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  • Author or Editor: Mathews Paret x
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Bacterial leaf spot on roses caused by a Xanthomonas sp. is a new disease affecting commercial rose production with the potential to cause major economic losses. In the past few decades, antimicrobial photocatalyst technology has emerged from basic research and development to provide convenient formulations of titanium dioxide (TiO2) nanoparticles, which have the ability to destroy bacteria on surfaces in the presence of light. In this study, a TiO2 nanoparticle formulation containing zinc (TiO2/Zn) was tested for management of bacterial leaf spot on Rosa ‘Noare’. TiO2/Zn caused significant reduction in the survival of Xanthomonas sp. strain Xr-1 on glass coverslips coated with the nanoparticles on exposure to light at 3 × 104 lux for 10 minutes. There was no reduction of bacterial viability in non-coated or non-illuminated controls. Field applications of TiO2/Zn at ≈500 to 800 ppm on Rosa ‘Noare’ significantly reduced bacterial spot severity compared with the untreated control. TiO2/Zn activity was better or on par with the ornamental industry standard for management of rose diseases.

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Rose rosette emaravirus (RRV, genus Emaravirus), the causal agent of rose rosette disease, is the topmost pathogen of concern for the rose industry in the United States. The only strategy available for disease management is early identification and eradication of the infected plants. Highly reliable, specific, and sensitive detection assays are thus required to test and confirm the presence of RRV in suspected plant samples. RRV is only a recently characterized virus and hence limits the diagnostic tools available for its early detection. With a U.S. Department of Agriculture (USDA) Specialty Crop Research Initiative (SCRI) project sponsorship, several diagnostic tools including end-point reverse transcription-polymerase chain reaction (RT-PCR) and RT-qPCR assays targeting single and multiple genes targets were developed for routine diagnostics. This review introduces an overall view of the different diagnostic tools developed, which are reliable, highly sensitive, and can be easily implemented for detection and identification in laboratories providing diagnostic services and confirmation of RRV-infected samples.

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Ralstonia solanacearum (Rs) race 4 strains cause bacterial wilt of edible ginger (Zingiber officinale). The survival of the pathogen was studied in plant-free soil and potting medium in the presence of plants inoculated by different methods (non-wounded, rhizome-wounded, and stem-wounded) and irrigated on different schedules (alternate and daily). Detection thresholds for Rs were determined for an enzyme-linked immunoabsorbent assay (ELISA), immunostrip assay, and polymerase chain reaction (PCR) using drainage water from soil and potting medium containing known concentrations of Rs. In the absence of a plant or in the presence of non-wounded plants, Rs populations declined rapidly in drainage water from potting medium during the first 9 days and were undetectable after 81 days. When plants were stem- or rhizome-wounded, Rs populations increased by two to three orders of magnitude from the initial population levels for the first 9 to 19 days and then gradually declined and became undetectable after 89 days. Results were similar in experiments with soil except for non-wounded ginger plants, where the initial decline in Rs populations was followed by an abrupt increase after day 11, reaching 7 log cfu/mL on day 21, then declining gradually to non-detectable levels after 137 days. The increase was attributed to natural infection of the plants followed by release of high populations of Rs into the irrigation water when plants wilted. When rhizome-inoculated plants were watered on alternate days, Rs was recovered from 97 to 129 days in soil and potting medium, but when the plants were watered daily, Rs was recovered in soil and potting medium up to 153 days after plant inoculation. ELISA using Ps1a monoclonal antibody detected the pathogen from >95% of the samples from soil and potting medium when viable populations were >5 log cfu/mL. The immunostrip assay (using the same antibody) detected the pathogen from 100% of the samples when viable populations were >3 log cfu/mL. PCR based on the flagellin gene fliC detected the pathogen from >95% of the samples from soil and from >74% of the samples from potting medium when viable populations were >4 log cfu/mL.

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Fourteen species of ginger belonging to Zingiberaceae and Costaceae were evaluated for susceptibility to the bacterial wilt pathogen Ralstonia solanacearum (Rs) race 4 (ginger strains) by several methods of inoculation, including tests to simulate natural infection. Twelve of 14 species tested were highly susceptible to all strains of Rs race 4 upon stem inoculation, and susceptible plants wilted within 21 days. In contrast to previous reports that Rs strains from an invasive alien species, kahili ginger (Hedychium gardenarium), are nonpathogenic on ornamental gingers, the kahili ginger strain wilted both ornamental and edible ginger (Zingiber officinale) species within 21 days. Pour inoculation to the base of 11 plant species to simulate natural infection confirmed the ability of Rs to invade all the tested species without root wounds. Shampoo ginger (Zingiber zerumbet) was the most susceptible (wilted in 26 days) whereas pink ginger (Alpinia purpurata) and red ginger (A. purpurata) were the least susceptible and wilted in 71 and 76 days respectively. Pathogen survival in potting medium was evaluated by enumerating viable cells in effluent water from drenched pots with and without infected edible ginger after stem or rhizome inoculation. Ralstonia solanacearum survived in plant-free potting medium for 120 days and for 150 to 180 days in potting medium with infected edible ginger. The ability of Rs race 4 to infect many ginger species without wounding and to survive for long periods indicates that high risks will be incurred if the kahili ginger strain is inadvertently introduced from the forest reserves into ginger production areas.

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Seven hybrid tomato rootstocks with possible resistance to bacterial wilt caused by Ralstonia solanacearum and a known resistant cultivar were tested as grafting rootstocks to impart resistance to a bacterial wilt-susceptible cultivar, BHN 602. Greenhouse studies showed resistance of all the rootstocks to bacterial wilt. The disease incidence and yield of ‘BHN 602’ grafted to these rootstocks were evaluated in open-field tomato production in Florida and Virginia over four seasons. Significant differences in bacterial wilt incidence were observed between grafted entries in three of the four trials. In these three trials, grafted entries consistently exhibited the least bacterial wilt incidence compared with the controls; the self-graft, and non-grafted entries. Over all the trials, tomato plants grafted onto ‘Cheong Gang’, ‘BHN 1054’, and ‘BHN 998’ displayed the least bacterial wilt incidence. Rootstocks had a significant effect on total marketable yield in all the trials with certain grafted entries yielding significantly greater than non-grafted ‘BHN 602’. Field studies show that grafting holds promise for decreasing the impact of bacterial wilt on tomato cultivars as well as increasing the overall productivity of tomato cultivars.

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Summer squash (Cucurbita pepo L.) is a major vegetable crop produced in Georgia and Florida during the fall season. This production is vulnerable to whitefly (Bemisisia tabaci Genn.)-transmitted viruses that lead to severe yield losses. Over the past several years, whitefly populations have increased during the fall, thus leading to an increase in whitefly-transmitted viruses such as Cucurbit leaf crumple virus (CuLCrV) and Cucurbit yellow stunting disorder virus (CYSDV). Whitefly management for summer squash relies on the use of insecticides and can be costly without providing adequate management of the viruses. Deployment of host resistance to whiteflies and their transmitted viruses (CuLCrV and CYSDV) is the best strategy for mitigating yield loss of summer squash; however, no resistant cultivars are commercially available. In the current study, resistance or tolerance to whiteflies, CuLCrV, and CYSDV was determined for squash germplasm from the U.S. Department of Agriculture (USDA) Germplasm Resources Information Network (GRIN), university breeding programs, and commercial companies in Georgia and Florida across 2 years. In both locations and years, visual virus symptom severity scores were collected and a quantitative polymerase chain reaction (qPCR) was used to determine the CuLCrV viral load and CYSDV presence in Georgia. Whitefly-induced feeding damage was evaluated by directly assessing the intensity of silverleaf symptoms and visual counts of whitefly adults on the foliage in the field or in photographs. Virus symptom severity was lower in C. moschata Duchesne ex Poir. genotypes, namely, PI 550689, PI 550692, PI 550694, PI 653064, and Squash Betternut 900, than in other evaluated genotypes. Two C. pepo accessions were common between both locations for viral severity (PI 442294) or viral severity and viral load (PI 171625). Lower CuLCrV loads were identified in C. ecuadorensis Cutler & Whitaker (PI 540895), and C. okeechobeensis (Small) L.H.Bailey (PI 540900) than other evaluated genotypes. Four genotypes tested negative for CYSDV during both years: C. pepo (PI 507882), C. moschata (PI 483345), C. ecuadorensis (PI 390455), and C. okeechobeensis (PI 540900); they are potential sources of resistance. Six C. moschata accessions (PI 211999, PI 550690, PI 550692, PI 550694, PI 634982, and PI 653064) showed high tolerance to silverleaf disorder and had the lowest adult whitefly counts. Collectively, the accessions identified in the current study are potential sources of resistance or tolerance to whitefly and whitefly-transmitted viruses (CuLCrV and CYSDV).

Open Access

The eriophyid mite, Phyllocoptes fructiphilus, vectors the causal agent, Rose rosette virus (RRV), that results in rose rosette disease. Parts of the southeastern United States have remained free of the disease, except for infected plant material introductions that were eradicated. A survey of sampling points through Alabama, Georgia, and Mississippi (n = 204) revealed the southeastern border of RRV. The presence of RRV in symptomatic plant tissue samples (n = 39) was confirmed by TaqMan-quantitative reverse transcription polymerase chain reaction (RT-qPCR). Samples were also collected at every plot for detection of eriophyid mites, specifically for P. fructiphilus. Three different species of eriophyid mites were found to be generally distributed throughout Alabama, Georgia, and Mississippi. Most of these sites (n = 60) contained P. fructiphilus, found further south than previously thought, but in low populations (<10 mites/gram of tissue) south of the RRV line of incidence. Latitude was found to be significantly correlated with the probability of detecting RRV-positive plants, but plant hardiness zones were not. Plot factors such as plant size, wind barriers, and sun exposure were found to have no effect on P. fructiphilus or the presence of RRV. The reason for the absence of RRV and low populations of P. fructiphilus in this southeast region of the United States are unclear.

Open Access