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Alexander Lang and Richard K. Volz

The effects of spur leaf removal on xylem sap flows and calcium accumulation in fruit of apple (Malus ×domestica Borkh. `Royal Gala') were determined 56 to 61 days after full bloom. Fruit calcium concentrations were reduced but fruit size was not influenced by partial spur defoliation at bloom. Apples exchanged xylem sap with the tree in daily cycles of flow reversal. The presence of local spur leaves promoted this exchange by accentuating the xylem sap drawn out of the fruit during the day, requiring more to flow back into the fruit at night to replace it. Calcium concentrations were lower in the xylem sap leaving the fruit than in that entering it. The reduced calcium accumulation in the fruit borne on defoliated spurs can therefore be attributed to the reduced volume of xylem sap exchanged between tree and fruit.

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Richard K. Volz, William V. Biasi and Elizabeth J. Mitcham

Apple (Malux ×domestica Borkh., cv. Fuji) fruit were harvested from two California orchards 190 and 210 days after full bloom and from an additional three orchards at 190 days after full bloom. Fruit were immediately exposed to 20 or 50 kPa CO2 in air at 20 °C. Area of flesh browning and tissue ethanol, acetaldehyde, and ethyl acetate concentrations for individual fruit were determined immediately before exposure and after 3 and 7 days (20 kPa) or 1 and 3 days (50 kPa) exposure to CO2. Area of flesh browning and concentrations of all compounds increased with increasing duration of exposure to high CO2, were greater in response to 50 kPa than to 20 kPa CO2, and were greater for fruit harvested later in the season. For individual orchards and for individual fruit within most orchards, greater flesh browning was associated with higher acetaldehyde concentrations after 7 days exposure to 20 kPa CO2 or 3 days exposure to 50 kPa CO2. Similarly, flesh browning was positively correlated with ethanol concentrations after 7 days at 20 kPa CO2, but was not related to tissue ethyl acetate concentrations at either CO2 partial pressure. However, higher production of ethanol, acetaldehyde, or ethyl acetate relative to flesh browning occurred during exposure to 50 kPa than to 20 kPa CO2. This suggests that the relationship between accumulation of these compounds and CO2-induced flesh browning in `Fuji' is not simply causal.

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Richard K. Volz, F. Roger Harker and Sandy Lang

Puncture force was measured in `Gala'apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] fruit from 16 to 175 days after full bloom over 2 years using a range of circular flat-tipped probes (1 to 11 mm diameter) to test the firmness of each fruit. The area-dependent (Ka) and perimeter-dependent (Kp) coefficients of puncture force were determined and were used to calculate the indicative puncture force approximating a standard 11.1-mm-diameter Effegi/Magness-Taylor probe for even the smallest fruit. Ka declined exponentially throughout fruit development with much greater changes occurring closer to bloom. In contrast, maximum Kp occurred at 107 to 119 days after full bloom before declining progressively. Estimated firmness (using a 11.1-mm-diameter probe) declined constantly from 16 days after full bloom. Ka was associated with developmental changes in cortical tissue intercellular air space, cell volume and cell packing density although relationships changed throughout fruit growth. However seasonal change in Kp was not associated with any obvious anatomical change in the cortex.

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Kathleen Delate, Andrea McKern, Robert Turnbull, James T.S. Walker, Richard Volz, Allan White, Vincent Bus, Dave Rogers, Lyn Cole, Natalie How, Sarah Guernsey and Jason Johnston

The global market for total organic product sales was $20 billion in 2005, continuing an annual growth rate of 20% to 35%. In the United States, there were 937,000 ha of certified organic land in 2003 with 5626 ha of organic apples [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.]. Increases in organic fruit production have been associated with improved pest management methods, the use of disease-resistant cultivars, and organic-focused marketing schemes. Often constrained by lower apple yields and smaller fruit size compared with conventional counterparts, key challenges for organic growers include regulation of nutrient cycling processes to maintain crop yields while minimizing the need for external inputs. In local or regional organic markets, disease-resistant apple cultivars, such as ‘Enterprise’, ‘Liberty’, ‘Redfree’, and ‘Gold Rush’, have gained increased acceptance, whereas exporting countries have continued their use of cultivars susceptible to scab [Venturia inaequalis (Cooke)]. Integrated insect pest management approaches, including the use of kaolin clay, codling moth granulosis virus, and spinosad-based insecticides, have been successfully developed to comply with export standards and quarantines, and to meet market demand. Key pests, such as codling moth [Cydia pomonella (L.)], have been managed at damage levels less than 5% using these approaches. Future pest management strategies in organic apple production will focus on development of scab-resistant cultivars with enhanced storage capability and reduction in inputs associated with negative environmental and health effects.

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Kathleen Delate, Andrea McKern, Robert Turnbull, James T.S. Walker, Richard Volz, Allan White, Vincent Bus, Dave Rogers, Lyn Cole, Natalie How, Sarah Guernsey and Jason Johnston

By 2003, organic apple [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] production had increased to 5626 ha in the United States and to 2964 ha in New Zealand by 2002. Common problems facing organic apple growers in the humid regions of New Zealand and the United States include effective management strategies for apple scab [Venturia inaequalis (Cooke)] and insect pests. Experiments conducted in Iowa in 2003–2004 demonstrated the effectiveness of a kaolin clay- and spinosad-based insecticide program in maintaining codling moth [Cydia pomonella (L.)] damage levels to less than 5% in the scab-resistant cultivars Enterprise, Liberty, Redfree, and Gold Rush. Similar pest management systems have been developed in New Zealand to comply with export standards and quarantines. The use of codling moth granulosis virus and a spinosad-based insecticide have led to reduced pest pressure and to an increase in organic exports with a 41% premium price over conventional apples. However, an association between spinosad use and woolly apple aphid [Eriosoma lanigerum (Hausmann)] population increase was observed in organic orchard surveys in 2006. An alternative to spinosad applications, insect disinfestation through controlled atmosphere (CA) treatment, was investigated to control quarantined pests and to extend the storage potential of scab-resistant cultivars. A CA treatment of 9 weeks of 2% O2 and 2% CO2 at 0.5 °C was determined to maintain firmness ratings to export standards in CA-stored, scab-resistant ‘Pinkie’ apples and to decrease internal ethylene concentration by 84% compared with apples stored in air. In addition, new scab-resistant cultivars with ‘Pinkie’ background under development in New Zealand show promise for organic production in humid regions. Few fruit quality differences were determined between ‘Pinkie’ fruits from integrated fruit production and organic production systems, although premium prices exist only for certified organic apples.

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Lidia Lozano, Ignasi Iglesias, Diego Micheletti, Michela Troggio, Satish Kumar, Richard K. Volz, Andrew C. Allan, David Chagné and Susan E. Gardiner

Single-nucleotide polymorphisms (SNPs) have been used for a range of genetic studies and are now starting to be applied for marker-assisted selection in plant breeding programs. To identify SNP markers associated with red fruit skin color, we conducted a genome-wide association (GWA) analysis in an apple (Malus ×domestica Borkh.) breeding population comprising 94 phenotyped individuals using a 384-plex SNP assay. Linkage disequilibrium (LD) analysis indicated that LD extends over a long physical distance in the population (17 Mbp), indicating that a small number of generations separates the individuals. No significant association of anthocyanin content, overcolor, and colorimetric measures (a*, b*, L*, a/b*, and hue angle) with a marker was identified, although the apple fruit skin color locus has been previously located on apple linkage group 9. Our trial of a small SNP panel for GWA in apple breeding material has demonstrated the limitation of this approach for marker trait association.