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Alvin M. Simmons and Amnon Levi

The B-biotype sweetpotato whitefly, Bemisia tabaci (Gennadius), feeds on and damages numerous vegetable crops including watermelon (Citrullus sp.). Seven watermelon cultivars, a triploid line, and 16 U.S. Plant Introduction accessions (PIs) of C. lanatus var. lanatus; 10 PIs of C. lanatus var. citroides; and eight PIs of C. colocynthis, were evaluated for resistance to B. tabaci. Bioassays were based on nonpreference and performance of the whiteflies on the 42 Citrullus genotypes. Most of the watermelon cultivars and C. lanatus PIs tested were highly susceptible to whitefly infestation, while the C. colocynthis accessions exhibited whitefly resistance. Among the C. colocynthis accessions tested, PI 386015, PI 386018, and PI 386024 were most resistant to B. tabaci. This study identified useful sources of germplasm that can be used for the improvement of watermelon for resistance to whiteflies.

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Chandrasekar S. Kousik, B.M. Shepard, Richard Hassell, Amnon Levi and Alvin M. Simmons

Two hundred nineteen U.S. plant introductions (PI) belonging to the watermelon core collection were evaluated for broad mite, Polyphagotarsonemus latus (Banks), infestation and injury that occurred naturally in a field planting. Of the 219 PIs, nine (4%) had no visible broad mite injury in the field compared with a commercial cultivar, ‘Mickey Lee’, which was severely injured. Injury mainly occurred on the growing terminals and the tender apical leaves. The growing terminals and the apical leaves were bronzed, grew poorly and, in some cases, they were distorted and curled upward. Broad mites were extracted by washing the growing terminals of 22 selected PIs with boiling water and counting the mites under a stereomicroscope. ‘Mickey Lee’ had more broad mites on growing terminals compared with some of the PIs with no visible injury. Fourteen selected PIs were further evaluated in the greenhouse to confirm their resistance by artificially infesting them with broad mites that had been cultured on susceptible watermelon plants. PIs in accessions belonging to Citrullus lanatus var. lanatus (PI 357708), Citrullus lanatus var. citroides (PI 500354), Citrullus colocynthis (PI 386015, PI 386016, PI 525082), and Parecitrullus fistulosus (PI 449332) had significantly lower broad mite injury ratings and counts compared with ‘Mickey Lee’ and other susceptible PIs. Broad mites have not been reported on watermelons in the United States before; however, it can emerge as a serious pest. The previously mentioned accessions can serve as potential sources of broad mite resistance for use in breeding programs aimed at enhancing pest resistance in watermelon.

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Chang-chi Chu*, Kai Umeda, Tian-Ye Chen, Alvin M. Simmons and Thomas H. Henneberry

Insect traps are vital component of many entomological programs for detection and monitoring of insect populations. We equipped yellow (YC), blue (BC) sticky card (BC) with 530 nm lime green (LED-YC) and 470 nm blue (LED-BC) light-emitting diodes, respectively that increased trap catches of several insect pests. The LED-YC traps caught 1.3, 1.4, 1.8, and 4.8 times more adult greenhouse whitefly Trialeurodes vaporariorum (Westwood), sweetpotato whitefly Bemisia tabaci (Gennadius) biotype B, cotton aphids Gossypium hirsutum (L.), and fungus gnats Bradysia coprophila (Lintner), respectively, compared with standard YC traps. The LED-YC traps did not catch more Eretmocerus spp. than the standard YC traps. Eretmocerus spp. are important B. tabaci parasitoids used in greenhouse biological control programs. For whitefly control in greenhouse the 530 nm lime green LED equipped plastic cup trap designed by Chu et al. (2003) is the better choice than LED-YC trap because it catches few Eretmocerus spp. and Encarsia spp. whitefly parasitoids released for B. tabaci nymph control. The LED-BC traps caught 2.0-2.5 times more adult western flower thrips Franklinella occidentalis (Pergande) compared with the standard BC traps.

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John L. Coffey, Alvin M. Simmons, B. Merle Shepard, Yaakov Tadmor and Amnon Levi

Selection for and incorporation of host plant resistance into cultivars is a fundamental strategy to control insects and diseases and may help reduce reliance on synthetic pesticides. The sweetpotato whitefly, Bemisia tabaci (Gennadius), is an important pest of watermelon [Citrullus lanatus (Thunberg) Matsum. and Nakai var. lanatus] and is among the most damaging pests in many agricultural systems worldwide. Citrullus colocynthis L., a perennial melon species indigenous to arid regions of northern Africa, the Mediterranean region, and southwestern Asia, is a valuable source of resistance to insect pests and diseases of watermelon. Laboratory and greenhouse experiments were conducted to evaluate selected C. colocynthis genotypes for sources of resistance to B. tabaci. Thirty genotypes of C. colocynthis, collected in different geographic regions, were evaluated against the heirloom cultivar Calhoun Gray using first a horizontal Y-tube olfactometer in the laboratory. A selected subset of the genotypes was evaluated in a second experiment in the laboratory using a vertical monitoring assay. In this assay, whiteflies could freely move upward to feed and oviposit on leaves placed in the upper portion of a Y-tube. In a third experiment, a choice assay was conducted on selected genotypes in cages in the greenhouse. Of the 30 C. colocynthis genotypes evaluated, PI 346082 (collected in Afghanistan) exhibited the highest level of resistance against B. tabaci based on all three experiments. PI 537277 (collected in Pakistan) exhibited a significantly high level of whitefly resistance based on low survival of adult whiteflies and a low ratio of nymphs to eggs. PI 346082 and PI 537277 should be a useful source for breeding projects aiming to improve whitefly resistance in watermelon cultivars.

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Rolando López, Amnon Levi, B. Merle Shepard, Alvin M. Simmons and D. Michael Jackson

The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), often causes serious damage to watermelon (Citrullus spp.), and there is a need to evaluate and identify watermelon germplasm resistant to T. urticae. Watermelon cultivars (Citrullus lanatus var. lanatus), and U.S. plant introduction (PI) accessions of C. lanatus var. citroides and C. colocynthis, were evaluated for preference by T. urticae (number of adults and eggs on leaves). In open-choice experiments in the greenhouse and in laboratory rearing cages, there was a significant preference by T. urticae for watermelon cultivars, Citrullus lanatus var. lanatus PIs, and C. lanatus var. citroides PIs over C. colocynthis PIs. All watermelon cultivars and PIs were infested, but the C. colocynthis PIs were significantly less infested with T. urticae. The C. colocynthis PIs may be useful sources for enhancing T. urticae resistance in cultivated watermelon.

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Amnon Levi, Judy A. Thies, Alvin M. Simmons, Howard Harrison, Richard Hassell and Anthony Keinath

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Kelechi Ogbuji, Gloria S. McCutcheon, Alvin M. Simmons, Maurice E. Snook, Howard F. Harrison and Amnon Levi

Whiteflies [Bemisia tabaci (Gennadius)] and aphids [Aphis gossypii Glover and Myzus persicae (Sulzer)] are serious threats to watermelon by direct feeding and by transmitting viruses of important virus diseases. The desert watermelon Citrullus colocynthis (L.) has been shown to exhibit resistance to these insect pests and could be a useful source for breeding resistance into watermelon [Citrullus lanatus var. lanatus (Thunbs) Matsum & Nakai]. Using high-performance liquid chromatography (HPLC), we found differences among the chemical profiles of two U.S. PIs of C. colocynthis, one PI of C. lanatus var. citroides, and two heirloom watermelon (C. lanatus var. lanatus) cultivars (‘Charleston Gray’ and ‘Mickey Lee’). Flavonoid and caffeic acid derivatives were identified in the leaf extracts by a combination of ultraviolet (UV) and mass spectrometry (MS) spectral analyses. Four phenolic derivatives of caffeic and/or ferulic acid were found to be essentially unique to C. colocynthis. Total flavonoid content was found to be approximately four to 18 times higher in C. colocynthis accessions and seven to nine times higher in C. lanatus var. citroides as compared with watermelon cultivars. Caffeoyl-glucose was also identified in the leaves of watermelon cultivars for the first time. Leaf sugar concentrations (198 to 211 mg·dL−1), read from a glucometer, were statistically the same among the various germplasm entries. These results will help in the development of pest-resistant watermelon.

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Amnon Levi, Claude E. Thomas, Judy A. Thies, Alvin M. Simmons, Kai-Shu Ling, Howard F. Harrison, Richard Hassell and Anthony P. Keinath

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Amnon Levi, Alvin M. Simmons, Laura Massey, John Coffey, W. Patrick Wechter, Robert L. Jarret, Yaakov Tadmor, Padma Nimmakayala and Umesh K. Reddy

Citrullus colocynthis (CC) is a viable source of genes for enhancing disease and pest resistance in common cultivated watermelon [Citrullus lanatus var. lanatus (CLL)] cultivars. However, there is little information about genetic diversity within CC or the relationship of CC accessions to C. lanatus. In this study, we examined genetic diversity and relationships among 29 CC accessions collected in northern Africa, the Middle East, and Asia, and their relationships to 3 accessions and 3 cultivars of CLL, 12 accessions of citron melon [C. lanatus ssp. lanatus var. citroides (CLC)], and 1 accession representing the desert perennial Citrullus ecirrhosus (CE). Twenty-three high-frequency oligonucleotides-targeting active gene (HFO–TAG) primers were used to produce a total of 431 polymorphic fragments that target coding regions of the genome. Cluster and multidimensional scaling plot analysis, separated the CC into five groups, in general agreement with their geographic origins. CC genotypes admixed with CLL and CLC also were identified. Major reproductive barriers resulted in significantly reduced fertility in CC × CLL hybridizations. However, several of the U.S. PIs of CC were successfully crossed with watermelon cultivars using traditional breeding procedures, and the seeds produced from these crosses were viable. This suggests that CC can be a viable source to introduce biotic and abiotic stress resistance genes into cultivated watermelon.