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Abstract
The cover is a color photograph of a painting of a ‘Tristar’ strawberry plant by Lynda Chandler at the end of the Beltsville 1980 fall harvest season. ‘Tristar’ and its sister cultivar, ‘Tribute’, are new strawberry introductions from the U. S. Department of Agriculture — University of Maryland breeding program (see also p. 792–795 for more details). These cultivars are the first to combine red stele and verticillium resistance from octoploid cultivated strawberry cultivars with a strong, cyclic expression of the everbearing character from a wild selection of octoploid Rocky Mountain strawberry. ‘Tribute’ and ‘Tristar’ are related through a common grandparent to the ‘Brighton’, ‘Hecker’, and ‘Aptos’ day-neutral cultivars recently introduced in California by Bringhurst and Voth.
Abstract
The response of lettuce (Lactuca sativa L.) to salinity was studied in greenhouse tests. Seven-day-old seedlings were grown in solution culture with salinity (NaCl + CaCl2, 2:1 molar ratio) ranging from 0.6 to 26 decisiemens per meter (dS m−1). Fresh weight (FW) declined as salinity increased; root weight was not affected as much as leaf weight. Regression analysis of 30-day-old seedlings showed that FW was significantly reduced at 4.6 dS m−1. FW decreased an additional 4.5% for each dS m−1 increase above 4.6 dS m−1. Results were similar 60 days after growth, and were comparable to previous field studies. A screening test for salt tolerance in lettuce was developed using sand culture. Considerable variability exists among 85 U.S. lettuce cultivars and breeding lines irrigated with 5.6 to 6.2 dS m−1 solution. ‘Climax’, ‘Climax 84’, ‘Shawnee’, ‘Tom Thumb’, ‘Fulton’, and ‘Wintergreen’ were the most tolerant cultivars tested. ‘Bibb’, ‘Ruby’, ‘Gustoverde’, ‘Prizehead’, and ‘Big Boston’ were significantly more sensitive to salinity.
Abstract
Young strawberry (Fragaria × ananassa Duch.) seedlings were screened over a 4-year period for resistance to a composite of races of the red stele root rot causal organism, Phytophthora fragariae. A mixture of mycelia of 5 races of the organism, agar and water used as a root dip at the time of planting into sand on 6- to 8-week-old seedlings gave uniform results in the elimination of susceptible seedlings approximately five months later. The technique lends itself to efficient and effective mass screening of strawberry seedlings for relative resistance to P. fragariae.
Abstract
Two methods of evaluating seedling drought resistance in Vaccinium (blueberry) spp. were examined. Twenty interspecific populations were greenhouse-grown and either matric-stressed in a dry 1 sand : 1 soil medium or osmotic-stressed in a nutrient solution containing polyethylene glycol (PEG). In both tests, population means were separated statistically by shoot damage ratings. The correlation (r = 0.46) between the two tests was positive and significant. Progenies of clones JU64 and JU62, which are sister seedlings (V. myrsinites Lamark × V. angustifolum Aiton), were the most drought-resistant. The soil screening test appeared more accurate because it grouped populations with common parentage. These tests indicated that the progenies differ in genetic capacity to resist drought.
Abstract
The U.S. Department of Agriculture strawberry breeding program has introduced 61 cultivars from 6 breeding locations in its 61-year history. Fifteen to 20 of these cultivars have been widely grown, and 5 of the recent releases show exceptional promise. From its inception, the program has been aimed at producing high-quality, multipurpose cultivars with disease resistance and broad regional adaptation. Because of disease pressures, genetic resistance has been emphasized, especially for red stele root rot. The USDA has become the principal source of red stele-resistant germplasm for breeding pruposes, and has constantly refined and improved its disease-screening techniques. Red stele and Verticillium resistance have now been incorporated into everbearing strawberries adapted to the eastern United States. Investigations concerning virus detection and eradication and the propagation of virus-free stocks have led to state virus-free plant certification programs, which assure the public of production of the cleanest possible strawberry nursery stocks. The work of the USDA and cooperating state stations has played a significant role in the improvement of strawberry cultivars and the development of modern strawberry production in the United States.
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.
The relationship between moisture stress and mineral soil tolerance was studied by placing 10 blueberry (Vaccinium) clones in a Berryland sand soil high in organic matter (Berryland) and a Galestown sandy clay loam soil (Galestown) and subjecting them to one of two moisture regimes. The Berryland and Galestown soils represent an excellent blueberry soil and a mineral soil, respectively. A moderate degree of water stress influenced biomass partitioning in blueberries in a similar manner as stress induced by culture on mineral soil. Berryland control plants on Berryland partitioned more biomass into leaves and produced more dry matter and leaf area than plants on Galestown or those moisture stressed. Net assimilation rate and relative growth rate were not significantly different between soil or moisture treatments. The primary reason for the reduction in absolute growth rate due to soil type or moisture stress was a significantly lower leaf area duration on Galestown soil and in-moisture stressed plants. Clones differed in instantaneous transpiration, leaf conductance, and apparent photosynthesis and the ability to partition biomass into various plant parts. By selecting for increased leafiness, a high photosynthetic rate, and a more energy efficient root system, improvement in mineral soil tolerance should be possible.
Abstract
Seedlings from a diallel set of crosses of inbred and noninbred strawberry (Fragaria X ananassa Duch.) selections resistant to Phytophthora fragariae Hickman were inoculated with the 5 principal Eastern U. S. races of the fungus. The inbred selections, when selfed or intercrossed to other inbreds or to the noninbreds, did not transmit resistance to a greater portion of their progeny than the control cross, a cross of 2 noninbreds. Specific combining ability was found to be important in transmission of resistance to progeny.
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.
Abstract
Most progenies from intercrossed or outcrossed inbred selections of strawberry (Fragaria × ananassa Duch.), germinated as well or better than a control outcross between 2 vigorous noninbred clones. Germination of an S3 progeny was lower than the control. In general, inbreeding reduced seedling vigor while intercrossing or outcrossing of inbreds restored vigor. Germination total and rate as expressed by an index were unrelated to subsequent seedling growth.