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Arlie A. Powell, James Pitts, and Robert Boozer

Early flowering of peach in the southeastern U.S. often results in some annual crop loss as a result of late winter–early spring freezes. It has been shown in peach and other prunus that a fall application of ethephon delays flowering 4 to 7 days and possibly affords increased bud hardiness. However, delayed harvest and smaller fruit size of certain varieties may occur. Hydrogen cyanamide replaces lack of chilling in peach, but can also advance harvest date and possibly enhance or maintain fruit size. A randomized complete-block experimental design was used to evaluate whether hydrogen cyanamide could offset the delayed harvest and smaller fruit size disadvantages of using ethephon without advancing bloom dates over a 3-year period. Treatment combinations of ethephon (at 20%, 50%, and 90% of required chilling) and hydrogen cyanamide (at 90% to 100% of required chilling) were applied as whole-tree foliar sprays to near point of drip. Results exhibited a possible trend toward hydrogen cyanamide overcoming smaller fruit size and delayed harvest.

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Arlie A. Powell, James Pitts, and Bobby Boozer

Early flowering of peach in the Southeast can result in annual crop loss as a result of late winter—early spring freezes. It has been shown in peach and other Prunus that a fall application of ethephon delays flowering several days. However, delayed harvest and smaller fruit size of certain varieties may occur. Hydrogen cyanamide replaces lack of chilling in peach but can also advance harvest date and possibly enhance or maintain fruit size. A randomized complete-block experimental design using 12-year-old `Redhaven' trees was used to evaluate whether hydrogen cyanamide could offset the delayed harvest and smaller fruit size disadvantages of using ethephon without advancing bloom dates. Treatment combinations of ethephon (at 20%, 50%, and 90% of required chilling) and hydrogen cyanamide (at 90% to 100% of required chilling) were applied as whole-tree foliar sprays to near point of drip. Although nonsignificant, there were trends toward hydrogen cyanamide overcoming both smaller fruit size and delayed harvest induced by ethephon.

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Arlie A. Powell, James Pitts, and Bobby Boozer

Early flowering of peach in the southeastern United States can result in annual crop loss as a result of late winter-early spring freezes. In peach and other prunus, a fall application of ethephon delays flowering several days; however, delayed harvest and smaller fruit size of certain varieties may occur. Hydrogen cyanamide replaces the late stage of chilling in peach but can also advance bloom and harvest date while maintaing or enhancing fruit size. A randomized complete-block experimental design using 13-year old `Surecrop' trees was used to evaluate whether hydrogen cyanamide could offset the delayed harvest and smaller fruit size disadvantages of using ethephon without advancing bloom dates. Treatment combinations of ethephon (at 20%, 50%, and 90% of required chilling) and hydrogen cyanamide (at 90% to 100% of required chilling) were applied as whole-tree foliar sprays to near point of drip. Although not significant, there were trends toward hydrogen cyanamide overcoming both smaller fruit size and delayed harvest induced by ethephon. This agrees with an earlier study using `Redhaven'. Dormex negated the late flowering effects of ethephon applied at 20% chilling but did not cause flowering earlier than the control.

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C.A. Powell, A. Hadidi, and J.M. Halbrendt

The ability of 32P-labeled transcribed cRNA probes to detect tomato ringspot virus (TmRSV) RNA in nucleic acid extracts from roots, bark, and leaves of nectarine (Prunus persica [L.] Batsch) trees with the Prunus stem-pitting disease was assessed and compared with detection of TmRSV antigen by enzyme-linked immunosorbent assay (ELISA) in the same tissues. Neither TmRSV-specific nucleic acid nor antigen was detected in nectarine leaf tissue. ELISA detected TmRSV antigen in root extracts from 71% of the diseased trees, while dot hybridization detected virus-specific nucleic acid in 18% of the same samples. However, ELISA detected TmRSV antigen in only 47% of bark extracts; whereas TmRSV-specific nucleic acid was detected in 100% of the bark extracts from samples collected at or near the soil line. When nucleic acid extracts from bark were prepared from various locations on diseased trees and tested for TmRSV-specific nucleic acid by dot hybridization, there was an almost perfect correlation between the presence of stem-pitting symptoms and the detection of TmRSV nucleic acid. Detection of TmRSV RNA from the bark tissue of rootstock suckers from TmRSV-infected `Delicious'/MM.lO6 apple (Malus × domestica Borkh.) trees was unsuccessful using dot hybridization. The viral RNA, however, was usually detected in either leaf or root tissue of these same trees.

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Arlie A. Powell, Robert T. Boozer, and James A. Pitts

Phenological studies were conducted over a 3-year period beginning in Winter 1993–94 to relate flowering and fruiting stages of peach to heat accumulation [growing degree hours (GDH)]. Mature trees of `Loring' and `Redhaven' peach in the same orchard were used annually. Some variation from year to year was apparent in GDH levels related to 50% flower and other stages of development. Major sources for this variation appear to be timing and severity of pruning, tree vigor, and shoot diameter. Temperature predict models were used successfully to properly forecast GDH accumulation and and various flowering and fruiting stages once rest was satisfied.

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Charles A. Powell, Mark A. Ritenour, and Robert C. Bullock

The trunk diameter of ‘Valencia’ sweet orange trees tested with seven insect control strategies was measured annually for the first 5 years after planting. Yield data (marketable fruit per tree) were collected after the fourth and fifth years. The insect control treatments were Admire (imidacloprid) applied at 12, 6, 3, or 2-month intervals; Temik (aldicarb) applied annually; Meta-Systox-R (oxydemeton-methyl) applied annually; or no insect control. Trunk diameter was significantly increased by Temik treatment at 1 and 2 years after planting. Six annual applications of Admire (at 2-month intervals) significantly increased trunk diameter 2 years after planting. None of the other treatments affected trunk diameters compared with the control. There were no trunk diameter differences among treatments at 3, 4, or 5 years after planting. Both Temik applied annually and Admire applied every other month or every 3 months significantly increased yield.

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Zhipeng Huang, Phyllis A. Rundell, Xiong Guan, and Charles A. Powell

Four field sources of citrus tristeza virus (CTV) (Y3, Y6, Y7 and Y23) collected from grapefruit trees at groves in Fort Pierce, Florida, and isolate T36 were used to evaluate the transmission and separation of different virus genotypes by single brown citrus aphids (BrCA). Analysis of the field sources of CTV by inoculation to indicator plants, ELISA and RT-PCR showed that Y6 was a decline-inducing isolate and Y23 a nondecline-inducing isolate. Assays of genotype by RT-PCR indicated that Y6 contained the T36 genotype while Y23 contained the T30 genotype. Both Y3 and Y7 were a mixture of decline-inducing and nondecline-inducing CTV isolates and were a mixture of T36 and T30 genotypes. When Y6 and Y23 were the acquisition host for single BrCA, only the T36 or T30 genotypes, respectively, were detected by RT-PCR in `Mexican' Lime receptor plants. Only the T36 genotype was transmitted to receptor plants from infected Y3 and Y7 plants although these acquisition plants contained more than one genotype. No T3 or VT genotypes were detected in any acquisition or receptor plants. CTV genotype mixtures in the various field sources were separated by single BrCA transmission and that the T36 genotype in T36/T30 mixtures was more easily transmitted than the T30 genotype when the acquisition plant was `Duncan' grapefruit and the receptor plant was `Mexican' lime.

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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.