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  • Author or Editor: Todd C. Wehner x
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Root knot, caused by Meloidogyne spp. is the most important disease of cucumber (Cucumis sativus L.) in North Carolina, causing an average annual yield loss of 12 %. A greenhouse study was conducted 10 screen 924 cultigens (728 accessions, 136 cultivars and 36 breeding lines of C. sativus, and 24 accessions of C. metuliferus Naud.] for resistance to 3 species of root knot nematodes, M. incognita r. 3, M. arenaria r. 2 and M. hapla, Plants were grown from seed in 150-mm diameter clay pots. Two-week-old seedlings were inoculated with 5000 nematode eggs per plant, then evaluated for resistance 9 weeks later. All cultigens evaluated were resistant to M. hapla. Little resistance was found in the cultigens of C. sativus to M. incognita r. 3 and M. arenaria r. 2. Most of the cultigens evaluated were susceptible to both. `Southern Pickler' was resistant to both nematodes (1 % average galls). `Green Thumb and LJ 90430 were resistant to M. arenaria r. 2, Two check cultigens, `Sumter' and Wis. SMR 18, had an average of more than 50% galls. All C. metuliferus cultigens evaluated were resistant to all root knot nematodes tested. PI 482452 was most resistant (1 % average galls), and PI 482443 was least resistant (5% average galls) of the C. metuliferus cultigens tested.

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The cucumber (Cucumis sativus L.) germplasm collection of 924 cultigens (accessions, breeding lines and cultivars) was evaluated for resistance to anthracnose (Colletotrichum orbiculare (Pass.) Ell. & Halst) in the field and greenhouse. The field test was run using 1 m plots grown in 4 environments (year-location combinations). The field was inoculated 3 weeks after planting using a backpack sprayer. A susceptible spreader cultivar (Wis. SMR 18) was planted every 5th row, and plots were overhead-irrigated 3 times/week. Plots were rated 1 and 2 weeks after inoculation. The greenhouse test was run using seedlings grown in flats of vermiculite, and inoculated with 104 spores/ml on one cotyledon. Plants were rated using the size of the chlorotic halo surrounding the lesion. There was no correlation (r=0.04 to 0.17) of seedling test with field test ratings, nor between any of the 4 field test environments. Correlations were significant among field tests when only cultivars and breeding lines were evaluated. We concluded that diversity within accessions resulted in the lack of correlation among tests. The cultigens that had high resistance in all tests were `Slice', NCSU M 21, Gy 14A, `Addis' and PI 164433 (India). Most susceptible were PI 175696 (Turkey) and PI 285606 (Poland).

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Suitability of commercial production of oriental trellis type cucumber (Cucumis sativus L.) was evaluated using 18 cultivars and breeding lines (referred to as cultigens hereafter). The cultigens (15 oriental trellis and three American slicers) were tested using two systems (trellis and flat bed production) in two seasons (spring and summer) in 1995 at Clinton, N.C. Traits evaluated included total yield, early yield, percentage culls, vine length, leaf area, gynoecious rating, fruit shape, fruit color, fruit length, seed cell size, anthracnose resistance, powdery mildew resistance, and fruit keeping ability. Highest yielding cultigens were `Summer Top', `Tasty Bright', and `Sprint 440'. Those with best fruit quality were `Sprint 440', `Tasty Bright', `Poinsett 76', and `Summer Top'. Most disease resistant was `Poinsett 76'. The best cultigens considering all traits measured were `Sprint 440', `Tasty Bright', and `Summer Top'. Production of cultigens on trellis rather than flat bed resulted in an average increase in total, marketable, and fancy yield of 130%, 160%, and 140%, respectively. Oriental trellis cucumbers can be produced as a less expensive alternative to European greenhouse cucumbers, and to supply consumer demand for this particular product type.

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The combining ability (hybrid performance) of breeding lines is often determined to measure selection progress for yield. Plant breeders utilize this information to develop breeding lines with higher combining ability. The objectives of this study were to measure the specific combining ability for yield traits over three selection cycles from four pickling cucumber populations with Gy 14, a popular pickling cucumber inbred; and to determine the change in specific combining ability for yield traits in four populations improved through recurrent selection. Four pickling cucumber populations, North Carolina wide base pickle (NCWBP), medium base pickle (NCMBP), elite pickle 1 (NCEP1), and hardwickii 1 (NCH1), were developed and improved through modified half-sib selection from 1983 to 1992 to improve yield per se and fruit quality in each population. Eleven families were randomly selected from each of 3 selection cycles (early, intermediate, advanced) from each populations and were hybridized to Gy 14. Plants were sprayed with Paraquat to defoliate them and to simulate once-over harvest. The experiment was a randomized complete-block design with 22 replications per population arranged in a split plot with the four populations as whole plots and the three cycles as subplots. The combining ability for fruit quality rating of NCWBP and NCMBP increased as the number of selection cycles increased. Conversely, selection for higher yield per se decreased the combining ability of the NCEP1 population for improved fruit quality. In most instances, the combining ability of each population exhibited a constant response over selection cycles for each measured yield trait.

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Significant loss in yield of cucumber (Cucumis sativus L.) due to fruit rotting caused by Rhizoctonia solani (Kuhn) is frequently observed in the Southeastern United States. Chemical controls are costly and provide only partial control. Currently there are no resistant cultivars. The objective of this study was to identify potential sources of resistance and develop efficient screening methods for use in a breeding program. In the summer of 1991, 105 cucumber cultigens representing a range from resistant to susceptible were grown in Clinton, NC. Those cultigens were screened using field and detached fruit methods. Resistant cultigens chosen for further study were PI 165509, PI 197086 and PI 197088, with 2 to 4 % of the fruit surface damaged. Susceptible cultigens were PI 419108, PI 178886 and PI 432855, with 13 to 16 % of the fruit surface damaged. Five methods were then evaluated on greenhouse grown cucumber seedlings to identify an efficient screening method. The methods evaluated were a soil drench, a leaf dip using a mycelium suspension, syringe inoculation, and potato dextrose agar disks of R. solani placed on the third true leaf or against the hypocotyl at the soil line.

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Fruit weight in the cultivated watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus] ranges from 1 kg to over 100 kg. In recent years, preference of consumers has shifted toward fruit of smaller sizes than the large sizes traditionally used for parties and picnics. This has produced increased interest in the genetics of fruit weight, especially among watermelon breeders. The objectives of this study were to determine the inheritance of fruit weight. Six adapted cultivars having very large or very small fruit weight were crossed in a half diallel. Field trials were conducted at two locations in North Carolina (Clinton and Kinston). Large-fruited parents had higher phenotypic variance than small-fruited parents. Environmental variance was higher than genetic variance (mean, 7.58 and 3.82, respectively) at Kinston, NC. At Clinton, NC, genetic and environmental variances were similar (mean, 9.45 and 8.99, respectively) for 67% of the families. Narrow- and broad-sense heritability estimates were low to intermediate (mean, 0.59 and 0.41, respectively). A high number of effective factors (mean, 5.4) was found to influence fruit weight in watermelon. Watermelon breeders should use quantitative methods such as recurrent selection for population improvement to change fruit weight in the development of new cultivars.

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Genes for watermelon [Citrullus lanatus (Thunb.) Matsumura & Nakai] fruit traits have been identified since the 1930s. We conducted a study of fruit traits including fruit stripe width, stripe color, rind color, fruit shape, and blossom end shape (concave vs. convex). Ten watermelon cultivars (inbred lines) were used as parents. Several new genes or alleles were discovered. A series of alleles at the g locus is proposed to explain the inheritance of fruit rind pattern: G (medium or dark solid green), g W (wide stripe), g M (medium stripe), g N (narrow stripe), and g (solid light green or gray). The dominance series is G > g W > g M > g N > g. Another series of alleles at the ob locus is proposed for the fruit shape: allele Ob E for elongate fruit, which is the most dominant; allele Ob R (not the same as the o gene for round) for the round fruit; and allele ob for oblong fruit, which is the most recessive. Gene csm is proposed for the clear stripe margin in the cultivar Red-N-Sweet and is recessive to the blurred stripe margin (Csm) in ‘Crimson Sweet’, ‘Allsweet’, and ‘Tendersweet Orange Flesh’.

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Downy mildew, caused by the oomycete pathogen Pseudoperonospora cubensis (Berkeley & Curtis) Rostov, is a major foliar disease of cucumber. Ten years after the reemergence of P. cubensis, downy mildew continues to be a major threat to cucumber production in the United States. Cucumber accessions with high levels of resistance have been identified. Development of cultivars with high levels of resistance remains an important objective of cucumber breeding programs. We tested a set of cucumber cultigens, including highly resistant PI accessions and susceptible control lines, to observe the effect of plant age on resistance. Cultigens responded differently to disease across plant developmental stages. In general, older plants had more disease symptoms, even those classified as resistant, such as PI 197088. However, PI 330628 and PI 605996 held their resistance even at late developmental stages. It is possible that these lines were resistant at late stages due to other factors, such as their rapid, indeterminate growth, that allows them to outgrow the disease. However, although PI 197088 appears to have a rapid, indeterminate growth habit, it did not have more resistance at later stages of plant maturity. Regardless of the mechanism involved, plant breeders should use the genetic resistance in PI 330628 and PI 605996 over PI 197088.

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The effects of leaf age, guttation, stomata and hydathode characteristics, and wounding on the symptom development of gummy stem blight [Didymella bryoniae (Auersw.) Rehm] of cucumber (Cucumis sativus L.) were studied to develop a useful germplasm screening method. Older cucumber leaves were more susceptible than younger leaves in field, greenhouse, and detached-leaf tests. Compared to seedlings with true leaves, seedlings at the cotyledon stage were less susceptible, had a smaller variance for ratings, and were more likely to escape infection. Stomata density and hydathode counts were not correlated with field ratings; but, stomata length on older leaves was highly correlated with susceptibility y. In greenhouse and field tests, susceptibility y increased as guttation increased and actively guttating plants were more susceptible than nonguttating plants. Phylloplane moisture and/or nutrition were more important in the infection process than was stomata] opening. Although important, guttation was not necessary for infection. Dawn inoculation of field or greenhouse tests increased leaf symptoms compared with dusk inoculation. The increase was likely due to the free water and nutrients provided by guttation. Genotype ranks and ratings for detached-leaf tests were not correlated with field results. A useful method, highly correlated (r = 0.82 to 0.96) with field ratings. for screening germplasm in the greenhouse was developed.

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