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W. Patrick Wechter, Ralph A. Dean and Claude E. Thomas

Two 24-mer primers, MUSKFOM I and MUSKFOM II, were developed that amplify a 1.5-kb DNA fragment in race 1 Fusarium wilt resistant muskmelon (Cucumis melo L.), but not in race 1 susceptible germplasm tested. Three race 1 resistant cultivars and two race 1 resistant breeding lines as well as eight race 1 susceptible lines were analyzed using the two sequence-specific primers in the polymerase chain reaction. These primers should prove valuable for nondestructive determination of Fom 2 gene introgression in breeding programs.

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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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W. Patrick Wechter, Chandrasekar Kousik, Melanie McMillan and Amnon Levi

Fusarium wilt (FW) is a major disease of watermelon in North America and around the world. Control of this disease is difficult because the soilborne causal agent Fusarium oxysporum f. sp. niveum (Fon) produces chlamydospores that remain infectious in the soil for many years. Although various levels of resistance to Fon Races 0 and 1 exist in watermelon cultivars, no resistance to Race 2 or 3 has been reported. In this study, we used seed and seedling inoculation procedures to screen 110 U.S. PIs of wild watermelon (Citrullus lanatus var. citroides) for resistance to Race 2 FW. Of these 110 accessions, 15 showed significantly higher resistance to Fon Race 2 than that found in the watermelon cultivars Sugar Baby or Charleston Grey as well as in the C. lanatus var. citroides PI 296341 that was reported to contain resistance to FW. PI 271769, another C. lanatus var. citroides that was previously reported as containing resistance to FW, is among the 15 resistant accessions described here. These 15 accessions are potential sources for resistance to Race 2 FW in watermelon breeding.

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Karen R. Harris, W. Patrick Wechter and Amnon Levi

Sixty-six watermelon (Citrullus lanatus var. lanatus) disease resistance gene analogs were cloned from ‘Calhoun Gray’, PI 296341, and PI 595203 using degenerate primers to select for the nucleotide binding sites (NBS) from the NBS–leucine-rich repeat (LRR) resistance gene family. After contig assembly, watermelon resistance gene analogs (WRGA) were identified and amino acid sequence alignment revealed that these groups contained motifs characteristic of NBS-LRR resistance genes. Using cluster analysis, eight groups of WRGA were identified and further characterized as having homology to Drosophila Toll and mammalian interleukin-1 receptor (TIR) and non-TIR domains. Three of these WRGA as well as three disease-related watermelon expressed sequence tag homologs were placed on a test-cross map. Linkage mapping placed the WRGA on linkage group XIII, an area on the watermelon map where resistance gene analogs cluster. In addition, these WRGA sequence-tagged sites (STS) were amplified from various genera of the Cucurbitaceae indicating that conservation of resistance gene analogs exists among cucurbits. These WRGA-STS markers may be useful in marker-assisted selection for the improvement for disease resistance in watermelon.

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Sandra E. Branham, Mark W. Farnham, Shane M. Robinson and W. Patrick Wechter

Bacterial leaf blight incited by Pseudomonas cannabina pv. alisalensis (Pca) is a devastating disease with incidence reports worldwide and a wide host range capable of infecting all commercially valuable Brassica crops. With no chemical control options available, the most effective form of disease control is host plant resistance, but thus far resistant germplasm has only been identified in Brassica juncea L. (mustard greens). We report the first screening of Brassica oleracea L. var. viridis germplasm, including leafy green collard and collard-like accessions, for resistance to bacterial leaf blight by artificial inoculation of Pca in greenhouse trials. All commercial cultivars tested displayed an intermediate disease response resulting in leaf lesion development that renders the product unmarketable. Two sources of significant resistance were identified in the U.S. Department of Agriculture (USDA) viridis collection, which provides a valuable source of resistance alleles for collard cultivar development and introgression into other B. oleracea crops.

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W. Patrick Wechter, Melanie M. McMillan, Anthony P. Keinath, J. Powell Smith and Mark W. Farnham

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W. Patrick Wechter, Melanie M. McMillan, Mark W. Farnham and Amnon Levi

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W. Patrick Wechter, Mark W. Farnham, J. Powell Smith and Anthony P. Keinath

Brassica leafy greens (Brassica juncea L. and Brassica rapa L.) represent one of the most economically important vegetable crop groups in the southeastern United States. In the last 10 years, numerous occurrences of a leaf-spot disease on these leafy vegetables have been reported in several states. This disease, known as peppery leaf spot, is now causing serious crop losses and has been attributed to the bacterial phytopathogen Pseudomonas syringae pv. maculicola (Psm). To date, it appears that all cultivars of the Brassica leafy greens are susceptible, and available pesticides for control of this disease appear unable to reduce the disease to acceptable levels. Thus, we undertook a search for potential resistance to this disease among accessions of B. juncea and B. rapa included in the U.S. Plant Introduction (PI) collection. In greenhouse trials, we screened commercial cultivars and 672 U.S. PIs (226 B. juncea and 446 B. rapa) for resistance to Psm with artificial inoculation. Although severity of disease symptoms was significantly different among inoculated accessions, no acceptable levels of resistance were found in any of the more than 400 B. rapa accessions tested. Only two B. juncea accessions (PI 195553 and G 30988) of 226 tested had acceptable levels of resistance that might prove economically useful.

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Amnon Levi, Judy A. Thies, Patrick W. Wechter, Mark Farnham, Yiqun Weng and Richard Hassell

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W. Patrick Wechter, Amnon Levi, Kai-Shu Ling, Chandrasekar Kousik and Charles C. Block

Bacterial fruit blotch (BFB) caused by the bacterium Acidovorax avenae subsp. citrulli (Aac) is a seed-borne disease that threatens most cucurbit crops. Although limited resistance has been found in a small number of Plant introductions (PIs) in watermelon (Citrullus spp.), there are no reports of high levels of resistance in germplasm lines of Cucumis spp. In this study, 332 Cucumis spp. PIs were screened for resistance to Aac using a newly developed seed vacuum–infusion assay. Significant differences in the reaction of the PI to BFB were observed. The majority of lines were found to be extremely susceptible to the disease. However, several PIs with lower levels of resistance were also identified. Variability in the reaction of plants within each PI was also observed. Of the 332 PI tested, 16 were selected for additional evaluation using a standard spray inoculation tests. PI 353814, PI 381171, PI 536573, and PI 614401, all belonging to C. melo, and PI 504558 (C. ficifolius) were found to have significantly greater levels of resistance than susceptible control cultivars or other PIs in two independent spray inoculation tests. Germplasm lines developed from these PIs may be useful as sources of resistance to BFB in Cucumis breeding programs.