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  • Author or Editor: Maurus V. Brown x
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Trials were established in Fayetteville, Ark., in 1993 and in 1994 to evaluate grape germplasm for downy mildew resistance. Accessions were obtained from the national grape repositories in Geneva, N.Y.; Davis, Calif.; and the Univ. of Arkansas fruit substation, Clarksville. The 1993 trial contained 26 cultivars, eight selections, and 24 Vitis species; the 1994 trial contained 37 cultivars. Each trial had four single-plant replications. Plants were rated on a scale of 0 to 5 for sporulation, chlorosis, and necrosis on 23 Aug. and 26 Sept. 1994. Fungicides were not applied to the vineyard to better determine the level of natural downy mildew resistance. The results from both trials indicated that several grape accessions showed little or no symptoms of downy mildew, which suggests these have resistance that would be beneficial to use in a breeding program.

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The influence of in-row plant spacing on the yield and fruit size of `Blueray' (erect growing) and `Bluecrop' (spreading) highbush blueberry (Vaccinium corymbosum L.) was studied. Plants of both cultivars, spaced at 0.61 m within the row, had significantly higher yields per hectare than plants grown at wider spacings (0.92 and 1.22 m) in each of five harvest years. On a per-plant basis, however, plants spaced at 1.22 m had higher yields in the last two harvest years of the experiment than plants spaced more closely, which indicated that interplant competition reduced per-plant yields of closely spaced plants as plants grew larger. Over the 5-year harvest period, plots with 0.61-m plant spacing produced a cumulative total yield of 17.24 t·ha more than plots with the conventional 1.22-m spacing. Plant spacing did not affect fruit size in this experiment.

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Plasmopara viticola infects and sporulates through stomata of susceptible grape leaves. Sporulation, chlorosis, and necrosis ratings were made in 1994 and 1995 on grape selections and cultivars and Vitis species grown in a fungicide-free vineyard. Cellulose-acetate impressions were made of the abaxial leaf surfaces and stomata were carefully counted within a circle 100 μm in diameter under a light microscope. Leaves were rated as either pubescent or glabrous. There were significant differences among genotypes for sporulation, chlorosis, and necrosis for 1994 and 1995, with highly significant correlations over both years. Stomatal densities were significantly different, but there were no correlations among levels of downy mildew and stomata! densities. Pubescent leaves had significantly higher sporulation, chlorosis, and necrosis ratings for downy mildew than glabrous leaves over both years.

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`Saturn', `Mars', and `Reliance' were compared based on their different Vitis vinifera and V. labrusca compositions. Disks (10 mm) from young leaves were placed abaxial side down on a standard media containing NAA or 2,4-D at 0.0, 1.0, and 2.0 mg/L with BAP at 0.0, 0.1, and 0.2 mg/L. Each treatment was replicated in 10 culture tubes and incubated at 25 ± 1C under cool-white fluorescent light for 10h photoperiods. Calli were compared by size, color, and occurrence of morphogenesis. NAA generally produced a larger callus by cultivar than 2,4-D. A greater quantity of callus was generally produced with the increase of the V. labrusca component. Callus produced on 2,4-D medium was round, compact and light to dark green in color. However, callus produced on NAA medium was amorphous, friable, and ranged in colors. Rooting occurred on some calli produced on NAA media.

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Disks (18 mm in diameter) from young, fully-expanded leaves of susceptible and resistant germplasm were inoculated on the abaxial surface by spray or pipette with 2 × 104, 5 × 104, 1 × 105 sporangia per milliliter of distilled water. Disks were placed on filter paper bridges in plastic containers with tap water to maintain high relative humidity and incubated at 24 C with a 14-hour photoperiod. Disease ratings were made five days after inoculations and disks were cleared in lactophenol (100 C) for two to three hours, stained with lactophenol/cotton blue, and examined by light microscopy. Uncleared disks of resistant material showed no symptoms, and susceptible material showed a range of symptoms and sporulation. Hyphal growth was evident in infected tissue of susceptible plants, but little or no growth was observed in resistant material. Results indicate that leaf disks can be used to evaluate downy mildew resistance. Further investigations will determine efficacy of this technique for screening seedling populations for field resistance.

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The influence of in-row plant spacing on highbush blueberry yield and fruit size was studied on two cvs., `Blueray' (erect growing) and `Bluecrop' (spreading). Plants of both cvs. spaced at 0.6lm in-row significantly outyielded plants at wider spacings (0.92, 1.22m) in each of the five harvest years when based on per hectare yields. On a per plant basis, however, plants spaced at 1.22m outyielded closer spaced plants in the last two harvest years of the experiment, indicating that interplant competition was reducing per plant yields on close spaced plants as plants grew larger. Over the 5-year harvest period, plots with 0.61m plant spacing produced a cumulative total yield increase of 17239 kg/ha more than plots with the conventional spacing of 1.22m. There were no effects of plant spacing on fruit size in the experiment.

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This research was conducted to compare an in vitro leaf disk technique with greenhouse and field evaluations for screening large populations of grape (Vitis sp.) seedlings for downy mildew (Plasmopara viticola Berk. & Curt. Berl. & de Toni) resistance. Seedlings produced by crossing resistant × resistant, resistant × susceptible, and susceptible × susceptible parents were rated for sporulation, chlorosis, and necrosis. Leaf disk sporulation ratings at the first and second rating were highly correlated with the second sporulation rating in the field. Necrosis ratings from the leaf disk evaluations were significantly correlated with field necrosis ratings, but leaf disk chlorosis ratings were not correlated with field ratings. Some correlations, including evaluations of chlorosis, between the greenhouse and field ratings were highly significant. Seedling ratings of 0 or 1 for sporulation, chlorosis, and necrosis in the leaf disk assay agreed with field evaluations 85.6% of the time vs. 80.3% agreement between greenhouse and field ratings. Sporulation was the parameter most highly correlated between leaf disk or greenhouse and field evaluation of resistance. The leaf disk procedure appeared to be a good predictor of field resistance, and is more practical than the greenhouse method for screening large populations.

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In the Spring and Summer 1997, severe die back of `Pinot Gris' and `Chambourcin' grape (Vitis vinifera) vines was observed by aerial surveillance in a commercial vineyard adjacent to Lake Erie. Vines grown over the tile lines grew well during 1997-99 following the excessively wet year of 1996. This was not the case for vines that were located betweentile lines. It was postulated that by digging and refilling the trench to insert the tile that either soil compaction or soil pH had been altered and could be responsible for the vine performance. Measurements indicated that these factors were not altered enough to explain the growth differences between vines growing over tile lines and those vines growing between tile lines. It appears that soil oxygen was improved by tiling and likely made the difference in cane dieback during the excessively wet year of 1996. By 1999, vines over tile and between tile had similar yields, and the pattern was no longer visible from the air. This study showed that heavy clay soils with naturally poor internal drainage caused cane dieback and poor growth of vines, especially in very wet years. Thus, it appears prudent on soils of this type, tile drainage is beneficial and spacing of lateral tile lines needs to be closer than 40 ft (12 m) in plateau silt loam soils to adequately protect vines from wet years.

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A study was conducted to determine how resistance to downy mildew [Plasmopara viticola (Bert. & Curt.) Berl. & de Toni] is inherited in germplasm (Vitis vinifera L., V. labrusca L., V. rupestris Scheele, and V. riparia Michx.) used for breeding table grapes. Crosses, including reciprocals, among parents possessing different levels of downy mildew resistance were evaluated in 1994 and 1995. The proportion of foliar tissue with sporulation, chlorosis, or necrosis was used to measure resistance. All genotypes were rated for these characters on two separate dates in 1994 and 1995. Hypersensitive flecking was also evaluated in the 1995 seedlings to determine its relationship with downy mildew resistance. Crosses with at least one resistant parent had a larger number of resistant offspring than crosses between two susceptible parents. General combining ability (GCA) effects were highly significant for 1994 and 1995. Specific combining ability effects were significant, but were relatively small compared to GCA, suggesting additive gene action was a primary influence on downy mildew resistance. Heritability estimates for sporulation, chlorosis, and necrosis were the highest at the second rating in 1994 (0.88, 0.74, and 0.57, respectively) and 1995 (0.50, 0.60, and 0.60, respectively). Reciprocal crosses indicated that maternal inheritance did not influence downy mildew resistance. A small percentage of progeny with hypersensitive flecking were identified from the germplasm. Seedlings with the flecking characteristic tended to have lower sporulation, chlorosis, and necrosis ratings earlier in the growing season.

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