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Squash leaf curl (SLC) is a virus disease of squash transmitted by the sweetpotato whitefly [Bernisia tabaci (Germ.)]. 'Cucurbita maxima Duch. ex Lam., C. mixta Pang, and C. pepo L. cultivars and the wild taxon. C. texana Gray exhibited severe symptoms in response to SLC in greenhouse and field tests. Symptoms on C. moschata (Duch.) Duch. ex Poir. cultivars were much more severe in greenhouse tests than in field tests. Three wild species, C. ecuadorensis Cutler and Whitaker, C. lundelliana Bailey, and C. martinezii Bailey, were virtually immune in greenhouse tests, but were infected in field tests. Cucurbita foetidissima HBK expressed moderate symptoms in a field test. Benincasa hispida (Thunb.) Cogn., C. ficifolia Bouche, Lagenaria siceraria (Mol.) Standl., Luffa acutangula (L.) Roxb., Luffs aegyptiaca Mill., and Luffs graveolens Roxb. were resistant to SLC in greenhouse and field tests.

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Abstract

Seedlings of 18 plant types from 5 genera of Cucurbitaceae were analyzed for cucurbitacins, total sugars, and fatty acids and these were correlated with spotted cucumber beetle feeding. The species were: Citrullus colocynthis L. (Schrad.), C. lanatus (Mansf.) Matsumara: Cucumis anguria L., C. dipsaceus Ehrenb., C ficifolius Bouche’C. longipes Hook f., C. melo L., C. myricarpus Naud., C. prophetarum L.: Cucurbita foetidissima H. B. K., C. pepo L.; Lagenaria siceraria (Mol.) Stamdl.; Luffa acutangula (L.) Roxb., and L. cylindrica Roem. Seedling injury ranged from 0 (none) to 3 (severe). Regression analysis indicated that cucurbitacins, total sugars, and the fatty acids (palmitic and linolenic) except linoleic contributed to insect feeding; correlation between concn and feeding was positive. Cucurbitacins, which cucumber beetles were able to locate without feeding, played the major role in seedling susceptibility; next in importance were palmitic acid, linolenic acid, and total sugars. All apparently related to the preference of beetles for specific strains or cultivars. In non-preferred seedlings, preference was induced by topical application of cucurbitacins A, B, C, D, E, and 1 and by the glycosides from C. foetidissimma roots. A barrier seemed to obstruct feeding of the beetles on the upper surface of the cotyledons, even when the attractant cucurbitacins were applied.

Open Access

Citrulline, arginine, and lycopene are naturally occurring compounds found in watermelon, Citrullus lanatus (Thumb) Matsum & Nakai, with beneficial effects on plant growth and human health. This study evaluated seven commercial cultivars and one breeding line for citrulline, arginine, and lycopene content in mature fruit grown at two locations in North Carolina. Correlations among these compounds and fruit quality traits (percent soluble solids and flesh pH) were evaluated. Watermelon cultigens evaluated were chosen for their fruit trait diversity. ‘Yellow Doll’ and NC-517 possessed the highest citrulline and combined concentration of citrulline and arginine of all cultigens evaluated. Lycopene content was highest in ‘Dixielee’, followed by ‘Sugar Baby’, and ‘Allsweet’, each of which have different shades of red flesh color. Location and its interaction with genotype had no significant effect on arginine or lycopene concentration. Broad-sense heritability was estimated for each trait. Arginine content (89%) and lycopene content (99%) had very high heritability. Citrulline content (41%), percent soluble solids (46%), and flesh pH (61%) had moderate heritability. Lycopene was positively correlated with flesh pH (r = 0.517) and negatively correlated with percent soluble solids (r = −0.344). Arginine content had a weak negative correlation with flesh pH (r = −0.343) and was not correlated with percent soluble solids.

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Dry heat treatment has been commonly used to inactivate some seed-borne pathogens in vegetable seeds. Virtually all the gourd seeds for watermelon rootstock are being treated with dry heat to inactivate cucumber green mottle mosaic virus (CGMMV, a strain of tobamovirus) and Fusarium. Seeds of five gourd and one squash cultivars were treated with dry heat (35 °C for 24 h + 50 °C for 24 h + 75 °C for 72 h) and, immediately after the dry heat treatment, the seeds (moisture content of 1% or lower) were allowed to absorb atmospheric moisture in a moisture saturated chamber until the seed moisture contents reached 2% to 8%. After the equilibrium obtained, the seeds were sealed in air-tight bags and stored for 1 day or 30 days at 20 °C. The seeds were then sown in cell trays and the emergence and seedling characteristics were evaluated. Dry heat treatment caused significant delay in emergence in all tested cultivars, but had little or no influence on the final emergence rate. Moderate to severe injury was observed in seedlings grown from dry heat-treated seeds in three out of six cultivars tested. However, little or no dry heat phytotoxicity was observed in other cultivars, thus suggesting the marked differences in cultivar susceptibility to dry heat treatment. Rapid humidification before sealing also appeared to reduce the early emergence rate in some cultivars, but had no effect on the final emergence rate in most cultivars. Storage of dry heat-treated seeds in sealed bags for 30 days before sowing was highly effective in minimizing the phytotoxicity symptoms in seedlings as compared to the seedlings grown from the seeds sown immediately after the dry heat treatment. This suggests that the reestablishment of metabolic process required for normal seed germination requires a long period after the dry heat treatment. Other characteristics associated with DH treatment will also be presented.

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Trials were conducted in 2002 and 2003 in California's San Joaquin Valley to determine the efficiency of reflective plastic and wheat straw in managing silverleaf whitefly and aphid-borne virus diseases in late planted cantaloupes. In 2002, the incidence of aphid-borne viruses was lowest in plants growing over reflective plastic followed by those growing over wheat straw and then those growing over bare soil. Wheat straw mulch was as effective as reflective plastic during the early part of the growing season in reducing the incidence of virus disease, but by mid-season, the reflective plastic was superior. The incidence of virus diseases in plants growing over wheat straw was significantly (P < 0.05) lower than that in plants growing over bare soil throughout the season. Whitefly numbers (nymphs per cm2) and aphid numbers were significantly reduced on plants growing over both reflective mulch and wheat straw mulch compared to those growing over bare soil. Yields of all sizes of melons were significantly higher in the reflective mulch plots and yield for the straw mulched and bare soil plots were not significantly different. Results in 2003 were similar to those of 2002. Both whitefly numbers and aphid numbers were significantly lower in plants growing over both mulches than in those growing over bare soil. Virus incidence was initially low but following an aphid flight in late August, the number of infected plants increased rapidly. Both the reflective plastic and straw provided equal protection form aphid-borne viruses throughout the growing season. Yields were highest in the reflective plastic plots, followed by the straw mulch and finally the bare soil. Differences were significant (P < 0.05) among all three treatments.

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Factors of crop management such as irrigation, cultivation, cultivar selection, and control of insect pests and plant diseases play important roles in watermelon production. To gain a better understanding of how intensity of crop management affects yield, we conducted a comparative study contrasting high and low intensity management in 1997, 1999, and 2000. High-intensity management (HM) included the use of trickle irrigation, black plastic mulch, insecticides, and fungicides, not used under low-intensity management (LM). We examined the effects of management intensity on watermelon productivity, the variation in such effects among watermelon cultivars, and the mediating effect of survival of watermelon plants, abundance of insect pests, and incidence of anthracnose (% leaves with anthracnose lesions). The results indicated that HM produced 100% greater marketable fruit yield per area and marketable fraction of total fruit than LM in 2 out of 3 years. The effect of management intensity on plant survival was related to this effect on yield in 1 out of 2 years, and contributed to the latter by increasing weight and number of marketable fruit per plant under HM. We detected no significant effect of abundance of insect pests and incidence of anthracnose on yield. There was variation in the effect of management intensity on yield among watermelon cultivars in 1 out of 3 years. The triploid `Gem Dandy' showed great differences in yield between HM and LM in 2 years, producing on average 28.9 Mg·ha-1 of marketable fruit yield under HM compared to 14.0 Mg·ha-1 under LM. `Gem Dandy' also produced 100% higher yield of marketable fruit per area, per plant, and marketable fraction of total fruit than the open-pollinated diploid `Allsweet' or the diploid hybrid `Sangria.' Each year during the 3-year study, all three cultivars had a similar density of insect pests, incidence of anthracnose, and plant survival after transplant and at harvest. This study provided information on the collective impact of multiple aspects of watermelon management on yield.

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RNA isolation from ripe fruit can be complicated by high concentrations of sugar and water. These sugars interfere with RNA extraction often resulting in low RNA quality and quantities, and high water concentrations dilute the RNA, making isolation difficult. We report a simple but novel method by which the majority of the excess sugar and water in mature fruit of tomato (Lycopersicon esculentum Mill.), watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai], and muskmelon (Cucumis melo L.) can be easily removed from tissue before RNA extraction. This method produced quality RNA in a shorter time than the currently accepted method for fruit tissue RNA isolation and does not require liquid nitrogen or a freeze dryer.

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The objective of this study was to measure honey bee (Apis mellifera L.) impact on seed set, fruit set, and yield of jack-o-lantern (Cucurbita pepo L.), large-sized (C. maxima Duch.), and processing pumpkins (C. moschata Duch. ex Poir.) under field conditions. There were sufficient natural pollinators [including bumblebees (Bombus spp.), carpenter bees (Xylocopa spp.), honey bees, and squash bees (Peponapis pruinosa Say)] provided under field conditions to induce fruit set of jack-o-lantern pumpkins as fruit number obtained per hectare was not affected by the addition of a honey bee colony. However, the addition of honey bees did increase fruit number per hectare of the C. moschata and C. maxima cultivars evaluated. Honey bee pollination resulted in larger-sized fruit, increasing individual fruit size of all but small-sized pumpkins (<0.5 kg). Individual pumpkin fruit weights of the Cucurbita pepo, C. moschata, and C. maxima cultivars evaluated increased by about, 26%, 70%, and 78%, respectively, when honey bee colonies were included. Natural pollination was insufficient to stimulate maximum fruit size development and seed number and seed weight per fruit. Although pumpkin fruit set will occur with natural pollinators, the addition of honey bee colonies will ensure the presence of pollinators to maximize fruit size. Since pumpkins are generally sold on a weight basis, growers may generate greater revenues with the addition of honey bee colonies in pumpkin fields.

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