Roses are economically important ornamental plants in the United States and contribute substantially to the ornamental horticulture sector. The wholesale value of potted roses in the United States, sold as florist roses for indoor and patio use, accounts for more than seven million pots with a value of $23 million (NASS, 2012), Roses sold as cut flowers account for more than $17 million (NASS, 2012). The wholesale value of roses sold as deciduous shrubs is more than $194 million (USDA, 2010). Among the many diseases reported on roses, bacterial leaf spot caused by Xanthomonas sp. is a relatively new disease affecting nursery production. The disease is widespread and documented in Florida on the shrub rose varieties ‘RADrazz’ (Knock Out®) and ‘RADtco’ (Double Knock Out®) (Vallad, 2009). This disease causes chlorotic and necrotic spots on the foliage rendering the plants unmarketable (Fig. 1). Florida’s high relative humidity of greater than 75% throughout the growing season provides a favorable condition for the development of the disease and the spread of Xanthomonas sp. on roses.
Most commercial nurseries use overhead irrigation for rose production that further contributes to disease incidence and spread. There is currently no available information on best management practices for bacterial leaf spot on roses. Copper bactericides may provide control as in the case of other bacterial diseases, although copper tolerance in Xanthomonas spp. is common (Canteros, 2002; Pernezny et al., 2008; Ritchie and Dittapongpitch, 1991). We currently have no information on copper tolerance or resistance of rose strains of Xanthomonas sp. The current nursery plant disease management plan involves use of fungicides/bactericide rotations for management of black spot, Cercospora leaf spot, and bacterial leaf spot. This rotation plan includes the use of azoxystrobin, mancozeb, thiophanate methyl, and copper hydroxide-based chemistries in rotation at 7- or 14-d intervals depending on existing weather conditions from early spring to late fall (Stewart Chandler, Monrovia, personal communication). Current studies by our group and other groups in Florida are evaluating fungicide/bactericide rotation options for management of bacterial leaf spot on roses. Preliminary studies on the use of Acibenzolar-S-methyl, a systemic acquired resistance (SAR) inducer, that activates plant defense systems by increasing the transcription of stress-related genes indicate its ability to reduce bacterial leaf spot severity (Vallad, 2009).
A new area of our focus is exploring the possibility of using light-activated antimicrobials for management of bacterial leaf spot on roses. Antimicrobial photocatalyst technology has emerged from basic research and development to the application level in engineering and medicine with the development of TiO2 nanoparticles (Chen and Mao, 2007; Matsunaga et al., 1985). In the presence of light, TiO2 nanoparticles create highly chemically reactive hydroxyl and superoxide free radicals, which are detrimental to bacterial cells. This photocatalytic process continues as long as sufficiently energetic light is available. Although TiO2 nanoparticles have antimicrobial properties, using different doped (materials inside the TiO2 structure) versions of TiO2 with Zn, silver, and copper can further enhance the functional properties (Colon et al., 2006; Namiki et al., 2005). Research studies have shown the antimicrobial potential of TiO2 nanoparticles on bacteria, fungi, and viruses (Adams et al., 2006; Anpo, 2000; Choi et al., 2009; Dancer, 2008; Page et al., 2009; Tsuang et al., 2008; Wu et al., 2010). A recent study demonstrated the effectiveness of applying macroparticle TiO2 as a foliar spray against bacterial blight of geranium and bacterial leaf spot of poinsettia caused by X. hortorum pv. pelargonii and X. axonopodis pv. poinsettiicola, respectively (Norman and Chen, 2011). That study indicated that a key limitation in the commercialization of macroparticle TiO2 was the higher level of white residue that remained on leaves after application, subsequently reducing the marketability of plants for ornamental use. Also, a large quantity of material may have to be used for effective disease control, which is not a sustainable approach. Also, TiO2 photocatalysts have been previously shown to have antifungal activity and yield improvement potential in many crops (Lu et al., 2006; Owolade and Ogulenti, 2008).
The objective of this study was to evaluate the photocatalytic effect of a TiO2/Zn nanoparticle formulation on a Xanthomonas sp. infecting roses and the impact of foliar applications on bacterial leaf spot. We hypothesize that the TiO2/Zn nanoparticle formulations may offer high bactericidal action in the presence of light and high efficacy in reducing bacterial leaf spot severity than the currently available industry practice.
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