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James A. Duthie, James Shrefler, Warren Roberts, and Jonathan V. Edelson

In each of seven field experiments, density of watermelon (cultivar Sugar Baby) plants was varied over the range 1000-9000 plants/ha by varying the distance between plants in single-row, replicate plots. Per unit area, reproductive biomass and marketable yield each increased linearly with density. An upper limit on these response variables at high density was not detected in any experiment. The rate of increase per 1000 plants/ha ranged from 1.1 to 3.2 Mg·ha-1, for reproductive biomass, and from 0.5 to 1.1 Mg·ha-1, for marketable yield. The linear effect of density explained >90% of the increase in reproductive biomass in most experiments. The effect on marketable yield was more variable because the marketable fraction of reproductive biomass often was highly variable. In most experiments, the marketable fraction did not vary systematically with density. The linear rate of change in the marketable fraction with density did not exceed 3% per 1000 plants/ha on average in any experiment. Intraspecific competition intensified rapidly as density was increased in some experiments. Intensity of competition appeared to vary among environments.

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Charles L. Webber III, Angela R. Davis, James W. Shrefler, Penelope Perkins-Veazie, Vincent M. Russo, and Jonathan V. Edelson

The increasing perception by consumers that organic food tastes better and is healthier continues to expand the demand for organically produced crops. The objective of these experiments was to investigate the impact of different weed control systems on yields of watermelon (Citrullus lanatus var. lanatus) varieties grown organically. Six watermelon varieties were transplanted at two locations (Lane and Center Point, Okla.). The six varieties included three seeded varieties (`Early Moonbeam', `Sugar Baby', and `Allsweet') and three seedless varieties (`Triple Crown', `Triple Prize', and `Triple Star'). The weed control system at Lane utilized black plastic mulch on the crop row, while the area between rows was cultivated to control weeds. The no-till organic system at Center Point used a mowed rye and vetch cover crop, hand weeding, and vinegar (5% acetic acid) for weed control. When averaged across watermelon varieties, Lane produced significantly more fruit per plant (4.2 vs. 2.3 fruit/plant), greater marketable yields (16.0 vs. 8.4 kg/plants), and higher average marketable weight per fruit (6.1 vs. 4.0 kg) than at Center Point. When comparing locations, four of six varieties had significantly greater number of fruit per plant and higher marketable yields at Lane than at Center Point. Except for `Early Moonbeam', all other varieties produced significantly heavier fruit at Lane than at Center Point. In contrast, the Center Point location produced a greater percentage of marketable fruit for all varieties except `Allsweet'. Fruit quality (lycopene and °Brix) was as good or greater when harvested from the weedier Center Point location.