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  • Author or Editor: Benny Bruton x
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This is the second year of research on the effects of grafting watermelon (Citrullus lanatus) onto rootstocks of squash and gourd. The study was conducted at Lane, Okla., in both 2004 and 2005. This report deals with the results from 2005. Treatments consisted of watermelon cultivars SF 800, SS 5244, SS 7167, SS 7177, and SS 7187 from Abbot & Cobb Seed Co., grown on their own roots, or grafted onto rootstocks of RS1330, RS1332, RS1420, or RS 1422. Additional controls consisted of nongrafted cultivars Sangria, Royal Sweet, Jubilee, and Jamboree. Two fields were planted, with three replications per field. Plants were grown on 1-m centers, with rows 3 m apart. Yields from grafted plants were higher than average farm yields in Oklahoma, but were lower than yields from the nongrafted plants in this experiment. Sugar content, measured as soluble solids, was in some cases lower with the grafted plants than with the nongrafted plants. Lycopene content of fruit from grafted plants was similar to that of fruit from nongrafted plants. Fruit firmness, as measured by a penetrometer, was significantly greater in the grafted fruit than in the nongrafted fruit. This increase in fruit firmness should be of significance to the fresh-cut fruit industry. Matching of scions with appropriate rootstocks was important, as interactions did occur. Rootstock 1332 generally had lower sugar content and yield than did the other rootstocks, but not with all scions. Certain combinations of rootstock and scion were significantly superior to other combinations.

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Grafting of watermelons has been used in many countries to provide control of, or resistance to, certain soil borne diseases such as Fusarium wilt. The impact of grafting on postharvest quality has not been thoroughly examined. This report deals with the comparison of the costs of production between grafted versus conventional watermelons and the potential net revenue of the two. A 2-year study was conducted on the effects of grafting watermelon (Citrullus lanatus) onto rootstocks of squash and gourd at Lane, Okla., in both 2004 and 2005. Details of the research methodology are outlined in “Year Two: Effects of Grafting on Watermelon Yield and Quality” by Roberts et al. Costs of using grafted transplants increased the costs of production from $1,209 to $1,914 or $705/acre at 1,500 plants/acre. Results of the 2-year study indicated grafted watermelons had slightly lower yields per acre, similar sugar in some grafted combinations the first year but slightly lower the second year, similar lycopene content, and much higher firmness. Results of a 10-day storage study indicated that firmness of fresh-cut flesh for all watermelons declined after ten days on the shelf. However, the grafted watermelon flesh was firmer after ten days than the nongrafted fruit at the beginning of the ten days. This improved shelf life should interest the cut-fruit industry and should lead to contract price enhancement for the growers. A market price of $0.02/lb for grafted watermelons above the market price of nongrafted watermelons would be needed to provide similar net revenues at the same yield per acre.

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Plots were established at the Lane Agricultural Center in Lane, Okla., in 2003 for the purpose of conducting research in certified organic vegetable production. A field was selected that had been in pine timber since 1985. The field was cleared, plowed, disked, and land-planed. To establish a baseline for future reference, soil samples were collected on a 30 × 30 ft grid. Lime was added to adjust the pH. Poultry litter was added to the field as a fertilizer, and was incorporated by disking. Turnips were grown as a cover crop during the winter of 2003–04. In Spring 2004, the field was divided into four equal sections, which were planted with either tomatoes, sweet corn, watermelons, or southern peas. Tomatoes were planted using both determinate and indeterminate types. Plants were selected based on reported properties of interest to organic growers, such as disease resistance, pest resistance, or heat-set capabilities. The cultivars with greatest yield were Sunny, Solar Set, Classica, Sun Leaper, and Mountain Fresh. Visual disease ratings were taken throughout the season. Copper sulfate was used as a fungicide. The cultivars with the lowest disease ratings were Amelia, Peron, Celebrity, Florida 91, and Mountain Fresh. The major insect pest throughout the season was aphids. Aphid counts reached 6.9 aphids per leaf on 11 June. Two applications of AzaDirect, a neem extract, reduced aphid populations to 1.0 aphid per leaf on 17 June, 0.1 aphid per leaf on 25 June, and 0 aphids on 9 July.

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Grafted cucurbits are commonly grown in various Asian and European countries, but only rarely in North America. Disease control in fields where crop rotation cannot be practiced is a common justification for grafting cucurbits. In the present study, grafting is being examined as a methyl bromide alternative, which may allow cucurbits to be grown in fields where heavy disease pressure would make production of nongrafted cultivars impractical. A study with watermelons (Citrullus lanatus) grafted onto rootstocks of squash and gourd was conducted at Lane, Oklahoma in 2004. Treatments consisted of watermelon cultivars SF 800, SS 5244, SS 7167, SS 7177, and SS 7187 from Abbot & Cobb Seed Co., grown on their own roots, or grafted onto rootstocks of RS1330, RS1332, RS1420, or RS 1421. Controls consisted of nongrafted cultivars Sangria, Royal Sweet, Jubilee, and Jamboree. Two fields were planted, with three replications per field. Plants were grown on 1 m centers, with rows 3 m apart. Yields of grafted plants were generally equal to or greater than the nongrafted plants. Sugar content, measured as soluble solids, was affected minimally, if any, by grafting. Lycopene content of fruit from grafted plants was equal to, or marginally better than, fruit from nongrafted plants. Fruit firmness, as measured by a penetrometer, was significantly greater in the grafted fruit than in the nongrafted fruit. The firmest fruit occurred with SS 7167 scions, grafted onto RS 1420 rootstock, which had a value of about 2.0 × 105 Pascals. The nongrafted plants had values of about 1.0 × 105 Pascals, or less. Matching of scions with appropriate rootstocks was important, as interactions did occur. Certain combinations were significantly superior to other combinations. We estimate that the cost to purchase a grafted seedling plant from a seedling supplier would be $0.75 to $1.00, which would include the cost of the seed and the grafting operation. This cost would compare favorably with the cost of applying methyl bromide to the soil and then planting nongrafted seeds or transplants. Higher plant survival due to disease resistance along with planting fewer plants per hectare is anticipated with grafted plants. The high values in fruit firmness in grafted fruit should be of particular interest to the fresh-cut industry.

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The shelf life and over-all quality of fresh-cut watermelon from two cultivars grafted onto four rootstocks were compared with fresh-cut fruit from the nongrafted cultivars. Fresh-cut cubic pieces of about 4.5 cm per side were prepared from ripe watermelons grown at the Lane Research Station and were stored at 5 °C in 35-oz PETE containers. Quality attributes of firmness, soluble solids content, lycopene content, and bacterial counts of the pieces were measured after 0, 5, and 10 days of storage. Sugar content of the cut fruit was independent of rootstock and remained constant over the ten days of storage. Lycopene content of the fruit decreased by 5% to 10% during the storage period, regardless of treatment. Bacterial count on the fruit from all treatments remained low and variable during the ten days at 5 °C. Firmness of cut pieces from fruit originating from the grafted plants was dependent upon the rootstock employed, and melons from grafted plants possessed firmer fruit than did those from the nongrafted plants. Overall, the firmness of fruit from all sources decreased 20% to 30% during the ten days of cold storage. However, the firmness of fruit from some of the rootstocks after 10 days of storage was equal to or significantly higher than that of the fruit from nongrafted plants when it was initially cut. Thus, these studies suggest that grafting to a proper rootstock will produce fresh-cut watermelon that is equal in sweetness and lycopene content to its nongrafted counterpart, but it will possess greater crispness throughout its storage on the supermarket shelf.

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Forty-one cultivars of triploid and diploid watermelons (Citrullus lanatus) were grown at Lane, Oklahoma in 2003. Seeds were placed in Jiffy-9 pellets in a greenhouse on 21 May. Fields were prepared with raised beds 1 m wide covered with black plastic. Plots were 3 m wide by 15 m long, with 4 replications, arranged as a randomized complete block. Seedlings were transplanted to the field on 4 June. From 4–9 June, rainfall occurred 5 days. Maximum soil temperatures at 5 cm, under bare soil, from 1–9 June were 34, 34, 35, 26, 22, 26, 31, 29, and 32 °C, respectively. On 9 June, 84% of the seedlings were dead. Lesions were observed on the roots and stems and isolations were made from symptomatic tissues. The predominant pathogen isolated from the seedlings was Pythium aphanidermatum. Some of the cultivars appear to have some degree of resistance to P. aphanidermatum. Mortality among the cultivars, averaged across all replications, ranged from 33% to 100%. The cultivars with the lowest mortality were “Tri-X Carousel” (33%), `Sunny' (40%), `WT-02-31' (53%), `Ole' (58%), and `Tri-X Palomar' (68%). New seeds were seeded in the greenhouse on 16 June, and transplanted to the field on June 30. The replacement seedlings were planted in the same field, in the same location as the previous plants. Maximum soil temperatures for the two week interval following the second planting ranged from 33 to 39 °C, with only one rain of 0.8 cm occurring 10 days after planting. There was no apparent plant loss due to P. aphidermatum in the second planting.

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