Host Effects on Natural Spread of Citrus Tristeza Virus in Florida

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 transmission 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 susceptibility to citrus tristeza virus (CTV). The mechanism of these differences is complex and not well understood. Garnsey et al. (4) have described differences in virus titer of different CTV isolates in different cultivars. Some varieties such as trifoliate orange, Swingle citrumelo, and Carrizo citrange are highly resistant to CTV infection, even by graft transmission, whereas sweet orange varieties support high titers of virus. Natural spread of CTV is by aphid vectors. The variety of the acquisition host and the variety of the receptor affected CTV transmission under experimental conditions (2, 7,9). Hence, natural spread of CTV depends on a complex of factors which includes susceptibility to inoculation, virus replication, vector feeding behavior, vector reproduction, and vector movement. The purpose of this study was to gain further insight into these interactions by comparing field spread of CTV into different cultivars with some laboratory tests of vector transmission, and to measure aphid reproduction on different citrus varieties. METHODS AND MATERIALS Trees and plot. A 13-row citrus plot was planted in a commercial citrus grove in 1976 in Indiantown, Florida. Ten rows were planted with Valencia orange trees infected with T-26, a mild CTV isolate, on different rootstocks. Greenhouse-grown, 1year-old seedlings or rooted cuttings of 12 cultivars (Table 1) were planted as tristeza-free trees in rows 1 ,8 , and 13 in this plot. These trees were rootstock varieties and were planted in a randomized complete block design with six replications and three trees per replication. The entire plot area covered 1.4 ha with 80 trees per row and a tree spacing of 5.2 x 7.6 m. The primary purpose of this plot test was to monitor citrus blight incidence in grafted and nongrafted trees. Since other commercial plantings near this plot were infected with CTV, a survey was conducted of all rootstocks trees in the plot after 10 years of field exposure. These trees provided an opportunity to measure CTV spread in the field under replicated conditions. Tenth IOCV Conference CTV assay. Fully expanded young leaves or twigs were harvested from two sides of each field tree in the spring of 1985 and 1986. Bark and/ or leaf midribs were excised from these samples and assayed for CTV infection by double antibody sandwich enzyme-linked immunosorbent assay (1) using antisera to whole, unfixed virus particles of CTV isolate T-4 (3). Vector transmission. Seedlings of nine citrus varieties were grown in the greenhouse. These plants were used as receptor plants when they were ca. 10-20 cm tall. highly transmissible CTV isolate, T-67, was propagated in Madam Vinous sweet orange as the inoculum source for virus acquisition by Aphis gossypii Glover (9). Twenty aphids (late instars and adults) were transferred to each receptor plant. The virus acquisition access and the inoculation periods were each 24 h. Donor and receptor plants were maintained in an air-cooled, partly shaded glasshouse. Daily diurnal temperatures fluctuated between 20-32 C. Aphid host range. Laboratory colonies of the melon aphid were reared on okra, Hibiscus esculentum L. var. Clemson spineless, while the spirea aphid, Aphis citricola van der Goot, was reared on C. hystrix DC. Both aphids were reared in environmental chambers at 25 C with a relative humidity of 80 + 10% and a 16 h photoperiod at 5000 foot candles. Two reproductively mature apterous aphids were transferred from the colony to detached leaves of each cultivar on 1% water agar, as described by Yokomi and Gottwald (10). There were 10 replications and the test was repeated twice. Host suitability was evaluated by counting the number of live nymphs on each detached leaf at day 7. This value was called the reproductive rate. Because host preconditioning was not included prior to the test, the early reproductive rate was often influenced by the colony host. After 7 days, however, the number of live nymphs was indicative of the host suitability of the aphid. Statistical analysis was conducted by SAS (Raleigh, NC).