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  • Author or Editor: Todd C. Wehner x
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Agricultural workers in many developed and developing countries are interested in collecting, evaluating, and maintaining germplasm of the crop species used in their particular agricultural production systems. However, opportunities to collect germplasm from centers of origin are being reduced as land is developed for agriculture, roads, housing, and other uses. Of the 240,000 angiosperm species thought to exist currently, it is estimated that 50,000 will become extinct by the year 2000 (2). The urgency to collect accessions of useful or potentially useful plant species is driving many germplasm institutions to expand. Thus, administrators of germplasm institutions are worried about their ability to maintain accessions unchanged for use in solving future problems (1). With such a large effort involved in the areas of germplasm collection and maintenance, it is important to use efficient collection methods, as well as proper means to preserve accessions.

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There are some 240,000 species of angiosperms in the world, of which 3000 or more have been used for food at one time or another. Of those, only 30 species are produced in quantities of at least 107 t/year, and 12 major crops supply the world's population with most of its plant- and animal-based calorie intake. Thus, humans are dependent for food on only a few species of the many thousands of flowering plants on the earth.

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Horticultural scientists often use field trials to determine the value of cultivars and experimental lines in specific production areas. Questions arise as to the most efficient methods for running such trials in order to gather as much information as possible with the least cost in both time and money.

Open Access
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Cucumber (Cucumis sativus L.) is one of the more chilling sensitive crops. Chilling resistance could provide growers with protection against late spring frost. Significant differences for chilling resistance were observed among a set of 9 diverse cucumber cultigens when grown at 22 C to 1st true leaf stage, then given a chilling treatment of 4 C for 7 hours in full light (PPFD 500 μmol.m-2.s-1). Two populations, NCWBP and NCES1, were used to measure narrow-sense heritability (estimated as twice the parent-offspring regression coefficient) for chilling resistance. Sets (256/population) of parents and offspring were evaluated in separate tests for seedling resistance. Plants were rated for damage 0 (none) to 9 (dead) on the cotyledons and 1st true leaf, 3 and 5 days after chilling. Ratings were corrected for position in the Phytotron chamber, and log transformations used to normalize the data. Generally, correction reduced the heritability estimates and transformation improved them. Heritability was highest for cotyledon ratings made 5 days after chilling, ranging from 0.35 for NCWBP to 0.70 for NCES1. Ratings of the 1st true leaf were more difficult to make, and produced lower estimates of heritability. Breeders should be able to make good progress in selecting for chilling resistance using this seedling test.

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Several major traits (yield, earliness, quality) of interest to cucumber (Cucumis sativus L.) breeders are quantitatively inherited. The objective of this study was to determine the progress made on such traits using recurrent selection in 4 fresh-market cucumber populations (NCWBS, NCMBS, NCES1, NCBA1). During population improvement, 1 to 2 replications of 200 to 335 half-sib families were evaluated for 5 traits: total, early and marketable fruits per plot, a quality rating, and a simple weighted index (=.2Total/2 + .3Early + .2%Marketable/10 + .3Quality). Families from each population were intercrossed in an isolation block during each summer using remnant seeds of the best 10% selected using the index. Progress was evaluated using a split-plot treatment arrangement in a randomized complete block design with 32 replications in each of 2 seasons (spring and summer). Whole plots were the 4 populations, and subplots were the 11 cycles (cycles 0-9 plus checks). Greatest gains were made for the NCBA1 population, with an average of 45% gain from cycle 0 to 9 over the 5 traits, and for early yield, with an average of 58% gain from cycle 0 to 9 over the 4 populations. Populations were improved for performance in a selected (spring season) as well as a non-selected environment (summer season).

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Fruit yield, earliness, and quality have low to moderate heritability, but are traits of major importance in cucumber (Cucumis sativus L.). The objective of this study was to determine the changes made in those traits using recurrent selection in three slicing cucumber populations (NCMBS, NCES1, and NCBA1). During population improvement, one or two replications of 200 to 335 half-sib families were evaluated in the spring season for five traits: total, early, and marketable fruit per plot, fruit shape rating, and a simple weighted index (SWI = 0.2(total yield)/2 + 0.3(early yield) + 0.2(% marketable)/10 + 0.3(fruit shape). Families from each population were intercrossed in an isolation block during the summer season using remnant seeds of the best 10% selected using the index. Response was evaluated using a split-plot treatment arrangement in a randomized complete block design with 32 replications in each of two seasons (spring and summer). Whole plots were the three populations, and subplots were the 11 cycles (cycles 0 to 9 plus checks). We measured improvement in performance of the populations in a selected (spring) and unselected environment (summer). Significant gains were made for all traits in all populations over the 9 to 10 cycles of recurrent selection. Greatest progress was made for the NCMBS population, with an average of 37% gain from cycle 0 to 9 over all five traits. The trait where most progress was made was early yield, with an average of 63% gain from cycle 0 to 9 over the three populations.

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Belly rot, caused by the fungal pathogen Rhizoctonia solani Kühn., is a severe disease in many regions that produce cucumber (Cucumis sativus L.). Annual crop loss to belly rot is commonly 5% to 10%, but losses as high as 80% can occur in individual fields. There are no resistant cultivars, so fungicides are used to provide partial control. Genetic resistance in an acceptable cultivar would be more desirable and economical. Studies were conducted in Summers 1991 and 1992 to screen promising germplasm for belly rot resistance using field and detached-fruit screening methods. In 1991, 105 cultigens (cultivars, breeding lines, and plant introduction accessions) were evaluated for belly rot resistance. The tests were repeated in 1992 with 63 cultigens, including the most resistant cultigens identified in 1991 and appropriate controls. Several cultigens were identified as potential sources of resistance genes. Pickling cucumbers showing resistance included PI 197085, PI 271328, and an F4 selection of PI 197087 × PI 280096. Slicing cucumbers with resistance included `Marketmore 76' and the F1 of Gy 14 × PI 197087. Belly rot resistance was not correlated with other horticultural traits measured, including fruit type, skin type, spine color, and firmness. The resistant cultigens identified should be useful for developing cucumber cultivars with enhanced resistance to Rhizoctonia solani.

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Progress was measured in four populations of cucumber (Cucumis sativus L.) improved by recurrent selection. The populations were the North Carolina wide base pickle (NCWBP), medium base pickle (NCMBP), elite pickle 1 (NCEP1), and hardwickii 1 (NCH1). Families from each of three cycles (early, intermediate, and late) from each population were randomly chosen and crossed with Gy 14 to produce gynoecious hybrids. Gy 14 is a gynoecious inbred used commonly as a female parent in the production of pickling cucumber hybrids. Once the plants had 10% oversized (>51 mm in diameter) fruit, plots were sprayed with paraquat to simulate once-over harvest. Selection cycles were evaluated for total, early, and marketable yield, and fruit shape. Testcross performance for fruit shape rating increased over cycles for the NCWBP and NCMBP populations when tested in either season. Testcross performance for total and early yield of the NCEP1 population tested in the spring decreased with selection, but remained constant over cycles in the summer season. The majority of yield traits in each population remained unchanged across selection cycles. Of the four populations studied, the NCMBP population had the greatest gain (7%) in testcross performance over cycles and averaged over all traits. In addition, testcross performance for fruit shape rating had the greatest gain (11%) with selection and averaged over populations. Years and seasons greatly influenced testcross performance for fruit yield and shape rating. In most instances, the fruit yield and shape of Gy 14 was higher than the testcross performance of population-cycle combinations. The performance of several families exceeded that of Gy 14 when testcross combinations were made. Those families could be selected for use in the development of elite cultivars. Chemical name used: 1,1'-dimethyl-4,4'-bipyridinium ion (paraquat).

Free access