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- Author or Editor: Todd C. Wehner x
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.
Leaf and stem resistance to gummy stem blight [Didymella bryoniae (Auersw.) Rehm.] in five resistant by susceptible crosses of cucumber (Cucumis sativus L.) was investigated using generation means analysis. No single gene of major effect controls either leaf or stem resistance to gummy stem blight in these five crosses. The mean number of effective factors controlling leaf resistance in the cross `Slice' × `Wis. SMR 18' was estimated to be at least five. Estimates of broad- and narrow-sense heritabilities indicated that environmental effects were larger than genetic effects. In general, additive variance was the larger component of genetic variance. Epistasis was significant in most crosses, and dominance was present in several crosses. Additive gene effects contributed more to resistance than to susceptibility in contrast with dominance gene effects. Reciprocal differences for leaf rating were detected in the crosses M 17 × `Wis. SMR 18' and `Slice' × `Wis. SMR 18'. Phenotypic correlations between leaf and stem ratings were moderate (r = 0.52 to 0.72). Estimates of genetic gain for resistance to gummy stem blight ranged from low to moderate. Breeding methods that make best use of additive variance should be used because much of the variance for resistance is additive, and dominance effects, at least in these crosses, tended to contribute to susceptibility.
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).
Root knot caused by Meloidogyne spp. is an important disease of cucumber. Resistance to M. javanica in cucumber (Cucumis sativus L.) is conferred by the newly discovered mj gene. The objective of this research was to determine whether mj was linked to other genes controlling morphological or disease resistance traits in cucumber. Four inbred lines homozygous for mj (LJ 90430, `Manteo', NCG-198, and NCG-199) were crossed with inbreds (`Coolgreen', M 21, NCG-101, WI 2757, and `Wisconsin SMR 18') to form six families: NCG-101 × LJ 90430, WI 2757 × LJ 90430, NCG-199 × `Wis. SMR 18', NCG-198 × M 21, `Manteo' × M 21, and NCG-198 × `Coolgreen'. F2 progeny were evaluated in all families, and BC1 progeny were evaluated only in the NCG-199 × `Wis. SMR 18' family. Meloidogyne javanica resistance and the 17 other traits controlled by simple genes were evaluated in greenhouse or field tests. None of the 17 genes were linked with mj. Therefore, cucumber breeders interested in nematode resistance should be able to incorporate the trait into lines without having to break linkages with the 17 genes used in this study.
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.
Mixtures of different cucumber cultivars or breeding lines (collectively called cultigens) has been used commercially in some parts of the U.S. The objective of this study was to determine whether mixtures of cultigens produce higher yields than pure stands. Three cultigen pairs: Gy 14A+ M 21, Gy 4 + WI 2757 and Regal+ Carolina (higher yielding cultigen listed first) were blended at 0, 25, 50, 75 and 100%, and sown in a spring and summer planting in 1989. Fruit were harvested 6 times in each study and graded according to N. C. standards. Data were collected on yield, earliness, vine size, sex expression and disease resistance. Yields from the spring planting were superior to those from the summer, For maximum fruit weight (Mg/ha), a mixture consisting of 25% of the lower yielding cultigen of each pair should be grown. However, for maximum fruit value ($/ha), the higher yielding cultigen of each pair should be grown in pure stand. We concluded that arbitrary mixtures of cultivars offer no advantage in most cases, although superior mixtures might be produced by evaluating cultigen pairs for complementation.
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.
Yield was evaluated in 817 plant introduction accessions of cucumber (Cucumis sativus L.) along with 19 check cultivars. The study was conducted in spring and summer seasons of 1997 and 1998 with three replications using recommended horticultural practices and optimized field plot trials. In order to get fruit from each cultigen regardless of sex expression, plants were sprayed with ethrel (2-chloroethyl phosphonic acid) to make them gynoecious. Plots were harvested once-over when 10% of the fruit in a plot were oversize. Data were collected on fruit weight (total, marketable, early and cull), fruit number (total, marketable, early and cull), fruit type, fruit quality, and days to harvest. Total fruit weight for all cultigens ranged from 4 to 214 Mg/ha, with 1 to 40 fruit per plot. Based on statistical analysis, fruit number was the most useful trait for yield evaluation. Stand corrections for yield were not found to be useful. The cultigens with the highest fruit numbers for pickling type were PI 215589, PI 179678, PI 249561, PI 356809, and PI 370643. Highest fruit number for slicing type were PI 344440, PI 422199, and PI 342951. Highest fruit number for middle-eastern type were PI 525150, PI 525153, PI 181910, and PI 534540. Highest fruit number for Oriental trellis type were PI 432849, PI 432866, PI 508455, PI 372893, and PI 532520. Several cultigens produced more than the check cultivars. High-yielding cultigens could be used in breeding programs to improve the yield of cucumber.
Currently, both hybrid and inbred pickling cucumber cultivars are being grown commercially in the United States. Heterosis for yield in pickling cucumber has been previously reported. However, heterosis has not been repeatable in other studies. The objective of this study was to determine the existence of heterosis and inbreeding depression for yield in pickling cucumber. Six pickling cucumber inbreds (`Addis', `Clinton', M 12, M 20, `Tiny Dill', `Wisconsin SMR 18') were hybridized to form four F1 hybrid families (`Addis × M 20, `Addis' × `Wis. SMR 18', `Clinton' × M 12, M 20 × `Tiny Dill'). Within each family, F2, BC1A and BC1B generations were also formed. Thirty plants of each generation within each family were grown in 3.1-m plots for four replications in the spring and summer seasons of 1996 at the Horticultural Crops Research Station in Clinton, N.C. Data were collected at once-over harvest for total, marketable, and early yield in terms of number (1000 fruit/ha) and weight (Mg/ha). In addition to yield, a fruit shape rating was collected for each plot. High parent heterosis for yield (total and marketable fruit weight) was only observed for `Addis' × `SMR 18' grown in the summer season. The three other families did not exhibit heterosis for total, marketable, and early yield. Heterosis for shape rating was not observed for any family. `Addis' × `Wis. SMR 18' also exhibited inbreeding depression for total fruit weight, marketable fruit weight, early fruit number, and early fruit weight during the spring season and for marketable fruit number and marketable fruit weight during the summer season.