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Ralph Scorza

The genetically available range in tree fruit architecture has not been fully utilized for tree fruit breeding or production. Higher planting densities, new training systems, high coats of pruning, the need to eliminate ladders in the orchard, and mechanized harvesting require a re-evaluation of tree architecture. Dwarf, semidwarf, columnar, and spur-type trees may be more efficient than standard tree forms, especially when combined with specific production systems. Studies of the growth of novel tree types and elucidation of the inheritance of growth habit components may allow breeders to combine canopy growth characteristics to produce trees tailored to evolving production systems.

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Stephen Miller* and Ralph Scorza

Peach trees with a pillar (P) (columnar) or upright (UP) growth habit were planted at four in-row spacings (1.5, 2.0, 4.0, and 6.0 m) in 1999 and trained to a central leader or multiple leader system to evaluate their performance in an orchard environment. A standard (S) form peach cultivar (`Harrow Beauty') was included for comparison. In this replicated study using a split-split-split plot design, one-half of the trees were summer pruned (SP) 6 weeks before harvest in each growing season from 2001 to 2003. Growth habit, tree spacing, and SP had a significant effect on tree growth and time necessary for dormant pruning. Growth habit and spacing also affected time required to summer prune. Total pruning time for all growth habits was significantly greater for SP trees compared to non-SP trees. Cumulative yields per tree were greater for UP and S habit trees than P trees over the first four seasons. Per tree yields increased as the in-row spacing increased but were decreased slightly by SP. UP trees consistently produced larger size fruit than P or S trees. Potential yields per ha and pruning times based on projected best tree spacings will be presented. UP form trees provide a good transition for growers going from low-density to high-density peach systems, with significant advantages in yield and fruit quality.

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Dongyan Hu and Ralph Scorza

Since the first report of the ‘A72’ semidwarf peach [Prunus persica (L.) Batsch] tree in 1975, no new information has become available on this genotype. We evaluated the growth habit and verified the inheritance of ‘A72’ in a population of 220 progeny derived from self-pollination. Detailed tree and branch measurements revealed a unique forked-branch (FBR) character of the ‘A72’ (Nn) phenotype. The progeny segregated into 1 NN:2 Nn:1 nn. NN trees were indistinguishable from standard peach trees, Nn were FBR, and nn were dwarf. Hybrids between ‘A72’ and columnar (brbr) peach trees confirmed that FBR is inherited as a monogenic trait that appears to express incomplete dominance. ‘A72’ (Nn) trees were later blooming than sibling NN trees. The relationship (linkage or pleiotropy) between the growth habit of ‘A72’ and late bloom is not known. The structure of ‘A72’ trees presents new opportunities to breeder/geneticists, physiologists, and horticulturists to further explore the plasticity of peach tree growth and architecture that can be achieved through breeding. Applications of the ‘A72’ growth habit for commercial fruit production and as an ornamental, particularly in the dwarf form (nn) and in combination with the columnar tree (brbr) form, present opportunities that await exploration.

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Ralph Scorza and Margaret Pooler

Doubled haploid peach [Prunus persica (L.) Batsch] lines were cross-pollinated to produce F1 hybrids. F1 hybrids were evaluated at 3, 7, 8, and 9 years after field planting for tree growth as measured by trunk cross-sectional area, and for fruit production as measured by total weight, total number, and production per unit trunk cross-sectional area. Fruit quality of most F1 hybrids was within the range of quality observed in progeny of standard peach cultivars, and tree growth and productivity were similar to those of standard cultivars. F1 hybrids present the possibility of developing scion varieties that can be produced from seed, thus eliminating the need for grafting scions onto rootstocks in situations where specific, adapted rootstocks are not necessary. They could also be used to develop genetically uniform seed-propagated rootstocks. The use of doubled haploid-derived F1 peach hybrids, however, would require reliable, efficient production techniques.

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Thomas Tworkoski and Ralph Scorza

Shoot and root characteristics of four peach tree [Prunus persica (L.) Batsch (Peach Group)] growth habits (compact, dwarf, pillar, and standard) were studied. In compact trees, leaf number (1350/tree) was twice, but leaf area (6 cm2/leaf) was half that of pillar and standard trees. The number of lateral branches in compact trees (34) was nearly three times more than in pillar and standard trees. Leaf area index (total one-side leaf area per tree divided by the canopy cross-sectional area of the tree) of pillar trees was greater than compact, dwarf, and standard trees (13 compared with 4, 4, and 3, respectively) due to a narrower crown diameter. Dwarf trees were distinct with few leaves (134/tree) and less than half the roots of the other growth habits. Compact trees produced more higher order lateral (HOL) roots than pillar and standard trees. More second order lateral (SOL) roots were produced by compact than standard trees (1.2 vs. 0.8 SOL roots per centimeter first order lateral root). Pillar trees had higher shoot: root dry weight (DW) ratios (2.4) than compact and standard trees (1.7 for both) due to lower root DWs. Root topology was similar among compact, pillar, and standard peach trees but root axes between branch junctions (links) were significantly longer in compact trees. Compact trees had more and longer HOL roots in roots originating near the root collar (stem-root junction) (i.e., more fibrous roots) and this appeared to correlate with more lateral branches in the canopy. These results indicate significant differences in root as well as shoot architecture among growth habits that can affect their use as scion or rootstock cultivars.

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Thomas W. Zimmerman and Ralph Scorza

This study examined stage I peach shoot growth under various photoperiods in combination with different vessel closures and compared the influence of BA and Thidiazuron (TDZ) on peach shoot growth during stage II. The basal salts were as described by Almehdi and Parfitt (1986) with 1.0 μM BA, 0.02 μM IBA, 2% sucrose, 0.1% gelrite and 0.4% agar. Shoot growth of peach clone B612615, as determined by leaf number after one month, was similar in vessels capped with Kim-Kaps, Kaputs or PM caps. Plastic foam Identi-Plugs resulted in desiccation of the medium and stressed shoots with reduced growth. A 4 h light/2 h dark photoperiod four times a day provided better growth during stage I than a 16 h light/ 8 h dark photoperiod. For stage II, established shoots of Suncrest, Georgia Bell and Evergreen were grown on MS medium supplemented with 0.02 μM IBA in combination with 1.0 or 10 μM BA or 0.1, 1.0 or 10 μM TDZ. TDZ produced excessive callus resulting in minimal shoot proliferation. Shoot proliferation from axillary buds was greatest with 10 μM BA.