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Warren F. Lamboy, Jing Yu, Phil L. Forsline, and Norman F. Weeden

One of the primary progenitors of the cultivated apple (Malus ×domestica) is M. sieversii, a species native to the forested regions of Central Asia. Despite the horticultural importance of M. sieversii, little is known about its genetic variation. In this study, isozyme diversity at 18 loci was determined for 259 open-pollinated offspring belonging to 31 different maternal half-sib families collected from 14 different populations in 4 regions of central Asia. Genetic diversity statistics were computed from the resulting allele and phenotype frequencies. Cluster analysis of half-sib families showed that there was some grouping based on geographic region, but 16 of the half-sib families were most closely related to half-sib families from other regions. AMOVA, the analysis of molecular variance, indicated that most of the enzyme variability (85%) was attributable to differences among half-sib families within populations, none could be assigned to populations within regions, and 15% was due to differences among regions. In addition, no alleles were found that were both fixed in a region and unique to that region. These results suggest that plants belonging to M. sieversii effectively form a single panmictic population. Thus, a thorough sampling of a few large populations will efficiently capture most of the genetic diversity present in wild M. sieversii.

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Fenton E. Larsen and Stewart S. Higgins

Tree size, cumulative yield, yield efficiency and anchorage of 6 micropropagated (MP) apple (Malus domestica Borkh.) cultivars were determined in 1991 after 5 years of production, as compared with trees on seedling (sdlg) or M 7a roots. Trees were planted in 1984, with crops harvested from 1987 through 1991. Trees were generally smallest (trunk cross-sectional area) on M 7a and were largest with 4 cultivars (`Delicious', `Jonathan', `Rome', `Spartan') when micropropagated. `Golden Delicious' (GD) was largest on sdlg. Cumulative yield was affected by a scion × rootstock interaction, with few trends in scion or rootstock effects. Mean cumulative yield was 84 kg tree-1, 71 and 58 for M 7a, MP and sdlg, respectively. Yield efficiency was also affected by a scion × rootstock interaction. In 1991, mean yield efficiency was 0.5 kg cm-2 for sdlg and MP trees, but was 1.05 for M 7a. Efficiency on M 7a was superior to other rootstocks with all scions except `GD', while sdlg and MP trees were statistically similar with all scions. All trees leaned in response to prevailing westerly winds, with trees on sdlg tending to be more upright than MP or M 7a trees.

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Amy F. Fulcher, Thomas G. Ranney, James D. Burton, James F. Walgenbach, and David A. Danehower

Japanese beetles (JB), Popillia japonica Newman, are destructive, highly polyphagous herbivores that show a general preference for Rosaceous plants. Choice and nochoice feeding assays were conducted to determine the level of resistance among 10 taxa of Malus spp. Mill. Under no-choice conditions, M. baccata (L.) Borkh. `Jackii', M. × `Hargozam' Harvest Gold and M. transitoria (Balatin) Schneider `Schmitcutleaf' Golden Raindrops were highly resistant, with <2 cm2 leaf area consumed in 24 hours. M. × `Radiant' was highly susceptible, with 7.6 cm2 consumed, and the remaining six cultivars were intermediate. Under choice conditions, eight taxa were resistant with <10% defoliation, M. × `Red Splendor' was intermediate with 26%, and M. × `Radiant' was susceptible with 73% defoliation. Feeding responses to eight individual phenolics were tested in artificial diets over a range from 0 to 100 mm. Phloridzin, phloretin, naringenin, and catechin were all feeding deterrents, whereas quercetin and rutin were feeding stimulants. Chlorogenic acid stimulated feeding at low concentrations and deterred feeding at higher concentrations (i.e., a peak response). Kaempferol had no effect. Analysis of endogenous foliar phenolics showed considerable variation in concentrations among taxa. Stepwise multiple regression analysis identified phloridzin as the only endogenous phenolic that was significantly related to resistance under both choice and no-choice feeding conditions.

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Nnadozie C. Oraguzie, Sue E. Gardiner, Heather C.M. Basset, Mirko Stefanati, Rod D. Ball, Vincent G.M. Bus, and Allan G. White

Four subsets of apple (Malus Mill.) germplasm representing modern and old cultivars from the repository and apple genetics population of the Horticulture and Food Research Institute of New Zealand Limited were used in this study. A total of 155 genotypes randomly chosen from the four subsets were analyzed for random amplified polymorphic DNA (RAPD) variation. Nine decamer primers generated a total of 43 fragments, 42 of which were polymorphic across the 155 genotypes. Pairwise distances were calculated between germplasm subsets using the distance metric algorithm in S-PLUS, and used to examine intra-and inter-subset variance components by analysis of molecular variation (AMOVAR). A phenogram based on unweighted pair group method with arithmetic average (UPGMA) cluster analysis was constructed from the pairwise distances and a scatter plot was generated from principal coordinate analysis. The AMOVAR showed that most of the variation in the germplasm (94.6%) was found within subsets, suggesting that there is significant variation among the germplasm. The grouping of genotypes based on the phenogram and scatter plot generally did not reflect the pedigree or provenance of the genotypes. It is possible that more RAPD markers are needed for determining genetic relationships in apple germplasm. Nevertheless, the variation observed in the study suggests that the current practice of sublining populations in the first generation to control inbreeding may not be necessary in subsequent generations. If these results are confirmed by fully informative molecular markers, germplasm managers should reassess the structure of their genetics populations. There may be a need to combine sublines in order to capture the maximum genetic diversity available and to streamline breeding efforts.

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James F. Harbage and Dennis P. Stimart

Many physiological responses in plants are influenced by pH. The present chemiosmotic hypothesis suggests that auxin uptake into plant cells is governed by pH. Since auxin is used widely to enhance rooting, the influence of pH on 1H-indole-3-butyric acid (IBA) induced adventitious root formation was examined. Roots were initiated aseptically in 5 node apical shoot cuttings of micropropagated Malus domestica 'Gala'. Initiation was induced using a four day pulse in IBA and 15 g/L sucrose at pH 5.6 and 30C in the dark. Observations showed pH rose to 7.0 or greater within 1 to 2 days from microcutting placement in unbuffered initiation medium. Root numbers from shoots in media containing 1.5 μM IBA buffered with 10 mM 2[N-morpholino] ethanesulfonic acid (MES) to pH 5.5, 6.0, 6.5 or 7.0 with KOH resulted in average root numbers of 14.2, 10.9, 8.7, and 7.1, respectively, while unbuffered medium yielded 7,6 roots per shoot. Comparison of MES buffered medium at pH 5.5, 6.25 or 7.0 in factorial combination with IBA at 0, 0.15, 1.5, 15.0, and 150.0 μM resulted in a significant pH by IBA interaction for root number. At 0, 0.15 and 1.5 μM IBA root numbers were greatest at pH 5.5. At 15.0 μM IBA, pH 6.25 was optimal and at 150.0 μM IBA all three pH levels produced equivalent root numbers. A calorimetric assay to measure IBA removal from the initiation medium by microcuttings of `Gala' and `Triple Red Delicious' showed more IBA removal at pH 5.5 than at pH 7.0. Possible reasons for the effect of pH on adventitious root formation will be discussed.

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H.S. Aldwinckle, P.L. Forsline, H.L. Gustafson, and S.C. Hokanson

Resistance to apple scab (Venturia inaequalis) in apple cultivar breeding has been derived mainly from the Vf gene from Malus floribunda 821, which introgresses horticulturally unfavorable characters. M. sieversii, now thought to be the primary progenitor of M. × domestica, grows wild in many diverse habitats in Central Asia and can have fruit quality comparable to commercial cultivars. Since 1989, four major collections of M. sieversii have been made in Central Asia, where scab is endemic. Some seed collections have been made from trees with superior fruit, that were not infected with scab. Over a 6-year period, 3000 seedlings from 220 wild M. sieversii trees representing 10 diverse ecosystems in Kazakstan, Uzbekistan, Kyrgyzstan, and Tajikistan have been inoculated with conidia of five races and two wild types of V. inaequalis. Suspensions (270,000 conidia/ml) were applied to 4- to 8-leaved seedlings, which were incubated for 48 h at 19°C with constant leaf wetness. Symptoms for three resistant reactions were assessed 2 to 4 weeks after inoculation: A = chlorosis with crinkling (Vf type reaction); B = stellate necrotic lesions (Vr type reaction), and N = large necrotic areas (uncharacterized resistant reaction). Results indicated that nearly 20% of the seedlings showed one or more of the resistant reactions. The range of resistance within seedling populations from each of the 220 single-tree sources ranged from 0% to 75%. Significant differences existed among seedlings from each of the ecosystems. Most resistance reactions appeared to be similar to those observed for Vr from “Russian seedling.” Resistant selections with superior horticultural traits may constitute a genepool for increased efficiency of breeding scab-resistant cvs. This genepool may also be useful to address the breakdown of resistance to V. inaequalis race 6.

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Fuad Gasi, Silvio Simon, Naris Pojskic, Mirsad Kurtovic, Ivan Pejic, Mekjell Meland, and Clive Kaiser

pomological characterizations ( Nybom and Weising, 2010 ), we chose this approach in our study. SSR molecular markers have shown great promise as tools for managing Malus ex situ germplasm collections as well as for collection and preservation strategies of

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Jan Bizjak, Nika Weber, Maja Mikulic-Petkovsek, Ana Slatnar, Franci Stampar, Zobayer Alam, Karl Stich, Heidi Halbwirth, and Robert Veberic

mechanism, by which P-containing compounds affect the red coloration of apples. Materials and Methods Plant materials. The experiment was carried out in 2011 on 10-year-old trees of striped ‘Braeburn’ ( Malus domestica Borkh.) cultivar clone Hillwell

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Philip L. Forsline, E.E Dickson, and A.D. Djangaliev

The USDA National Plant Germplasm System (NPGS) sponsored a 1993 collection of wild Malus in Kazakhstan and Kyrgyzstan which followed a collection in 1989 from sites in Tajikistan and Uzbekistan. There is strong evidence that the domestic apple originated in the foothills of the Tian Shari mountains where Malus sieversii (Ldb.) M. Roem remains as a primary forest species. The goal of the recent expedition was to obtain additional genetic diversity of apple from some of the remote sites in that area with the assistance of the Kazakh hosts. While there, isolated pockets of other fruit in the wild (especially Vitis) were discovered and collected. Seed collections from the expedition are stored with the NPGS and seedling populations are being evaluated for valuable traits.

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Roberto Hauagge and James N. Cummins

Relationships among estimates for the length of bud dormancy over a wide range of cultivars of Malus × domestics Borkh. and related Malus spp. are reported. Flower bud dormancy state was estimated after fitting quadratic models for the number of days for 50% terminal budbreak (D50), bud development stage (BDS), and percent terminal budbreak (BB) after 21 days of forcing as a function of chilling unit (CU) accumulation. Cultivar, linear, and quadratic components of regression, as well as all interactions, were significant for D50, BDS, and BB when regressed against CU accumulation. Consequently, cultivars differ not only as to when changes in “dormancy intensity take place, but also in the patterns and rates at which these changes proceed. The number of CU needed to reach arbitrary stages of each characteristic at the end of dormancy was calculated and compared for all genotypes. Rates of CU accumulation until BB, BDS, and D50 reached 50%, 1.5 and 15 days, respectively, were well-correlated. These CU values were considered the end of bud dormancy or genotype chilling requirement (CR). Estimations of D50 were less variable and less subjective, and a wider range of data points could be used in the analysis. In addition, values for D50 can indicate the growth potential of buds when other indices do not show changes. A few sampling times during the dormant season may give a preliminary idea about the CR of a given genotype.