Search Results

You are looking at 1 - 10 of 18 items for

  • Author or Editor: Patrick Wechter x
Clear All Modify Search

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.

Free access

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.

Free access

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.

Free access

Fatty alcohol treatments can be used to eliminate the meristem of cucurbit (Cucurbitaceae) rootstocks, which prevents regrowth when grafting, but the effects of the treatment on the rootstock have not been documented. Two rootstock types, ‘Emphasis’ bottle gourd (Lagenaria siceraria) and ‘Carnivor’ interspecific hybrid squash (Cucurbita maxima × C. moschata) commonly used in watermelon (Citrullus lanatus) grafting significantly increased in cotyledon and hypocotyl size over 21 days after treatment (DAT) with a 6.25% fatty alcohol emulsion. There was a significant increase in total soluble sugar (glucose, sucrose, and fructose) content for each rootstock hypocotyl and cotyledon. Starch concentrations of hypocotyls and cotyledons also increased significantly in both rootstocks. This increase in stored energy could greatly increase the success rate of the grafting process. Increased rootstock energy reserves could overcome the need for keeping the rootstock cotyledon intact when grafting.

Full access

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.

Free access

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.

Free access

Anaerobic soil disinfestation (ASD) is a preplant pest management technique that involves amending the soil with a labile carbon source, irrigating the soil to stimulate decomposition, and then covering the soil with polyethylene film (polyfilm) to limit gas exchange. During the ASD process, soil microorganisms shift from aerobic to anaerobic metabolism and release phytotoxic byproducts such as organic acids and gases. Although it has been shown that these phytotoxic by-products have a negative impact on weed survival, questions remain about whether commercial-level weed control can be achieved using ASD alone or in combination with other chemicals. Greenhouse and field studies were conducted to evaluate ASD with mustard (Brassica sp.) meal, molasses, and herbicide applications for yellow nutsedge (Cyperus esculentus) control in tomato (Solanum lycopersicum). The treatments in these studies included factorial of two carbon sources [mustard meal + molasses (MMM) or no carbon amendment], three herbicide treatments [halosulfuron applied preemergence (PRE), halosulfuron applied postemergence (POST), and no herbicide] and two polyfilm treatments (polyfilm cover or polyfilm uncover). In field trials two polyfilm cover treatments were punctured and nonpunctured. Soil treatments included molasses at 14,000 L·ha−1 and mustard meal at 2100 kg·ha−1. Halosulfuron was applied at a rate of 1 oz/acre for PRE or POST applications. Greater anaerobic conditions were achieved in polyfilm cover treatments amended with MMM. In greenhouse and field trials, the most effective treatments for reducing yellow nutsedge populations were ASD with MMM or combined with halosulfuron application (PRE- or POST-ASD), which delivered significantly higher weed control than all other treatments tested or controls. In field trials, ASD with MMM caused plant growth stunting 14 d after transplantation (DAT); however, plants recovered, and stunting or injury was often not observed at 42 DAT. These studies demonstrated that ASD using MMM can be an effective strategy for reducing yellow nutsedge populations; however, the more research is needed to ensure crop safety while using ASD technology.

Open Access

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.

Free access

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.

Free access

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.

Free access