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Jonathan R. Schultheis

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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Mike Stanghellini and Jonathan R. Schultheis*

In 1999 and 2000, a total of 28 diploid watermelon cultigens (released cultivars and advanced breeding lines) were evaluated for staminate flower and pollen grain production to assess their potential to serve as pollenizers (pollen source plants) in triploid watermelon production systems. Male reproductive output (staminate flower and pollen production) was quantified during the peak flowering and fruit setting phase of the cultigens under field conditions. The number of staminate flowers produced per plant per day, number of pollen grains produced per flower, and total number of pollen grains produced per plant per day (staminate flowers per plant x pollen grains produced per flower) differed greatly between cultigens (for all tests, P < 0.01). Staminate flower production by cultigens differed by year (P < 0.0003) and days-within-years (P = 0.0094), but pollen production between years by cultigens was stable (P = 0.3845). Total male reproductive output ranged from 134,206 pollen grains per plant per day for `Jamboree' to 321,905 pollen grains per plant per day for `Summer Flavor 500'. These studies demonstrate the genotypic variability in watermelon male reproductive output potential, and may assist growers in selecting an optimal diploid pollenizer for triploid watermelon production.

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S. Alan Walters and Jonathan R. Schultheis

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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Jonathan R. Schultheis and Robert J. Dufault

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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Jonathan R. Schultheis and David W. Monks

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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Jonathan R. Schultheis and C. Ray Campbell

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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Robert J. Dufault and Jonathan R. Schultheis

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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Allan C. Thornton and Jonathan R. Schultheis

The goal of this research was to determine the effects of in-row spacing and planting time on yield and root grade of NC 98-608 over time. Two plantings were made in two grower locations (four total). An early planting was made 19 and 25 May and a late planting 19 and 24 June. NC 98-608 was evaluated at the following in-row spacings; 23, 31, 38, and 46 cm. `Beauregard' spaced at 23 cm in-row and was used as the standard comparison. Roots were harvested and graded into canner, number one, jumbo and cull grades 90, 105, and 120 days after planting for each of the planting dates and locations. Each grade was weighed. An early planting in late May resulted in roots reaching the highest percentage grade of U.S. number one roots as early as 100 days after planting, while the late planting in June resulted in roots never reaching their full number one yield potential in some cases. For an early harvest after planting (90 days after planting) the 38 cm in-row spacing produced the most marketable number one yields compared with the 23, 31, and 46 cm in-row spacings. For a later harvest time after planting (105 days or later), it appeared as though the 31 cm in-row spacing was the most economical spacing to use. Roots from the early plantings (late May) and finer textured soils appeared to have shorter roots than roots harvested from later plantings (after 15 June) or coarser textured soil. Root shape and yield was more uniform with NC 98-608 than with the Beauregard clone. With yields comparable to Beauregard, the NC 98-608 clone provides an excellent opportunity to produce a quality sweetpotato with consistent shape.

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Jonathan R. Schultheis and Donald N. Maynard

Market demand for diploid fruit has been declining over the past ten years, especially the past three years. Thus, the watermelon industry is looking for ways to produce triploid fruit more efficiently. Several companies have developed pollinizers for planting in-row so as not to take up space like a commercial diploid cultivar. The objective of our experiment was to determine the effects of `Companion' and `SP-1' pollinizers on triploid yield compared with a traditional diploid pollinizer which until recently were routinely sold to grocery chain stores. The experimental locations were Bradenton, Florida and Kinston, NC. Treatments included two triploid cultivars; Tri-X-313 (Syngenta, Rogers Brand Seeds) and Olympia (Seminis Seed Co.), and three pollinizers; `Companion' (Seminis Seed Co.), `SP-1' (Syngenta, Rogers Brand Seeds), and `Summer Flavor 800' (Abbott & Cobb Seed Co.). A seventh treatment included `Tri × 313' in which no pollinizer was planted next to the triploid cultivar. `Companion' was interplanted every two triploid plants. `SP-1' was interplanted every three triploid plants, and `Summer Flavor 800' planted every third hill. All fruit were harvested when ripe and each watermelon was weighed. Results in the Florida location were compromised due to the close proximity of pollen from adjacent watermelon cultivar trials. In North Carolina, some pollen movement did occur between treatment plots as evidenced by fruit set in the `Tri-X-313' plots which did not contain a pollinizer. Yields; however, were 25% to 33% of those treatments which contained a pollinizer. `Tri × 313' yielded greater fruit numbers but individual fruit weights were less than those harvested from `Olympia'. Early fruit yield was similar regardless of pollinizer, while late yields were greater using `SP-1' rather than `Companion' or `Summer Flavor 800'. Cumulative yields (three total harvests) were highest with `SP-1' than the other pollinizers. The use of `SP-1' provided the greatest potential for improved yields over traditional diploid pollinizers.

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Jonathan R. Schultheis and Dennis E. Adams

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.