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physical properties: Citrus exocortis viroid (CEVd), Citrus bent leaf viroid (CBLVd), Hop stunt viroid (HSVd), Citrus dwarfing viroid (CVd-III), and Citrus bark cracking viroid (CVd-IV). Recently, a new citrus viroid species, tentatively named

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covalently closed, single-stranded RNA molecules that undergo autonomous replication and have the ability to induce diseases in susceptible hosts ( Diener and Raymer, 1971 ; Semancik, 1979 ). Citrus viroids are classified into: Citrus exocortis viroid

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Abstract

The ‘Poorman’ orange (New Zealand grapefruit) has been tested against other rootstocks in Israel during 15 years. After an initial slow development, the trees on ‘Poorman’ are at present the largest trees in the trial. Compared with the standard rootstock (sour orange, sweet lime and rough lemon) trees on ‘Poorman’ excel in the quantity of their yields during 10 years of fruiting. Fruit quality on this rootstock does not differ from that on the standard stocks. This rootstock seems to be fairly resistant to Phytophthora, and tolerant to the viruses of xyloporosis and exocortis, which are universally present in the old clone budwood in Israel. ‘Poorman’ appears adapted to the heavier soil types.

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Abstract

Virus-free nucellar budlines of red grapefruit and an old budline selection carrying exocortis and xyloporosis were grafted on sour orange rootstocks and grown under commercial conditions in the orchard. No differences were found in fruit quality. The old line trees produced the greatest tonnage of fruit for the first 7 years. By the tenth year the old budline trees were only 55-62% as large as the nucellar trees and were producing less fruit. Counting numbers of fruit above and below 96 ring size revealed no distinct differences in fruit size. This information should be vital to those concerned with early returns from citrus groves.

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Tree growth, yield, and fruit quality of eight lemon cultivars [Citrus limon (L.) Burro. f.] on macrophylla (Alemow) (C. macrophylla Wester) rootstock were compared when grown in sandy soil in the arid climate of south-western Arizona. `Foothill Lisbon' had higher cumulative yield and titratable acids than `Monroe Lisbon', `Prior Lisbon', `Eureka', and `Villafranca', and had larger fruit than other `Lisbon' cultivars. `Prior Lisbon' produced a larger tree canopy with lower yield efficiency than all other cultivars and did not show any decline due to sieve tube necrosis 12 years after planting. Overall, `Eureka' cultivars and `Villafranca' had lower relative cumulative yields, canopy volumes, total soluble solids content, titratable acids, and seed content, but higher tree decline than `Lisbon' cultivars. Overall, `Foothill Lisbon', in spite of carrying exocortis viroid, produced good yields and fruit quality and `Prior Lisbon' had a satisfactory growth-performance.

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Abstract

A 30 to 50% frequency of successful grafts was obtained by using 2-week-old dark grown seedlings as rootstocks and 0.14 to 0.18 mm long shoot tips as scions. The shoot tip was inserted into an inverted-T made at the top of the decapitated rootstock epicotyl. Most scion cultivars gave satisfactory grafts on ‘Troyer’ citrange, whereas lemon, lime and citron yielded successful grafts only on ‘Rough’ lemon. The grafted plants were allowed to develop in vitro under 16 hr daily exposure to 1000 lux Grow Lux illumination and were provided with a nutrient solution containing a high concentration (7.5%) of sucrose. The best source of shoot tips was the flush from defoliated branches of field trees or glasshouse plants. It was also possible to use shoot tips from flushes arising in excised lateral buds cultured in vitro. Grafted plants were transplantable to soil 5 to 8 weeks after grafting with over 95% survival. Preliminary data indicated recovery of cultivars freed from tristeza and psorosis viruses, stubborn spiroplasma and exocortis viroid. Pathogen-free plants showed no reversion to the juvenile phase.

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learned that exocortis viroid was the reason for that reputation. The commercial prospects for trifoliate orange were also improved when it became apparent that there were selections categorized primarily by flower size that affected budded tree size

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, including the ability to produce good yields of high-quality fruit that store on the tree longer than on most other citrus rootstocks. Furthermore, it is tolerant to foot rot ( Phytophthora nicotianae Breda de Haan), exocortis, and xyloporosis viroids; is

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are free of exocortis, a degenerative viroid disease to which Poncirus and their hybrids are very sensitive. Taking into account this overall set of characteristics, the objective of this trial was to determine the potential of FD dwarfing rootstock

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’ grapefruit scion, clone F57-4-X-E, for which the budwood source had been tested for viruses and viroids and no CTV, xyloporosis (X), or exocortis (E) was detected. Table 1. Height (n = 6) of ‘Marsh’ grapefruit trees grown in the Florida Indian River region on

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