated with the same letter are not (0 to 1.5 nymphs). For spirea aphid, statistically different (P < .05) by Duncan's the highest reproduction was 25.6 multiple range test. TABLE 2 nymphs on Valencia followed by C. APHID TRANSMISSION OF CTV TO DIF- FERENT RECEPTOR CULTIVARS OF hystrix, navel, and Temple with 20.9, CITRUS IN THE GREENHOUSEz 18.5, and 16.9 nymphs, respectively.
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