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M.K. Ehlenfeldt and M.R. Hall

T 286, a rabbiteye blueberry selection from a `Delite' × `Tifblue' cross, generally has been regarded as producing semi-seedless fruit. A comparison of nonpollinated flowers of T 286, `Delite', and `Tifblue' showed no differences in ovule count, and comparisons of ovules at 10, 20, and 40 days from manual cross-pollination showed no obvious evidence of embryo abortion. Manually cross-pollinated flowers contained 85, 60, and 38 seeds per fruit for `Delite', `Tifblue', and T 286, respectively. Open-pollinated fruit of T 286 had a seed count similar to that of open-pollinated `Tifblue' but possessed significantly heavier fruit. The number of seed in T 286 and `Tifblue' indicates a tendency toward parthenocarpy.

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M.K. Ehlenfeldt and A.W. Stretch

Resistance to blighting by Monilinia vaccinii-corymbosi (Reade) Honey was evaluated under greenhouse conditions in multiple populations of the diploid species Vaccinium boreale Hall & Aalders, V. corymbosum L., V. darrowi Camp, V. elliottii Chapm., V. myrtilloides Michx., V. myrtillus L., V. pallidum Ait., and V. tenellum Ait., as well as in accessions of the polyploid species 4x V. hirsutum Buckley and 6x V. corymbosum f. amoenum Aiton. Significant species differences were found in mean blighting levels averaged over 2 years, with values ranging from 3.5% for V. boreale to 49.2% for 2x V. corymbosum, compared with 27.5% for the resistant 4x V. corymbosum check, `Bluejay', and 64.3% for the susceptible 4x V. corymbosum check, `Blueray'. Wild Vaccinium species may serve as new sources of resistance to blighting, if resistance can be transferred easily and horticultural type recovered.

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M.K. Ehlenfeldt and A.W. Stretch

The resistance of 26 rabbiteye blueberry (Vaccinium ashei Reade) cultivars to the blighting phase of mummy berry disease was evaluated under controlled conditions. In 1997, blight levels ranged from 31% to 84%, and averaged 61.3% across all cultivars. In 1998, blight levels ranged from 71% to 99%, and averaged 89.9%. Several cultivars, including `Coastal', `Delite', `Centurion', `Walker', `Callaway', and `Garden Blue', exhibited significantly lower levels of mummy berry blight infection in both years. Blighting levels were significantly correlated with new shoot length in 1997, but not in 1998. Rabbiteye blueberry, in general, is less resistant to mummy berry blight than is highbush blueberry (V. corymbosum L.), but several options exist for potential improvement.

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A.W. Stretch and M.K. Ehlenfeldt

Sixty-eight highbush blueberry (Vaccinium corymbosum L.) cultivars and selections were evaluated over 3 years for their resistance to the fruit infection phase of mummy berry disease [Monilinia vaccinii-corymbosi (Reade) Honey]. Average incidence of fruit infection under test conditions was 34.3% in 1995, 14.4% in 1996, and 27.9% in 1997, with significant differences occurring among clones in all 3 test years. Several cultivars exhibited consistent resistance to mummy berry fruit infection across all years of testing. `Northsky', `Reka', `Northblue', `Cape Fear', `Bluegold', `Puru', and `Bluejay' were among the most resistant, and `Atlantic', `Berkeley', `Herbert', and `E-176' were among the most susceptible. The consistent resistant reaction of certain cultivars indicates that they may be suitable as parents for introducing resistance into a breeding program. No significant correlation was observed between blighting resistance and fruit infection resistance.

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M.K. Ehlenfeldt, A.W. Stretch, and A.D. Drape

A group of 1031 genotypes representing 245 different crosses from a joint U.S. Dept. of Agriculture-New Jersey Agricultural Experiment Station blueberry breeding program was evaluated for blueberry red ringspot virus (BBRRSV) symptoms after 8 years of field exposure. Among 41 parents represented by 10 or more progeny, significant differences were observed in offspring BBRRSV expression. The species Vaccinium lamarckii Camp. (4x) and V. amoenum Ait. (6x) and the cultivars Woodard (6x) and Earliblue (4x) seem to have high frequencies of alleles for BBRRSV resistance. Significant differences were also found among 21 different crosses. The most resistant cross was `Elizabeth' x `Earliblue', which had a 23% BBRRSV incidence. Progeny evaluation revealed that none of the parents involved produced families in which all plants were resistant; hence, resistance to this virus may be under polygenic control.

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Y.D. Park, A.A. Boe, and M.K. Ehlenfeldt

Leaf disks of potato cv. Kennebec and ND 860-2 (North Dakota potato breeding clone) were cultured on Murashige Skoog (MS) medium supplemented with 6 levels of indole acetic acid (IAA) and 7 levels of zeatin riboside (ZR). Shoots were induced at various combinations of hormone levels. The medium containing 3.5 mg/l IAA and 4.0 mg/l ZR produced the most shoots. Rooted plantlets were grown in the greenhouse. The growth of regenerated plants obtained from the MS medium supplemented with 7.0 mg/l IAA and 3.0 mg/l ZR was significantly greater than those grown from nodal explants. In ND 8602, a leaf chimera with chlorophyll deficient (light yellow) sectors was found in plants regenerated from leaf disks (grown on MS medium supplemented with 3.5 mg/l IAA and 3.0 mg/l ZR) but not in plants grown from nodal explants. Phenotypic variability was also observed for tuber number, size and weight.

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A.W. Stretch, M.K. Ehlenfeldt, and V. Brewster

In vitro conidia production by Monilinia vaccinii-corymbosi (Reade) Honey, the cause of mummy berry disease in blueberry, was significantly enhanced by cellulose acetate membranes placed on the surface of V-8 juice agar for most of the pathogen isolates tested, compared to V-8 juice agar alone. Temperature and light affected conidia production, but the effects were not consistent. Higher temperature (22 vs. 15 °C) yielded better sporulation, but the effects of light environment were variable. When 55 isolates from various sources were rated visually for sporulation on cellulose acetate membranes at 22 °C under ambient light/dark cycles, a wide range of conidium production was observed, and three of 55 isolates (6%) were identified as having very high conidia production.

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A.W. Stretch, M.K. Ehlenfeldt, and V. Brewster

Fifty-five highbush blueberry (Vaccinium corymbosum L.) cultivars and selections were evaluated over 2 years for their resistance to the shoot blighting phase of mummy berry disease [Monilinia vaccinii-corymbosi (Reade) Honey]. Blight incidence in 1993 ranged from 1% to 78% and differences among cultivars were significant. In 1994, infection levels were lower and ranged from 0% to 43%, again with significant differences among the entries. Several cultivars exhibited mummy berry blight resistance in both years. Ranking most resistant to less resistant were `Jersey', `Elliott', `Bluejay', `Duke', `Stanley', `Darrow', `Meader', and `Angola'. Among the cultivars consistently blightsusceptible were `Bluehaven', `Bluegold', `Northblue', `Croatan', `Northsky', `Sierra', `Harrison', `Coville', and `Murphy'. The consistent resistant reaction of certain cultivars indicates that they may be suitable as parents for introducing resistance into a breeding program. The evaluation methodology developed in these tests should be useful in screening germplasm for new sources of resistance and evaluating segregating progeny from crosses.

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M.K. Ehlenfeldt, A.W. Stretch, and V. Brewster

The resistance of 48 highbush blueberry cultivars and selections to the blight phase of mummy berry disease, incited by the fungus Monilinia vaccinii-corymbosi (Reade) Honey, was examined in relation to percent Vaccinium angustifolium Ait. ancestry, season of fruit maturity, and shoot growth during the primary infection phase. Correlations of percent blighting with percent V. angustifolium ancestry were significant across 3 years, but correlations with fruit maturity were significant in only 2 of 3 years. Correlations of percent blighting with early shoot growth were significant in both years measured, with r values of 0.54 in 1994, 0.83 in 1995, and 0.83 across years. A multiple regression found only shoot growth highly significant for susceptibility and rendered V. angustifolium ancestry and season of fruit maturity nonsignificant. Resistant cultivars exhibiting early shoot elongation suggest that resistance can be either biochemically or escape based.

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M.K. Ehlenfeldt, A.D. Draper, and J.R. Clark

In the 1970s, the U.S. Department of Agriculture (USDA) began developing low-chill-adapted highbush blueberry (Vacchizium corymbosum L.) for the southern United States (lat. 29° to 32°N) by using germplasm of the native southern species, V. darrowi Camp. This breeding work resulted in the release of several low-chill southern highbush blueberry (SHB) cultivars in the mid-1980s. These cultivars have been evaluated for yield and adaptation at several locations through the southern regional blueberry germplasm evaluation trials. These trials have shown that organic mulch is required for good performance of SHB. The one-fourth V. darrowi composition of SHB cultivars presents problems of freeze damage at some locations. This problem may be resolved by breeding cultivars through several alternative approaches.