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.
W. Patrick Wechter, Ralph A. Dean, and Claude E. Thomas
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.
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.
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.
W. Patrick Wechter, Melanie M. McMillan, Mark W. Farnham, and Amnon Levi
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.
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.
W. Patrick Wechter, Melanie M. McMillan, Anthony P. Keinath, J. Powell Smith, and Mark W. Farnham
Karen R. Harris, Kai-Shu Ling, W. Patrick Wechter, and Amnon Levi
Zucchini yellow mosaic virus (ZYMV) is one of the most economically important viruses affecting watermelon [Citrullus lanatus (Thunb.) Matsun & Nakai var. lanatus] in the United States. The ZYMV-Florida strain (ZYMV-FL) is considered a major limitation to commercial watermelon production in the United States. Inheritance of resistance to ZYMV-FL is conferred by a recessive gene. This report describes the identification of single-reaction, polymerase chain reaction-based markers linked to the ZYMV-FL resistance gene in watermelon. In this study, we identified a marker ZYMV-resistant polymorphism (ZYRP) linked to the ZYMV-FL resistance gene locus (genetic distance of 8 cM) in an F2 population, and in a backcross one to the resistant parent population (BC1R) (genetic distance of 13 cM). The identification of a single nucleotide polymorphism within the ZYRP marker for the parental genotypes allowed the development of a sequence-characterized amplification region marker linked to the ZYMV-FL resistance gene. Experiments using a BC2F2 population derived from the U.S. Plant Introduction 595203 (C. lanatus var. lanatus) and the recurrent parent ‘Charleston Gray’ indicated that the ZYRP marker can be used in marker-assisted selection to identify genotypes containing the gene conferring ZYMV-FL resistance in watermelon.
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.