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Hong Lin and M. Andrew Walker

The DNA extracted from cambium tissues of grape (Vitis spp., Muscadinia rotundifolia Small) rootstocks was found to be suitable for molecular analysis. Its quality was equivalent to that of DNA extracted from leaf tissues, although the yield was higher from leaves. The use of cambium tissue allows DNA extractions during dormancy or from grafted rootstocks where leaves are not available. The DNA extracted was suitable for restriction enzyme digestion and for analysis by restriction fragment length polymorphism (RFLP), randomly amplified polymorphic DNA (RAPD), and simple sequence repeats.

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Peter Cousins and M. Andrew Walker

The grape Vitis champinii Planchon is one source of nematode resistance in grape rootstocks. Several selections valued for their resistance to the root-knot nematode (Meloidogyne incognita), a serious pest of grape production, are used as rootstocks and in rootstock variety development. However, V. champinii-based rootstock varieties are faulted for their excess vigor and susceptibility to other root pests. Root-knot nematode populations with the ability to damage important V. champinii-based rootstocks have been identified and may become more common. Other V. champinii accessions might be sources of nematode resistance genes with different specificities or might have more suitable horticultural characteristics than V. champinii varieties in commercial use. Nine V. champinii accessions from the National Clonal Germplasm Repository, Davis, Calif., and a V. champinii rootstock variety were screened for resistance to M. incognita. Resistance was assessed by counting eggs produced per root system. Eight of ten V. champinii accessions did not support nematode reproduction. Susceptible accessions supported lower nematode reproduction than susceptible V. vinifera control varieties. Progeny testing from crosses of resistant and susceptible accessions suggests that a dominant and a recessive gene may condition root-knot nematode resistance.

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Kevin Fort, Joaquin Fraga, Daniele Grossi, and M. Andrew Walker

Recent and severe droughts in major grape (Vitis)-growing regions of the United States and Australia underscore the importance of more efficient agricultural use of water. Grape rootstock breeding for increased drought tolerance could contribute to continued sustainable yields as fresh water supplies decline. Rhizotron containers were used in a greenhouse to investigate the predictive measures of drought tolerance in young grapevine rootstocks. Deeper rooting distributions were found for the drought-tolerant rootstocks ‘110R’ (Vitis berlandieri × Vitis rupestris) and ‘Ramsey’ (Vitis champinii, a natural hybrid of Vitis candicans × V. rupestris) as opposed to shallower distributions observed in the more drought-sensitive rootstocks ‘101-14Mgt’ (Vitis riparia × V. rupestris) and ‘Riparia Gloire’ (V. riparia). Production of new roots during a 6-day nonirrigated period declined 45% to 53% for ‘Riparia Gloire’ and ‘101-14Mgt’, respectively, but showed no change in ‘110R’ and ‘Ramsey’. Slow growth, a hallmark of abiotic stress tolerance, was evident in the drought-tolerant rootstocks in their relatively slow shoot growth before drought stress and their relatively slow new root growth during recovery, especially for ‘Ramsey’. High stomatal conductance (g S) corresponded with drought tolerance and distinguished rootstocks best during the first 3 days of recovery, with a mean value for ‘Ramsey’ 2.7 times higher than ‘101-14Mgt’. Stomatal conductance during recovery may serve as the most efficient means of predicting drought tolerance capacity in a breeding program.

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M. Andrew Walker, Lloyd A. Lider, Austin C. Goheen, and Harold P. Olmo

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Summaira Riaz, Alan C. Tenscher, Brady P. Smith, Daniel A. Ng, and M. Andrew Walker

The North American muscadine grape (Muscadinia rotundifolia Small) is a valuable source of resistance to powdery mildew [Uncinula necator (Schw.) Burr], root-knot nematode (Meloidogyne Goeldi), dagger nematode (Xiphinema index Thorne and Allen), grape phylloxera (Daktulosphaira vitifoliae Fitch), and Pierce's disease (Xylella fastidiosa Wells et al.). Efforts to breed muscadine grapes commenced in the early 1900s and have generated a large number of cultivars and a limited number of hybrids with Vitis vinifera L. and other Vitis L. species. Collections of this germplasm are currently maintained with accession identity based on declared identity when collected, breeding records, and comparisons of morphological traits. This study reports on the first use of DNA-based simple sequence repeat (SSR) marker profiles to authenticate M. rotundifolia cultivars and hybrids. A total of 57 accessions [39 M. rotundifolia cultivars, 3 V. vinifera cultivars, 3 Vitis spp. hybrids, and 12 V. vinifera × M. rotundifolia (VR) hybrids] from collections at the U.S. Department of Agriculture National Clonal Germplasm Repository and the University of California (Davis) Department of Viticulture and Enology were analyzed with 14 SSR markers. The fingerprint profiles were used to verify published breeding records of 31 M. rotundifolia cultivars and hybrids by comparing the shared alleles of parents and progeny. Marker data indicated that four cultivars were incorrectly identified; their alleles did not match respective parent/progeny relationships at more than five loci. Two M. rotundifolia accessions had the same fingerprint profile as a third accession at all 14 markers, implicating a likely planting error. The M. rotundifolia cultivars exhibited 88 unique alleles that were not present in a database of more than 600 V. vinifera cultivars.