Progenies and clones of interspecific hybrid blueberries were evaluated for annual fraction of canopy volume (FCYV) and for difference in fraction of canopy volume between control and stressed plants [FCYV(C) - FCYV(S)] in a moderate water-deficit environment. The FCYV(C) - FCYV(S) data were used to determine combining ability effects. In addition, physiological processes of attached leaves of the clones were monitored with a portable photosynthesis apparatus. Specific combining ability (SCA) effects were significant for FCYV(C) - FCYV(S). The clone with the lowest mean for FCYV(C) - FCYV(S) was US75, a hybrid of Vaccinium darrowi Camp × V. corymbosum L. Clone JU64 (V. myrsinites Lamark × V. angustifolium Aiton) also had a low FCYV(C) - FCYV(S) mean, and its two progenies (JU64 × JU11 and G362 × JU64) had low progeny means. Stomatal conductance was lowered when blueberries were exposed to atmospheric and/or soil moisture stress that resulted in lower transpiration and photosynthesis and increased or equal water-use efficiencies (WUE). Blueberry plants adjusted to moisture stress as the season progressed by lowering stomatal conductance and increasing WUE. In particular, stressed plants of US75 and JU64 had equal or higher WUE values than control plants. US226 was the most drought-susceptible clone in the study, and its stomata did not appear to be as responsive to moisture stress as the other clones. Breeding for higher WUE in a dry environment appears possible with the germplasm used in this study.
Data from a four-parent diallel, involving one highbush (Vaccinium corymbosum L.) clone and three interspecific hybrids grown on mineral soil unamended with organic matter, were analyzed to determine combining ability effects for six traits: plant size, berry size, the number of days between flowering and fruiting (# DBF&F), the ratio of total fruit weight to canopy volume (TFW: CYV), days to fruit ripe, and yield. General combining ability effects were significant for all characters tested, except yield and berry size in 1984. Specific combining ability effects were significant for plant size in 1983, #DBF&F in 1984, TFW: CYV in 1984, and berry size in 1985. Vigorous and productive highbush cultivars can be developed for mineral soils by using the interspecific clones from this study and their selected recombinant to combine the genes for plant vigor with the high-quality fruit traits of highbush cultivars.
The comparative self-compatibility, intra-, and interspecific crossability of representative clones of V. atrococcum (Gray) Heller, V. caesariense Mackenzie, V. darrowi Camp, and V. tenellum Aiton was determined. The number of germinated seeds and number of vigorous seedlings proved to be the best criteria for evaluation of crossability. The 4 species were largely self-incompatible. Generally, intraspecific and interspecific crossability levels were singificantly higher than those of the self-pollinations. Interspecific crossability was significantly lower than intraspecific crossability. Crossability levels within species and the range in crossability in individual combinations among species were extremely variable. Although F1 hybrids were produced in each of the 6 species hybridizations attempted, the rate of success was highly variable, and the direction in which the cross was made was usually critical. Specific combining ability was considerably more important than general combining ability in achieving species hybridizations. The hypothesis of complete homoploid interfertility in Vaccinium was not valid for these 4 species. The ranking of species crossability (number of vigorous seedlings per 100 pollinations) from highest to lowest was: V. atrococcum – V. caesariense, V. atrococcum – V. darrowi, V. darrowi – V. tenellum, V. caesariense – V. tenellum, V. caesariense – V. darrowi, and V. atrococcum – V. tenellum.
Adaptation to upland soils was investigated in a diallel study involving progeny of 4 blueberry (Vactinium) clones consisting of 1 highbush blueberry (V. corymbosum L.) clone and 3 interspecific hybrids. Both general and specific combining ability were significant. Seedling progenies of NJUS 11 (V. ashei × V. atrococcum) and US75 (V. darrowi × V. corymbosum) grew well on an upland soil. Although not included in the diallel crosses, NJUS 64 (V. myrsinites × V. angustifolium) progenies also grew well on upland soil.
Two clones of Vaccinium showed high field-resistance to Phytophthora cinnamomi and remained unaffected during 3 years of observation in naturally infested soil: Me-US 32, a productive, cultivated highbush selection (V. australe), and a selection from the wild of the diploid species (V. atrococcum). However, the clone, E-22 selection of V. australe, growing in adjacent row to the resistant clones was severely damaged by the pathogen. Phytophthora cinnamomi was readily isolated from soil surrounding both resistant and susceptible clones.
Diploid plants in Vaccinium section Cyanococcus, including plants of V. darrowi Camp, V. atrococcum. Heller (diploid V. corymbosum L.), a V. atrococcum × V. darrowi F1 hybrid, and a V. atrococcum × V. elliottii Chapmn. F1 hybrid, were hand pollinated in a greenhouse with pollen from diploid V. arboreum Marsh. (Section Batodendron). The resulting seeds were germinated and the seedlings were transplanted to a high-density field nursery. Forty of these F1 intersectional hybrids were selected after 2 1/2 years and transplanted to a 1.5 × 4-m spacing. Most of these plants were vigorous and flowered heavily in subsequent years, but only a small percentage of the flowers produced fruit. In 1990, however, >4000 berries were harvested from the 35 surviving plants. Open-pollinated seed from a much smaller number of berries was planted in Dec. 1987; these seeds produced ≈200 seedlings, some of which had moderate to high fruit set in a field nursery in 1989. Six of these seedlings, which were selected for high vigor, high fruit set, and characteristics intermediate between section Cyanococcus and section Batodendron, had fruit set ranging from 19.4% to 92.7% when pollinated with pollen from tetraploid V. corymbosum cultivars. One of the six seedlings was highly self-fruitful, and some intercrosses among the six seedlings produced much viable seed. Large-scale introgression of V. arboreum genes into tetraploid highbush cultivars likely will be possible by the methods used in this study.
A clone of the diploid blueberry species Vaccinium atrococcum Heller, was previously found to be highly resistant to the fungus tetraploid highbusy blueberry. The tetraploid V. atrococcum was highly root-rot resistant in a greenhouse study. It was crossed with ‘Earliblue’ and 85 seedlings were obtained. In general, the seedlings were fertile, had small, dark fruit with mild flavor, good scars and quite soft flesh consistency.
Blueberry selections Me-US 32 and Michigan Lowbush 1, and cultivars ‘Berkeley’, ‘Bluecrop’, ‘Earliblue’, and ‘Dixi’ were screened in the greenhouse for resistance to P. cinnamomi. Michigan Lowbush 1 was highly resistant to the root-rot fungus. Me-US 32 was resistant, but all the cultivars were susceptible. Michigan Lowbush 1 is a grandparent and probably the source of resistance of Me-US 32.
Interspecific blueberry (Vaccinium spp.) progenies were examined to determine combining abilities and genetic variability for seedling root system size and shoot vigor and to establish whether a large root system is correlated with good growth when plants are grown on a mineral soil and exposed to a moderate soil water deficit. General combining ability (GCA) variance components for root system size and shoot vigor and specific combining ability variance components for shoot vigor were significant. US226, a tetraploid hybrid of V. myrtilloides Michaux × V. atrococcum Heller, had the highest GCA effect for root system size and the lowest GCA effect for shoot vigor. US75 (V. darrowi Camp × V. corymbosum L.) had the highest GCA effect for shoot vigor and was second in GCA effect for root system size. Comparison of the crosses containing G111 (V. corymbosum) with those containing G362 (V. corymbosum) indicates that selecting for the best V. corymbosum clone to start a breeding program seems as important as selecting the mineral soil-adapted parent. Root system ratings were highly correlated with total dry weight of field-grown plants (r = 0.89). The method used in this study to evaluate seedlings for root system size and shoot vigor could be used to eliminate the less vigorous plants from a population before field planting and to evaluate mineral soil adaptability.
Bulk A horizon samples of 4 soils, with or without the addition of peatmoss, and 5 blueberry crosses were used in a study of the adaptability of blueberries to upland soil conditions under 3 fertilization regimes and trickle irrigation in outdoor pots. Blueberry progenies ranged from essentially pure highbush (Vaccinium corymbosum L.) to interspecific hybrids containing varying amounts of evergreen (V. darrowi Camp), lowbush (V. augustifolium Aiton), black highbush (V. atrococcum Heller), and rabbiteye (V. ashei Reade) blueberry germplasm. Blueberry growth, as measured by plant volume, initially was greatest on Manor clay loam, a Piedmont soil high in clay (30%), but by the 2nd growing season, growth was superior on Berryland soil. Various fertilizer sources affected small differences in growth. Generally those progenies that contained less highbush (V. corymbosum) parentage produced more vigorous growth. Depth of rooting and estimated root distribution were affected significantly by soil, but the addition of peatmoss had no consistent effect. Berryland sand and Manor loam soils, which represent extremes in clay content, both produced the deepest root systems. Fruiting and fruit characteristic data from the 2nd growing season indicated a significant effect of peatmoss on the Pope and Galestown soils, which resulted in lowered total fruit acidity. The Berryland soil produced fruit with the lowest total acidity. Blueberry plant growth over the first 2 seasons indicates that soil type can have pronounced effects on plant growth and rooting. These growth differences were due to soil characteristics other than particle size distribution, with fertilizer source having minimal effects on growth.
A range of soils, with or without the addition of peatmoss, and seedlings of blueberry progenies were used in an outdoor pot study to examine the adaptability of blueberries to upland soil conditions with controlled fertilizer additions and trickle irrigation. Blueberry progenies ranged from essentially pure highbush (Vaccinium corymbosum L.) to interspecific hybrids containing varying amounts of evergreen (V. darrowi Camp), lowbush (V. angustifolium Aiton), black highbush (V. atrococcum Heller), and rabbiteye (V. ashei Reade) blueberry germplasm. The soils represented the 3 physiographic regions of the eastern United States with Berryland sand used as a comparative control. Leaf analysis for N, P, K, Ca, and Mg showed significant effects of soil, but no consistent effect of peatmoss addition or fertilizer source in the 2 years of the experiment. There were significant differences among progenies. Foliar Fe, B, Al, Zn, and Cu concentrations varied independent of soil material, progeny, or fertilizer source. Leaf Mn was significantly increased from solid 10N-4P-8K fertilizer and a significant soil by progeny interaction existed. Those progenies containing some V. angustifolium tended to have increased foliar Mn levels. The reduced vigor of the blueberry progenies grown on soils other than the Berryland sand was tentatively ascribed to induced nutrient imbalances, involving Ca, Fe, and Mn, possibly being governed by soil cation exchange capacity and organic matter reactivity.