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Howard F. Harrison Jr., W. Patrick Wechter, and Chandrasekar S. Kousik

Watermelon [Citrullus lanatus var. lanatus (Thunb.) Matsum & Nakai] seed and root exudates inhibit germination and seedling growth of plants and growth of pathogenic fungi and bacteria. This study was conducted to determine if extractable components in the testa (seedcoat) contribute to the inhibition previously reported. Testae of eight genetically diverse Citrullus genotypes were extracted first with dichloromethane to remove less polar components and then with 70% methanol to remove more polar components. The dichloromethane extracts were not inhibitory in a Proso millet radicle growth bioassay; however, they were highly inhibitory to the growth of the fruit blotch bacterial pathogen Acidovorax avenae subsp. citrulli (Aac). All dichloromethane extracts were highly inhibitory to Aac except those from a watermelon breeding line, 406-1-x 7 and a C. lanatus var. citroides accession, PI 500354. The more polar components extracted in 70% methanol inhibited Proso millet radicle and Aac growth and Phytophthora capsici zoospore germination. The greatest inhibition of radicle growth was found with 70% methanol extracts from two watermelon relatives, C. lanatus var. citroides [Bailey (Mansf.)] (PI 532738) and C. colocynthis [(L.) Scrad.] (PI 432337). They reduced radicle elongation by 90% at an extract concentration of 250 mg of tissue extracted per mL water. The 70% methanol extracts of several genotypes partially inhibited Aac colony formation, but the C. lanatus var. citroides accession, PI 532738, was the only genotype with 70% methanol extracts that completely inhibited the bacterium at 100 mg·mL−1. The 70% methanol extracts of Charleston Gray, 406-1-x 7, PI 500354, PI 532738, and PI 167125 were highly inhibitory in a Phytophthora capsici zoospore germination bioassay. These results indicate that the testae of Citrullus genotypes contain at least two compounds that are inhibitory to microorganisms and plants in bioassay, and the amount of inhibition caused by the extracts varied among Citrullus genotypes.

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Howard F. Harrison Jr., Chandrasekar S. Kousik, and Amnon Levi

Clomazone herbicide is registered for use in watermelon; however, crop tolerance is marginal, and the recommended use rates (0.17 to 0.28 kg a.i./ha) are lower for watermelon than for other crops. In a greenhouse germplasm evaluation experiment including 56 germplasm accessions and watermelon cultivars, three Citrullus lanatus var. citroides PI accessions (PI 482324, PI 5003540, and PI 532624) were not injured by clomazone, whereas most of the other accessions and cultivars were moderately or severely injured. A greenhouse concentration response experiment demonstrated that the clomazone concentration required to cause moderate injury to the tolerant ‘PI 500354’ was approximately three times the concentration required to cause similar injury to the susceptible citroides accession ‘PI 244017’, and the concentration required to cause 50% shoot biomass reduction was approximately five times greater for ‘PI 500354’ than for ‘Charleston Gray’ watermelon. Subsequent field experiments demonstrated that two tolerant accessions (‘PI 500354’ and ‘PI 482324’) were injured less initially by clomazone and recovered more rapidly from clomazone injury than two susceptible accessions (‘PI 244017’ and ‘PI 271773’) and two watermelon cultivars (‘Charleston Gray’ and ‘Crimson Sweet’). Tolerant germplasm accessions like ‘PI 482324’ and ‘PI 500354’ may be useful as sources of clomazone tolerance in watermelon breeding. Chemical name used: clomazone {2-[(2-chlorophenyl)methyl]-4,4-dimethyl-3-isoxazolidinone}.

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Ryan S. Donahoo, William W. Turechek, Judy A. Thies, and Chandrasekar S. Kousik

Phytophthora capsici is an aggressive pathogen that is distributed worldwide with a broad host range infecting solanaceous, fabaceous, and cucurbitaceous crops. Over the past two decades, increased incidence of Phytophthora blight, particularly in eastern states, has threatened production of many vegetable crops. Cucumis melo L. (honeydew and muskmelon), although especially susceptible to fruit rot, is also highly susceptible to crown rot. Currently, little is known about host resistance to P. capsici in C. melo. To assess crown rot resistance in C. melo seedlings, 308 U.S. PIs, and two commercial cultivars (Athena and Dinero) were grown under greenhouse conditions. Seedlings with three to four true leaves were inoculated with a five-isolate zoospore suspension (1 × 104 zoospores per seedling) at the crown and monitored for 6 weeks. All the susceptible control plants of Athena died within 7 days post-inoculation. The majority of the PIs (281 of 308) were highly susceptible to crown rot and succumbed to the disease rapidly and had less than 20% of the plants survive. Several PIs (PI 181748, PI 182964, and PI 273438) succumbed to crown rot earlier than the susceptible melon cultivars. Eighty-seven PIs selected on the basis of the first screen were re-evaluated and of these PIs, 44 were less susceptible than cultivars Athena and Dinero. Twenty-five of the 87 PIs were evaluated again and of these six PI, greater than 80% of the plants survived in the two evaluations. Disease development was significantly slower on these PIs compared with the susceptible checks. High levels of resistance in S1 plants of PI 420180, PI 176936, and PI 176940 were observed, which suggests that development of resistant germplasm for use in breeding programs can be accomplished. Further screening and careful selection within each of these PIs can provide a framework for the development of resistant germplasm for use in breeding programs.

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Chandrasekar S. Kousik, Scott Adkins, William W. Turechek, Craig G. Webster, and Pamela D. Roberts

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Chandrasekar S. Kousik, Jennifer L. Ikerd, Patrick Wechter, Howard Harrison, and Amnon Levi

Phytophthora fruit rot, caused by Phytophthora capsici, is prevalent in most watermelon-producing regions of southeastern United States and is known to cause pre- and post-harvest yield losses. A non-wound inoculation technique was developed to evaluate detached mature fruit belonging to U.S. watermelon PIs for resistance to fruit rot caused by P. capsici. Mature fruit were harvested and placed on wire shelves in a walk-in humid chamber [greater than 95% relative humidity (RH), temperature 26 ± 2 °C] and inoculated with a 7-mm agar plug from an actively growing colony of P. capsici. Twenty-four PIs that exhibited resistance in a preliminary evaluation of 205 PIs belonging to the watermelon core collection in 2009 were grown in the field and greenhouse in 2010 and 2011 and evaluated in the walk-in humid chamber. Fruit rot development was rapid on fruit of susceptible controls ‘Black Diamond’, ‘Sugar Baby’, and PI 536464. Several accessions including PI 560020, PI 306782, PI 186489, and PI 595203 (all Citrullus lanatus var. lanatus) were highly resistant to fruit rot. One C. colocynthis (PI 388770) and a C. lanatus var. citroides PI (PI 189225) also showed fruit rot resistance. Fruit from PIs that were resistant also had significantly lower amounts of P. capsici DNA/gram of fruit tissue compared with the susceptible commercial cultivars Sugar Baby and Black Diamond. The sources of resistance to Phytophthora fruit rot identified in this study may prove useful in watermelon breeding programs aimed at enhancing disease resistance.

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Chandrasekar S. Kousik, Jennifer Ikerd, Mihir Mandal, Scott Adkins, and William W. Turechek

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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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William B. Rutter, Chandrasekar S. Kousik, Judy A. Thies, Mark W. Farnham, and Richard L. Fery

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Chandrasekar S. Kousik, Kai-Shu Ling, Scott Adkins, Craig G. Webster, and William W. Turechek

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Chandrasekar S. Kousik, Amnon Levi, Kai-Shu Ling, and W. Patrick Wechter

Powdery mildew (Podosphaera xanthii) can cause severe damage to cucurbit crops grown in open fields and greenhouses. In recent years, there has been an increased interest in the United States in grafting watermelon plants onto various cucurbit rootstocks. Bottle gourd plants (Lagenaria siceraria) are being used throughout the world as rootstocks for grafting watermelon. Although gourd plants are beneficial, they may still be susceptible hosts to various soilborne and foliar diseases. Bottle gourd plant introductions (PI) resistant to diseases and pests can be a valuable source of germplasm in rootstock breeding programs. We evaluated 234 U.S. PIs of L. siceraria for tolerance to powdery mildew in two greenhouse tests. Young seedlings were inoculated by dusting powdery mildew spores of melon race 1 on the cotyledons. Plants were rated 2 weeks after inoculation using a 1 to 9 scale of increasing disease severity. Although none of the L. siceraria PIs were immune to powdery mildew, several PIs had significantly lower levels of powdery mildew severity compared with susceptible watermelon cultivar Mickey Lee. The experiment was repeated with 26 select PIs on whole seedlings and cotyledon disks. Significant variability in the level of resistance to powdery mildew on plants within PI was observed. Moderate resistance in several PIs to powdery mildew was confirmed. PI 271353 had consistently lower ratings in the various tests and can be considered the most resistant to P. xanthii race 1 among the L. siceraria accessions evaluated in this study. A few other PIs with moderate resistance to powdery mildew included PI 271357, PI 381840, and PI 273663. These results suggest that novel sources of resistance could be developed by careful selection and screening of several of the PIs with moderate resistance described in our study.