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  • Author or Editor: Patrick J. Conner x
  • Journal of the American Society for Horticultural Science x
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A detached leaf screening technique was developed for studying specific interactions between pecan [Carya illinoinensis (Wangenh.) C. Koch] cultivars and isolates of the pecan scab fungus, Cladosporium caryigenum. Monoconidial isolates were obtained from leaf scab lesions on `Wichita', `Desirable', `Cape Fear', and `Elliot'. Each isolate was then inoculated onto detached leaves of each of the four cultivars and fungal growth was observed under the microscope after eight days. `Wichita', `Desirable', and `Cape Fear' isolates produced subcuticular hyphae at a much higher frequency when inoculated back onto the cultivar from which they were isolated in comparison to the other cultivars. The `Elliot' isolate was able to produce a high frequency of subcuticular hyphae when inoculated onto `Elliot' and `Cape Fear', but not when inoculated onto `Desirable' and `Wichita'. Field inoculations conducted with the `Wichita' and `Desirable' isolates validated the detached leaf protocol. The results obtained indicate that pecan scab is composed of multiple races with a high degree of specificity for host cultivars. A rapid whole-leaf staining system is presented which appears to have wide applicability to assessing fungal growth in leaves.

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Genetic variation among pecan [Carya illinoinensis (Wangenh.) C. Koch] cultivars was studied using randomly amplified polymorphic DNA (RAPD) markers. Using a combination of primers, a unique fingerprint is presented for each of the pecan genotypes studied. The genetic relatedness between 43 cultivars was estimated using 100 RAPD markers. Genetic distances, based on the similarity coefficient of Nei & Li, varied from 0.91 to 0.46, with an average value of 0.66 among all cultivars. The phenetic dendrogram developed from cluster analysis showed relatively weak grouping association. However, cultivars with known pedigrees usually grouped with at least one of the parents and genetic similarity estimates appear to agree with known genetic relationships.

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Muscadine grape (Vitis rotundifolia) is the first native North American grape to be domesticated. During the past century, breeding programs have created a large collection of muscadine cultivars. Muscadine cultivars are usually identified by evaluating morphological traits and checking breeding records, which can be ambiguous and unauthentic. During this study, simple sequence repeat (SSR) markers were used to generate DNA fingerprinting profiles to identify muscadine cultivars and verify their reported pedigrees. Eighty-nine Vitis accessions were genotyped using 20 SSRs from 13 linkage groups. From these, 81 unique subgenus Muscadinia accessions were identified, and a core set of five SSR markers was able to distinguish all of them. Eighteen misidentifications were found, and five previously unknown accessions were matched with cultivars in the dataset. The profiles of 12 cultivars were not consistent with their reported parentage–progeny relationships. Genetic diversity was analyzed at four levels: all V. rotundifolia cultivars (N = 67); current cultivars (N = 39); historical cultivars (N = 28); and wild V. rotundifolia accessions (N = 9). There was substantial genetic diversity in both wild and historically cultivated muscadines. The principle coordinate analysis (PCoA) showed clear separation among subgenus Vitis cultivars, wild muscadine accessions, and cultivated muscadines, with PCoA1 and PCoA2 explaining 11.0% and 9.3% of the total variation, respectively.

Open Access

Genetic linkage maps were created for three apple (Malus ×domestica Borkh.) cultivars using data from two progenies (`Wijcik McIntosh' xNY 75441-67 and `Wijcik McIntosh' xNY 75441-58). The maps consist primarily of randomly amplified polymorphic DNA (RAPD) markers, but also contain six isozyme loci and four morphological markers (Rf , fruit skin color; Vf , scab resistance; Co, columnar growth habit; Ma, malic acid). Maps were constructed using a double pseudotestcross mapping format and JoinMap mapping software. An integrated `Wijcik McIntosh' map was produced by combining marker data from both progenies into a single linkage map. Homologous linkage groups from paternal maps were paired with their counterparts in the `Wijcik McIntosh' map using locus bridges composed of markers heterozygous in both parents of a progeny. The `Wijcik McIntosh' map consists of 238 markers arranged in 19 linkage groups spanning 1206 cM. The NY 75441-67 map contains 110 markers in 16 linkage groups and the NY 75441-58 map consists of 183 markers in 18 linkage groups. The average distance between markers in the maps was ≈5.0 cM.

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Stenospermocarpic seedlessness from Vitis vinifera L. is being introgressed into muscadine grape (Vitis rotundifolia Michx.) germplasm through the use of a cross-fertile hybrid of the two species. Recently, a sequence-tagged site (STS) molecular marker, p3_VvAGL11, has been developed which enables detection of the dominant allele controlling stenospermocarpic seedlessness in V. vinifera. This marker was evaluated in six Euvitis Planch. × Muscadinia Planch. hybrid progenies to determine its association with seedlessness in this material. The presence of the 214-bp seedlessness-associated p3_VvAGL11 allele in seedling vines resulted in a nearly 3-fold reduction in mean seed fresh weight (MSFW) and significantly reduced mean seed weight per berry (MSWB), percent berry weight composed of seed (BWCS), and mean berry weight (MBW). When the lack of lignified seed was used as the determinant of seedlessness, the p3_VvAGL11 marker was able to correctly judge seedlessness in ≈85% of the progeny. Analysis of seedlessness in the progenies was hampered by poor vigor and fruiting ability of the hybrid seedlings. The p3_VvAGL11 marker shows potential to speed the introduction of the stenospermocarpic seedlessness into Muscadinia germplasm by identifying seedless progeny at the seedling stage.

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The columnar mutation `Wijcik McIntosh' has attracted much attention because of its compact growth habit, which is compatible with high-density plantings. Using bulked segregant analysis, we identified several randomly amplified polymorphic DNA (RAPD) markers that displayed a close linkage with the columnar locus (Co). The RAPD marker that displayed the closest linkage was end sequenced to develop a sequence tagged site for rapidly screening segregating populations. A simple sequence repeat (SSR) of (GA)17 was identified within the DNA fragment. Four allelic forms, including an apparent null allele, could be distinguished among the cultivars tested. The null allele displayed close linkage with Co in two progenies, and we used this marker to identify the location of the gene on the apple linkage map.

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Muscadine grape (Vitis rotundifolia var. rotundifolia) is a rare crop in that it has transitioned from a wild fruiting plant to a domesticated fruit within the past 150 years. Furthermore, this domestication process was carried out by just a few institutions that published copious records of the origin and traits of the first wild selections, goals and methods of the breeding programs, and the pedigrees of releases. We thus have a near complete record of the domestication of this interesting fruit crop. Early breeding efforts made use of fewer than a dozen wild selections, most of which were collected from the coastal plain of North Carolina and South Carolina. This narrow germplasm base has led to increasing levels of inbreeding in the most recent muscadine cultivar releases. To better understand the germplasm base of muscadine, the pedigrees of 54 muscadine cultivars released since 1970 were examined. Only 15 founders (founding clones) were identified that appeared in more than two cultivars, and five of these represent open pollination events that may not indicate the addition of new genetic material. By far the most used founder was ‘Scuppernong’, which appeared in 53 of 54 pedigrees and had an average genetic contribution of 22.8%. The remaining founders varied from 0.9% to 14.8% in their average genetic contribution. Coancestry coefficients between cultivars averaged 0.18, but were often much higher among recent fresh-market releases. Analysis of vine vigor as measured by trunk caliper in seedling progenies suggests that coancestry coefficients greater than 0.23 result in below average seedling vigor. The University of Georgia muscadine breeding program is evaluating multiple wild muscadine accessions to reduce inbreeding and increase the genetic diversity of its germplasm.

Open Access