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  • Author or Editor: Jonathan Schultheis x
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Commercially grown sweetpotato contain virus. Hill selection is practiced to maintain quality and trueness to type of a variety. Three field plantings of Beauregard and Jewel were made in 1993 to compare the yield of virus-free planting stock obtained from micropropagared plants (VFM); virus-infected planting stock obtained from micropropagated plants (VIM); foundation, registered, grower seed stock; and a selected California Jewel clone in which the virus was removed, then micropropagated (CVFM). For Beauregard, VFM had significantly more yield of marketable and number 1 roots at the 0.06 level of significance than plants not micropropagated. The yield of number 1 roots was also greater with VFM compared with VIM. Marketable yields of Beauregard were superior when registered versus grower seed (nor in certification program) were compared. For Jewel, marketable yields were increased from VFM versus plants not micropropagated. VFM and the VIM yielded similarly as did registered and grower seed stock. The VFM Jewel clone from North Carolina outyielded CVFM. Yield was at least as good when obtained from VFM compared with the other planting stocks. A yield increase of 10 to 20% was common when using VFM, hill selected sweetpotatoes.

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Yellow and zucchini squash (Cucurbita pepo L.) cultigens (breeding lines and cultivars) were evaluated over a 2-year (1995 and 1996) period in North Carolina. Yellow squash cultigens that performed well (based on total marketable yields) were `Destiny III', `Freedom III', `Multipik', XPHT 1815, and `Liberator III' in Fall 1995 and HMX 4716, `Superpik', PSX 391, `Monet', `Dixie', XPH 1780, and `Picasso' in Spring 1996. Some of the yellow squash cultigens evaluated had superior viral resistance: XPHT 1815, XPHT 1817, `Freedom III', `Destiny III', `Freedom II', TW 941121, `Prelude II', and `Liberator III' in Fall 1995 and XPHT 1815, `Liberator III', `Prelude II', and `Destiny III' in Fall 1996; all these cultigens were transgenic. The yellow squash cultigens that performed well (based on total marketable yields) in the Fall 1995 test had transgenic virus resistance (`Destiny III', `Freedom III', XPHT 1815, and `Liberator III') or had the Py gene present in its genetic background (`Multipik'). Based on total marketable yields, the best zucchini cultigens were XPHT 1800, `Tigress', XPHT 1814, `Dividend' (ZS 19), `Elite', and `Noblesse' in Fall 1995; and `Leonardo', `Tigress', `Hurricane', `Elite', and `Noblesse' in Spring 1996. The zucchini cultigens with virus resistance were TW 940966, XPHT 1814, and XPHT 1800 in Fall 1995 and XPHT 1800, XPHT 1776, XPHT 1777, XPHT 1814, and XPHT 1784 in Fall 1996. Even though TW 940966 had a high level of resistance in the Fall 1995 test, it was not as high yielding as some of the more susceptible lines. Viruses detected in the field were papaya ringspot virus (PRSV) and watermelon mosaic virus (WMV) for Fall 1995; while PRSV, zucchini yellow mosaic virus (ZYMV), and WMV were detected for Fall 1996. Summer squash cultigens transgenic for WMV and ZYMV have potential to improve yield, especially during the fall when viruses are more prevalent. Most transgenic cultigens do not possess resistance to PRSV, except XPHT 1815 and XPHT 1817. Papaya ringspot virus was present in the squash tests during the fall of both years. Thus, PRSV resistance must be transferred to the transgenic cultigens before summer squash can be grown during the fall season without the risk of yield loss due to viruses.

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Two field studies were conducted in 1997 (Clinton, N.C.) and 1998 (Carbondale, Ill.) to determine if replanting (at 1, 2, 3, or 4 weeks after the initial seeding) into stand deficiencies of 10%, 30%, and 50% affected `Athena' muskmelon (Cucumis melo L. var. reticulatis) melon size and yield. Muskmelon numbers were higher for 1997, but there was no interaction of treatment by year for any of the melon sizes (small, medium, or large) or total melon number. Based on the two experiments conducted, `Athena' muskmelons stand deficiencies up to 30% do not reduce total or marketable numbers compared to a complete stand. Replanting into 10%, 30%, and 50% stand deficiencies increases early-season melon numbers regardless of the replant times. For main-season and total-season harvests, there was no advantage of replanting into 10% deficient stands and in most cases, replanting reduced total and marketable melon numbers. In the 1997 experiment, replanting into 30% and 50% stand deficiencies improved yields but this did not occur in the 1998 experiment. Based on this information, `Athena' muskmelon should be replanted only if a field has a stand reduction of more than 30%. Melon numbers were generally higher if replanted in 1 or 2 weeks after the initial seeding compared to 3 or 4 weeks. However, the timing of replanting does not appear to have significant influence on total or marketable melon numbers.

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Boron has been used to overcome the disorder blister in varieties such as `Jewel'. `Hernandez' is an attractive, good-yielding variety with uniform shape that will consistently pack out at 80% to 90%. Over time in storage, however, roots develop blister-like symptoms, rendering roots unmarketable for fresh market. Our objective was to evaluate the effect of different B rates and application times on the yield and quality of `Hernandez' roots. Rates were varied up to 2.24 kg actual B/ha 6 days after planting, while various soil and foliar application times (6, 34, and 69 days after planting) were evaluated at 1.12 kg·ha–1. In 1994, three row plots were arranged in a randomized complete block design and replicated four times. Planting was on a deep sand to maximize the effect of the B carrier Solubor. Roots were harvested, graded, and weighed 120 days after planting and storage roots evaluated for blister-like symptoms in Mar. 1995. No significant differences in yield were attributed to B rate or application method. Blister-like symptoms were more severe when no B was applied; however, application of B did not eliminate symptoms, as most roots had the blister-like appearance. Boron application did not solve the problem, but symptoms were less apparent when some B was applied.

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Yellow and zucchini squash (Cucurbita pepo L.) cultivars/elite lines (cultigens) were evaluated over two seasons (fall 1995 and spring 1996) in North Carolina. Different cultigens were tested over the 2-year period for both yellow and zucchini squash, although some cultigens were tested both years. Cultigen recommendations are based on yield, quality, disease resistance, and season grown. Yellow squash cultigens that yielded well include: Destiny III, Freedom III, Multipik, TW 941141, Liberator III (fall 1995); and HMX 4716, Superpik, PSX 391, Monet, Dixie, Picasso, and XPH 1780 (spring 1996). Superior-yielding zucchini squash cultigens were: TW 940981, Tigress, TW 940982, ZS 19, Elite, and Noblesse (fall 1995); and Leonardo, Hurricane, Elite, HMX 4715, Noblesse, and Tigress (spring 1996). Virus ratings for fall 1995 indicated that some transgenic plants with virus resistance withstood virus infection better than those without resistance. These were Freedom III, Destiny III, Freedom II, Liberator III, Prelude II, and TW 941121 (yellow), and Tigress, TW 940982, TW 940981, TW 940866 (zucchini). Virus-infected plants were assayed and viruses were determined to be zucchini yellow mosaic, watermelon mosaic II, and papaya ringspot.

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Pretransplant nutritional conditioning (PNC) of transplants during greenhouse production may improve recovery from transplanting stress and enhance earliness and yield of watermelon [Citrullus lanatus (Thumb.) Matsum. & Nakai]. Two greenhouse experiments (Expts. 1 and 2) and field experiments in South Carolina and North Carolina (Expt. 3) were conducted to evaluate N and P PNC effects on watermelon seedling growth and their effects on fruit yield and quality. `Queen of Hearts' triploid and `Crimson Sweet' diploid watermelon seedlings were fertilized with N from calcium nitrate at 25, 75, or 225 mg·liter–1 and P from calcium phosphate at 5, 15, or 45 mg·liter–1. In the greenhouse, most variation in the shoot fresh and dry weights, leaf count, leaf area, transplant height, and root dry weight in `Queen of Hearts' and `Crimson Sweet' was attributed to N. Cultivar interacted with N, affecting all seedling growth variables, but not leaf area in Expt. 2. To a lesser extent, in Expt. 1, but not in Expt. 2, P interacted with cultivar, N, or cultivar × N and affected shoot fresh and dry weights, leaf count and leaf area. In the field, transplant shock increased linearly with N, regardless of cultivar or field location. The effect of PNC on plant growth diminished as the growing season progressed. For both cultivars at both locations, N and P PNC did not affect time to first staminate flower, fruit set, fruit width or length, soluble solids concentration, or yield. Vining at Charleston for both cultivars was 2 days earlier when N was at 75 rather than 25 mg·liter–1, without further change with the high N rate. At Clinton, the first pistillate flower was delayed linearly the higher the N rate for `Crimson Sweet'. At Charleston, hollow heart in the `Queen of Hearts' increased nearly 3 times when N PNC rate was tripled (from 75 or 225 mg·liter–1), while N had no effect on hollow heart in `Crimson Sweet'. In contrast, at Clinton, hollow heart in either cultivar was affected by P PNC, not N. PNC with 25N–5P (in mg·liter–1) can be used to reduce seedling growth and produce a more compact plant for easier handling, yet not reduce fruit quality or yield.

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Seedling losses shortly after emergence in muskmelon (Cucumis melo L.) can be potentially devastating to growers. Muskmelon growers often have problems with obtaining adequate stands and need to understand the affects of replanting seed into poor stands. Field studies were conducted over 2 years to determine if replanting (at 1, 2, 3, or 4 weeks after the initial seeding) into stand deficiencies of 10%, 30%, and 50% affected `Athena' muskmelon size and yield. `Athena' muskmelon stand deficiencies up to 30% does not appear to reduce total or marketable numbers, but stand deficiencies of 50% or more will decrease total and marketable melon yields. Replanting into 10%, 30%, and 50% stand deficiencies will increase early season melon numbers regardless of the replant times used. For main-season and total-season harvests, there was no advantage of replanting into 10% deficient stands, and in most cases, replanting reduced total and marketable melon numbers. In the 1997 experiment, replanting into 30% and 50% stand deficiencies improved yields but this did not occur in the 1998 experiment. `Athena' muskmelon should be replanted only if a stand reduction of ≈50% or more occurs. Melon numbers were generally higher if replanted by 1 or 2 weeks compared to 3 or 4 weeks, but the timing of replanting does not appear to have significant influence on total or marketable melon numbers.

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Bedded sweetpotatoes are often covered with a rowcover to enhance sprouting. Our study was conducted to evaluate several rowcovers for earliness, plant yield, and plant quality (weight). In 1993 and 1994, variety (`Beauregard' and `Jewel') and rowcovers (clear plastic; black plastic; photodegradable plastic; infrared transmissible plastic; Reemay polyester cover; and black plastic, which was then covered with a black plastic tunnel) were evaluated for their effects on plant production. Holes ≈1 cm in diameter were punched in the plastic ≈2 weeks after planting to prevent exposing the seed roots to excessively high temperatures. Rowcovers were removed when plants began emerging from the soil, except for Reemay and the black plastic tunnels, which remained in place over the bedded plants until first plant harvest. Black plastic tunnels were placed back over the treatment bed each time plants were harvested. When 50% of the plants were 25 cm tall, all plants from the plot were cut 1 to 2 cm above the soil line. Plots were arranged in a randomized complete-block design and replicated five times. `Jewel' produced plants earlier than `Beauregard'. Covering beds with black plastic mulch and tunnels resulted in the first plant cutting being up to 42 days earlier than the other treatments, with no rowcover treatments producing plants the latest. The second earliest cutting was obtained when beds were covered with Reemay rowcover. Plant fresh weight was greater with the no cover treatments; black plastic tunnel treatments produced the lowest weight plants. Using black plastic tunnels consistently produced more plants than the other treatments. In the future, using rowcovers to enhance slower sprouting varieties should be examined.

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To reduce transplant shock of bell peppers (Capsicum annuum L.), we tested the effectiveness of pretransplant nutritional conditioning (PNC) as a promoter of earliness and yield. In Expt. 1, `Gatorbelle' bell pepper seedlings were fertilized with N from Ca(NO3)2 at 25, 75, or 225 mg·liter-1 and P from Ca(H2PO4)2 at 5, 15, or 45 mg·liter-1. Nitrogen interacted with P, affecting shoot fresh and dry weight, leaf area, root dry weight, seedling height, and leaf count. In Expt. 2, transplants conditioned with N from 50, 100, and 200 mg·liter-1 and P at 15, 30, and 60 mg·liter-1 were field-planted in Charleston, S.C., and Clinton, N.C. Nitrogen- and P-PNC did not greatly affect recovery from transplant shock. Although N- and P-PNC affected seedling growth in the greenhouse, earliness, total yield, and quality were similar in field studies among all PNC treatments at both locations. PNC with 50 mg N and 15 mg P/liter can be used with this variety and not have any long-term detrimental effects on yield and quality.

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In the last 3 years, ≤50% of the North Carolina sweetpotato crop has been produced with the variety Hernandez. A brown to black discoloration on the epidermis of the `Hernandez' sweetpotato may develop when maintained in storage for several months. The symptoms resemble blister—blister is caused by a boron deficiency. Preliminary studies in 1994 indicated that boron reduced the discoloration on `Hernandez' but did not eliminate the problem. To help confirm these findings and further define the role of boron in defining skin discoloration, boron was applied in 1995 at several rates (0 to 5.6 kg·ha–1) and stages of plant development using two application methods (foliar or soil). Yields and plant analysis data were obtained. Marketable yields ranged from 18.4 to 29.3 mt/ha. Leaf boron concentration ranged from 50 to 100 mg·kg–1 throughout the production season when 1.1 kg·ha–1 boron was soil applied shortly after planting. Excessive levels of boron (200+ mg·kg–1) were measured in plant tissue when application levels exceeded 2.2 kg·ha–1 regardless of timing. Soil application appeared to be an adequate method for boron application. Roots were examined for symptoms of discoloration after 5 months. Results indicated no affect of boron on incidence or severity of the symptoms.

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