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Charles A. Powell and Robert R. Pelosi

Sixty-eight percent of the `Pineapple', 52% of the `Navel', 46% of the `Valencia', 38% of the `Hamlin', and 0% of the `Ambersweet' orange [Citrus sinensis (L.) Osh.] trees in five Florida citrus nurseries were infected with severe strains of citrus tristeza virus (CTV), as demonstrated by reaction with a monoclinal antibody specific for severe strains of the virus. Severe strains of CTV infected 4%, 46%, 76%, 30%, and 48% of the trees at each of the five nurseries, respectively, indicating a considerable difference in severe strain prevalence among the nurseries. Thirty-five percent of the trees in the scion blocks (budwood source) of the nurseries also contained severe strains of CTV.

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Charles A. Powell, Phyllis A. Rundell, and Robert R. Pelosi

Bark chips from six container-grown citrus trees, infected with nondecline-inducing citrus tristeza virus (CTV) isolates and maintained in a vector-free greenhouse for 10 years, 15 commercial grapefruit (Citrus paradisi Macf.) trees, and 16 commercial sweet orange [C. sinensis (L.) Osbeck] trees were used to inoculate three indicator plants each of `Madam Vinous' sweet orange [C. sinensis (L.) Osbeck], sour orange (C. aurantium L.), `Duncan' grapefruit (C. paradisi Macf.), `Mexican' lime [C. aurantifolia (Christm.)], Swingle citrumelo [C. paradisi Macf. × Poncirus trifoliota (L.) Raf.], and sour orange grafted with `Hamlin' sweet orange [C. sinensis (L.) Osbeck]. All plants providing bark chips had repeatedly tested positive by enzyme-linked immunosorbent assay (ELISA) for CTV [reacted with monoclonal antibody (MAb) 17G11], but tested negative for Florida decline-inducing isolates of CTV (did not react with MAb MCA13). After 6 months in vector-free greenhouses, all in oculated trees (except Swingle citrumelo, which is considered CTV resistant) were positive for CTV by 17G11 ELISA. In addition, some indicator plants inoculated from nine (two container, two commercial grapefruit, and five commercial orange trees) of the 37 bark chip source trees also were positive for decline-inducing CTV by MCA13 ELISA. Some of these positive indicators also showed vein-clearing symptoms characteristic of infection with a severe isolate of CTV. No control, noninoculated indicators in the same greenhouse, became infected with either decline-inducing or nondecline-inducing CTV. These results indicate that decline-inducing isolates of CTV can be present as a minor component of a mixture at levels undetectable by ELISA, and that these decline-inducing isolates can become detectable by ELISA and sometimes by symptoms when inoculated into indicator plants.

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Charles A. Powell, Robert R. Pelosi, and Phyllis A. Rundell

None of 4190 sweet orange [Citrus sinensis (L.) Osb.] nursery trees of `Hamlin', `Midsweet', `Navel', and `Valencia' sampled from five Florida citrus nurseries were infected with a decline-inducing isolate of citrus tristeza virus (CTV) as judged by enzyme-linked immunosorbent assay (ELISA) using isolate-specific monoclonal antibodies. Two of the nurseries had a relatively high level of infection (37% to 100% of composite samples containing tissue from 10 trees) with nondecline-inducing (mild) isolates of CTV, depending on the cultivar. Three of the nurseries had a lower incidence of mild CTV (0% to 22% of 10 tree composite samples). No nursery was CTV-free. ELISA of individual trees used as budwood sources by the nurseries revealed that one tree out of 260 tested contained decline-inducing CTV, and 83 contained mild CTV. These results suggest that the budwood certification program adopted in 1997 has virtually eliminated decline-inducing CTV from commercial budwood supplies.

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Charles A. Powell, Michael S. Burton, Robert R. Pelosi, Phyllis A. Rundell, Mark A. Ritenour, and Robert C. Bullock

The population densities of the brown citrus aphid (BrCA) (Toxoptera citricidus Kirkaldy) and the spirea aphid (SA) Aphis spiraecola Patch were monitored by scouting weekly for 6 years in a replicated citrus plot treated with 7 insect control regimes: Admire (imidacloprid) applied at 12, 6, 3, or 2 month intervals; Temik applied annually; Meta-Systox-R applied annually; or no insect control. The numbers of both aphid species varied greatly from month to month and year to year. The brown citrus aphid was normally only detected in the fall (August through December) with populations peaking in September, October, or December depending on the year. The spirea aphid could be detected throughout the year during years when overall populations were high. Spirea aphid populations often peaked both in the spring and fall. Annual applications of Temik or Metasystox were ineffective in reducing aphid populations. Generally, all four Admire treatment regimes controlled aphids, although at least 2 annual Admire treatments per year were required to control the spirea aphid during some years.

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Mark A. Ritenour, Robert R. Pelosi, Michael S. Burton, Eddie W. Stover, Huating Dou, and T. Gregory McCollum

Studies were conducted between November 1999 and April 2003 to evaluate the effectiveness of compounds applied preharvest for reducing postharvest decay on many types of fresh citrus (Citrus spp.) fruit. Commercially mature fruit were harvested two different times after the compounds were applied, degreened when necessary, washed, waxed (without fungicide), and then stored at 50 °F (10.0 °C) with 90% relative humidity. Compared to control (unsprayed) fruit, preharvest application of benomyl or thiophanate-methyl resulted in significantly (P < 0.05) less decay of citrus fruit after storage in nine out of ten experiments, often reducing decay by about half. In one experiment, pyraclostrobin and phosphorous acid also significantly decreased total decay by 29% and 36%, respectively, after storage compared to the control. Only benomyl and thiophanate-methyl significantly reduced stem-end rot (SER; primarily Diplodia natalensis or Phomopsis citri) after storage, with an average of 65% less decay compared to the control. Though benomyl significantly reduced anthracnose (Colletotrichum gloeosporioides) in two of four tests with substantial (>20%) infection and phosphorous acid significantly reduced it once, thiophanate-methyl did not significantly reduce the incidence of anthracnose postharvest. The data suggests that preharvest application of thiophanate-methyl may reduce postharvest SER and total decay similar to preharvest benomyl treatments.