Lettuce (Lactuca sativa L.) is one of the top ten most valuable crops in the United States with an average annual value of $2.1 billion (USDA, 2014). There are eight types of lettuce grown in the world, whereas four of them including crisphead, butterhead, romaine, and leaf are the major market types in the United States. On average from a 5-year data (2009 through 2013), head, romaine, and leaf lettuce accounted for 55.0%, 30.7%, and 14.3% of the U.S. lettuce production, respectively (USDA, 2014). California and Arizona are the two largest growing states of lettuce, producing 96% of head and romaine lettuce, and 98% of leaf lettuce in the United States (Glaser et al., 2001). Lettuce is an important winter vegetable crop in Florida. The EAA is the main production area of the state with an annual acreage of 10,000–11,000 and a farm gate value of $50–$60 millions. The warm and humid subtropical climate in Florida favors occurrence of biotic stresses such as diseases, which have significant, adverse impacts on lettuce production.
Bacterial leaf spot (BLS), caused by Xcv, is a devastating disease of lettuce in Florida. This disease was first reported in South Carolina and Virginia in the United States in 1918 (Brown, 1918). Since 1990s, substantial outbreaks of BLS have been reported with increasing frequency in some of the major lettuce growing regions, such as California (Barak et al., 2001; Bull et al., 2007; Bull and Koike, 2005; Umesh et al., 1996), Florida (Lu and Raid, 2013; Pernezny et al., 1995), and Ohio (Sahin and Miller, 1997). Currently, there are no chemicals available to control the disease. A severe epidemic usually results in significant crop damage and economic losses.
Initial symptoms of BLS occur on mature, outer leaves with small water-soaked angular leaf lesions, and later become brown to black necrotic and papery (Koike and Gilbertson, 1997). The symptoms then spread to other parts of the lettuce plants, resulting in unmarketable heads or hearts. Epidemiology of the disease has been investigated on a number of factors associated with dispersal and survival of Xcv. The pathogen is thought to be seedborne and can survive on dry seeds for extended periods (Sahin and Miller, 1997). Xcv can be transmitted by lettuce plant debris in soil or leaves of symptomless weed species after the summer and winter fallow periods in California (Barak et al., 2001). Xcv has also been demonstrated to be spread rapidly by overhead irrigation in greenhouse from only a few infected plants (Toussaint, 1999). A broad list of potential host plant species has been provided after inoculation of these plants with Xcv (Tsuchiya et al., 1981). Robinson et al. (2006) identified pepper as a potential crop host of Xcv of lettuce in Florida and also determined that the optimum temperature for Xcv infection is 22.7 °C under growth chamber conditions.
Nevertheless, virulence diversity of Xcv strains and variation of host responses among lettuce genotypes are very important epidemiological aspects for understanding and controlling BLS outbreaks. Structure differences of DNA, RNA, and other biochemical factors among Xcv strains have been investigated (Barak and Gilbertson, 2003; Sahin et al., 2003). Pernezny et al. (1995) found that symptoms in plants inoculated with the five Florida strains individually were more severe than other three Xcv reference strains from outside of the state, whereas, although the three lettuce genotypes tested were susceptible to Xcv, the disease reactions of crisphead cv. South Bay was less severe than romaine cv. Tall Guzmaine and butterhead breeding line B-1190. Different levels of host resistance have been reported in commercial lettuce cultivars or germplasm by inoculation with single Xcv strain (Carisse et al., 2000; Lu and Raid, 2013; Pernezny et al., 2000) or a mixture of Xcv strains (Bull et al., 2007; Hayes et al., 2014; Sahin and Miller, 1997). However, information is limited on responses of resistant lettuce genotypes to different Xcv strains. In addition, there have been no reports on change of pathogenic virulence of Xcv strains recovered in severe epidemic years and nonepidemic years in the same region.
The objectives of this study were to 1) investigate pathogenic differences of Xcv strains isolated in various years in the EAA of Florida with BLS outbreaks or without a severe disease occurrence and 2) determine reactions of susceptible and resistant lettuce genotypes to the different Xcv strains.
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