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Unaroj Boonprakob and David H. Byrne

Diploid plums such as Prunus salicina, P. simonii, P. cerasifera, P. americana, P. angustifolia, P. mexicana, and their hybrids have a high level of RAPD polymorphisms. Of 71 successfully used primers, there are 417 reproducible RAPD markers and only 55 (13%) markers are not polymorphic. Genetic relationships of these diploid plums based on RAPD data is estimated using genetic distance (GD) defined as GDij = 1 – Sij, where Sij is similarity coefficient. Two similarity coefficients, Jaccard's and simple matching coefficient, are compared. Simple matching always yields higher similarity coefficients. Genetic distance within and between each gene pool: California, southeastern U.S., foreign, is estimated. Genetic distances of these diploid plums ranged from 0.32 to 0.68, and agreed well with the natural geographic distribution of the species. The cluster analysis using unweighted pair-group methods using arithmetic averages (UPGMA) was used to construct phenograms to summarize the relationships among these cultivated diploid plums and plum species.

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W.A. Retzlaff, W.W. Barnett, L.E. Williams and T.M. DeJong

Japanese plum (Prunus salicina Lindel. `Casselman') trees exposed to three atmospheric ozone partial pressure treatments were sprayed with a summer application of Volck Supreme oil (1% aqueous solution) to control an outbreak of spider mites (Tetranychus spp.). Phytotoxic effects were observed on the foliage of trees in the plots exposed to ambient or higher atmospheric ozone partial pressures 5 days following spray application. Foliage on trees exposed to 0.044 and 0.081 μPa·Pa-1 ozone [12-h mean (8 Apr. to 12 June 1992)] partial pressures developed water spotting and more foliage abscission than trees exposed to charcoal-filtered air (0.024 μPa·Pa-1 ozone). Thus, ozone air-pollution stress may predispose plants to increased phytotoxicity from summer oils.

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Marcia Vizzotto, Luis Cisneros-Zevallos, David H. Byrne, David W. Ramming and W.R. Okie

Nineteen peach [Prunus persica (L.) Batsch] genotypes and 45 plum (Prunus salicina Erhr. and hybrids) genotypes with different flesh and skin color were analyzed for their antioxidant content and activity. Anthocyanin content, phenolic content, and antioxidant activity were higher in red-flesh than in light-colored flesh peaches. Carotenoid content was higher in yellow-flesh peaches than in light-colored ones. Red-flesh plums generally had higher anthocyanin and phenolic contents than the other plums but not necessarily greater antioxidant capacity. The total phenolic content had the most consistent and highest correlation with antioxidant activity, indicating that it is more important in determining the antioxidant activity of peaches and plums than are the anthocyanin or carotenoid contents. In general, the wide range of phytochemical content and antioxidant activity found indicates that the genetic variability present can be used to develop cultivars with enhanced health benefits.

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Esmaeil Fallahi, Brenda R. Simons, John K. Fellman and W. Michael Colt

Influence of various concentrations of hydrogen cyanamide (HC) on fruit thinning of `Rome Beauty' apple (Malus domestica Borkh.), `Friar,' and `Simka' plums (Prunus salicina Lindley) were studied. A full bloom application of HC at all tested concentrations decreased `Rome Beauty' apple fruit set and yield, and increased fruit weight. Hydrogen cyanamide at 0.25% (V/V) resulted in adequate apple thinning, indicated by the production of an ideal fruit weight. Prebloom and full bloom applications of HC at greater than 0.75% reduced plum fruit set and yield in `Friar.' Full bloom application of HC at 0.25% to 0.50% showed a satisfactory fruit set, yield, and fruit size in `Friar' plum. Full bloom application decreased fruit set and yield in `Simka' plum. Hand thinning, as well as chemical thinning, is recommended for plums.

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G.E. Boyhan, J.D. Norton and J.A. Pitts

The dwarfing characteristics of St. Julien and Pixy rootstocks, measured by shoot growth, were evident with `AU-Amber' and `AU-Producer' plum (Prunus salicina Lindl.) scions. Dwarfing did not occur with `AU-Rubrum'. Trunk cross-sectional area (TCA) was reduced with `AU-Amber', `AU-Producer', and `AU-Rubrum' scions on St. Julien and Pixy rootstocks. After 3 years, tree survival was 94% for Lovell; 89%, Halford; 57%, Nemaguard; 75%, Nemared; 83%, St. Julien; and 47%, Pixy. Tree survivability was significantly lower on Nemaguard and Pixy rootstocks than on Lovell and Halford. Multiple regression of total shoot growth, TCA, and survivability against foliar nutrient content resulted in the following significant equations: 0.460Mg - 0.210Mn, 0.236B - 0.487Mn, and 0.359N + 0.398Ca - 0.267P - 0.360Fe for each, respectively. Growth, survivability, and foliar nutrient content are significantly affected by rootstock in plum production.

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Stephen M. Southwick

Commercially grown apricots (Prunus armeniaca), peaches (Prunus persica), nectarines (Prunus persica), plums (Prunus salicina and Prunus domestica), and pluots (Prunus salicina × Prunus armeniaca) have a tendency to produce high numbers of flowers. These flowers often set and produce more fruit than trees can adequately size to meet market standards. When excessive fruit set occurs, removal of fruit by hand-thinning is common to ensure that fruit size meets market standards. Over the years there have been numerous attempts to find chemical or physical techniques that would help to reduce costs associated with and improve efficiencies of hand-thinning; however, using alternate strategies to hand-thinning have not been widely adopted in stone fruit production. In the past 10 years, through the continuing efforts of scientists throughout the world in public and private institutions and business, it appears that there are chemical sprays capable of reducing the need for hand-thinning in stone fruit. Management of flowering by reducing the number of flowers on apricot, peach, nectarine, plum, and prune has shown promise under climatic conditions such as those found in the San Joaquin Valley of California. By applying gibberellins during May through July, flowers in many stone fruit cultivars can be reduced in the following season. The reduction in flower number does not generally lead to an increase in fruit set. As a result, fruit numbers are reduced, the need for hand thinning can be reduced, and in some cases eliminated. There are risks associated with reducing flower number before climatic conditions during bloom or final fruit set are known. However, given the changes in labor costs and market demands, especially in the developed world, the benefits may outweigh the risks. The application and implications of these summer gibberellin applications toward reducing flower numbers will be discussed as it relates to commercial stone fruit growing.

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Noboru Muramatsu, Naoki Sakurai, Ryoichi Yamamoto and Donald J. Nevins

A nondestructive, acoustic method was applied to evaluate firmness of nectarines (Prunus persica Batch.), apricots (Prunus mume Sieb. et Succ.), plums (Prunus salicina Lindl.), and tomatoes (Lycopersicon esculentum Mill. `Beiju'). Sound with frequencies from 200 to 2000 Hz, generated by a miniature speaker attached to the fruit surface, was received by a small microphone attached to the opposite side. The signal was monitored by an oscilloscope. Sound frequency did not change during propagation in the fruit. However, as the microphone was moved along the circumference of the fruit, a phase shift in the received signal was observed. When the distance the microphone was displaced along the surface of the fruit corresponded to a shift of exactly one wavelength, the sound wavelength propagated within the fruit could be determined. The number of sound waves within the fruit over half its circumference was calculated as a function of this distance. Mature fruit propagated shorter wavelengths and consequently more sound waves than immature fruit, indicating that the sound velocity in the mature fruit was lower than in immature fruit. This relatively simple method for measuring lower frequency suggests that the sound velocity propagated through fruit can be determined without measuring the absolute velocity.

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Dangyang Ke and Adel A. Kader

Fruits of `Bing' cherry (Prunus avium L.), `Red Jim' nectarine (Prunuspersica L.), `Angeleno' plum (Prunus salicina, L.), `Yellow Newtown' and `Granny Smith' apples (Malus domestica Borkh.), and `20th Century' pear (Pyrus serotina L.) were treated with 0.25% or 0.02% O2 (balance N2) at 0, 5, or 10C to study the effects of these insecticidal low-O2 atmospheres on their postharvest physiology and quality attributes. Development of alcoholic off-flavor was associated with ethanol accumulation, which was the most common and important detrimental effect that limited fruit tolerance to low O2. Relatively higher storage temperature (T), higher respiration rate (R), and greater resistance to gas diffusion (r) enhanced while relatively higher O2 concentration (C) and higher soluble solids concentration (SSC) reduced off-flavor development. Using a SAS computer program to do multiple regression analysis with T, C, R, r, and SSC as variables, models were developed for prediction of fruit tolerance to insecticidal low-O, atmospheres. Comparison of fruit tolerances and published information on the times required to completely kill specific insects by O2 levels at or below 1% suggests that low-O2 atmospheres have a good potential for use as postharvest quarantine treatments for some fruits.

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Carlos H. Crisosto, F. Gordon Mitchell and Zhiguo Ju

The susceptibility to chilling injury (CI) or internal breakdown (IB) was evaluated in the most currently planted yellow- and white-flesh peach [Prunus persica (L.) Batsch] and nectarine [Prunus persica var. nectarine (L.) Batsch] and plum [Prunus salicina Lindel] cultivars from different breeding sources and fruit types. Cultivars were segregated into three categories (Cat. A, B, and C) according to their susceptibility to CI or IB symptoms (mealiness and flesh browning) when exposed to 0 °C or 5 °C storage temperatures. Cultivars in Cat. A did not develop any symptoms of CI after 5 weeks of storage at either temperature. Cultivars in Cat. B developed symptoms only when stored at 5 °C within 5 weeks of storage. Cultivars were classified in Cat. C when fruit developed CI symptoms at both storage temperatures within 5 weeks of storage. Most of the yellow- and white-flesh peach cultivars developed IB symptoms when stored at both storage temperatures (Cat. C). Most of the new nectarine cultivar introductions did not develop CI symptoms when stored at 0 °C or 5 °C after 5 weeks (Cat. A). Three out of six plum cultivars tested had CI symptoms within 5 weeks storage at 0 °C. However, all of the plum cultivars tested developed CI symptoms when stored at 5 °C (Cat. B). The importance of proper temperature management during postharvest handling was demonstrated.

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Jorge Pinochet, Carolina Fernández, Cinta Calvet, Adriana Hernández-Dorrego and Antonio Felipe

Twenty-nine commercial and experimental Prunus rootstocks, most with incorporated root-knot nematode [Meloidogyne javanica (Traub.) Chitwood] resistance, were evaluated against mixtures comprising nine populations of the root-lesion nematode Pratylenchus vulnus Allen and Jensen. Nearly all tested materials were susceptible. Five cultivars with high resistant levels were further challenged with seven P. vulnus populations individually. `Redglow' (Prunus salicina Lindl. × P. munsoniana Wight and Hedrick) was the only rootstock that showed broad resistance to all populations. The rootstocks `Torinel' (P. domestica L.), AC-595 (P. domestica × P. insititia L.), `Marianna 4001' (P. cerasifera Ehr. × P. munsoniana), and `Felinem' [P. dulcis (Mill.) D. A. Webb × P. persica (L.) Batsch] showed resistance to one or a few P. vulnus populations. Several supposedly resistant sources proved to be susceptible. Tests of crosses made between parents of diverse genetic background with partial resistance to P. vulnus indicate that a descendant with potential P. vulnus resistance is difficult to obtain. Pathogenic diversity among P. vulnus populations appears to be high.