Bacterial diseases cause significant economic losses in the ornamental and floricultural industries (Daughtrey and Benson, 2005). In today's competitive marketplace, plants with spotted leaves, retarded growth, or wilted stems are not acceptable to consumers. To produce disease-free plant materials, modern facilities rely on strict sanitation and cultural practices to prevent bacterial outbreaks. Among them, culture indexing is a commonly used practice (Raju and Olson, 1985). Indexing programs can systematically detect and eliminate propagation materials contaminated with pathogens to ensure that only healthy plant materials are used for production (Klopmeyer, 2000; Oglevee-O'Donovan, 1993). However, even with strict sanitation guidelines, bacteria can still enter production facilities through contaminated seeds, propagative materials, aerosols, wind, alternate hosts, human contacts, or human error (Daughtrey et al., 1995). In many cases, the initial mode of introduction of the pathogen into the production facility is never identified. Once introduced, bacteria become established and spread rapidly through contact or through irrigation, causing significant crop loss (Daughtrey and Benson, 2005).
Bacterial blight on zonal geranium (Pelargonium ×hortorum) is considered to be the most serious problem limiting geranium production (Nameth et al., 1999). Bacteria initially enter plants through stomates, pores, or small wounds on leaf surfaces. Symptoms of this disease may vary greatly between cultivars of geranium. First visible symptoms are usually small, dark brown, sunken leaf spots. In some cultivars, however, symptoms are expressed with formation of V-shaped yellow lesions that occur in the leaf edge and taper toward the center of the leaf. Plants sometimes become systemically infected and die.
The use of culture-indexed plants greatly reduces the incidence of bacterial blight in geranium production. However, if greenhouse temperatures are cool and humidity low, infected geraniums can remain asymptomatic for months. Increasing temperatures or environmental stress will eventually trigger wilt and death of systemically infected plants. Most index block production systems use drip irrigation systems to keep foliage dry and limit spread between plants.
Xanthomonas leaf spot of poinsettia (Euphorbia pulcherrima) is another important floral crop disease. It was first described in India in 1951 (Patel et al., 1951) and has since been found in nearly every country where poinsettia is produced. Initial symptoms are small pinpoint lesions on leaves. In some cultivars, these lesions remain small, whereas in others, they coalesce to form larger angular leaf spots. Spread of Xanthomonas axonopodis pv. poinsettiicola within production facilities is similar to that observed for Xanthomonas hortorum pv. pelargonii on geranium. Strict sanitation is considered the best method of excluding this disease from production (Benson et al., 2002). However, because poinsettia is predominantly propagated through cuttings, overhead irrigation allows for easy spread of Xanthomonas during propagation.
In addition to strict sanitation, bactericides are used for controlling Xanthomonas species in geranium and poinsettia. The most commonly used bactericides contain copper, manganese, and/or zinc, but very few bactericidal control products are effective against established infections (Benson et al., 2002). Development of pathogen resistance to chemicals is also a concern. Recently, several studies have suggested that TiO2 application suppresses bacterial and fungal pathogens of field crops. In cereal crops, severity and incidence of Curvularia leaf spot (Curvularia lunata) and bacterial leaf blight (Xanthomonas oryzae pv. oryzae) were reduced with TiO2 applications (Chao and Choi, 2005). Similar results were observed in field applications of TiO2 on cowpea, in which significant reduction of both incidence and severity of Cercospora leaf spot and brown blotch was documented (Owolade and Ogunleti, 2008). Bowen et al. (1992) were able to control both downy and powdery mildew on grapes with combinations of TiO2 with aluminum and silica. Yao et al. (2007) showed the potential of using TiO2 in recycled irrigation water to eliminate both fungal and bacterial pathogens. It is believed that TiO2 destroys cell membranes of pathogens, causing leakage and death through oxidation. Despite this strong oxidizing power, TiO2 has been shown to be safe on plant surfaces (Frazer, 2001).
As far as we know, there has been no report on TiO2 application to floricultural crops for disease control. The objective of this study was to determine if TiO2 could be used to control Xanthomonas hortorum pv. pelargonii on geranium and Xanthomonas axonopodis pv. poinsettiicola on poinsettia. Both geraniums and poinsettias are among the most popular floricultural crops in the United States with wholesale values at $9,385,000 and $145,088,000 in 2009, respectively (USDA, 2009).
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