Bacterial leaf spot of lettuce (Lactuca sativa L.), caused by Xanthomonas campestris pv. vitians, is an economically important disease of lettuce in the world (Barak et al., 2001; Patterson et al., 1986; Pennisi and Pane, 1990; Sahin, 2000; Sahin and Miller, 1997; Toussaint, 1999). A widespread and damaging outbreak of BLS in lettuce was first observed in Florida in the winter of 1992–93 (Pernezny et al.,1995). This disease has become problematic in Florida’s lettuce production in recent years. In 2010, a severe epidemic caused significant crop damage and economic losses.
Typical BLS symptoms are brown to black, greasy-appearing lesions, which render the entire lettuce head unmarketable as a result of the unsightly leaf blemishes. There are no chemicals available for effective control of the disease. Therefore, use of host-plant resistance is the most efficient and desirable method to manage BLS in lettuce. Different types of lettuce have been found to vary in response to BLS with romaine and butterhead being highly susceptible (Carisse et al., 2000; Pernezny et al., 1995; Toussaint, 1999). Among crisphead, green, and red leaf lettuce, relatively resistant cultivars were identified in field (Carisse et al., 2000) and greenhouse experiments (Sahin and Miller, 1997). However, these cultivars were found to be susceptible to BLS in subsequent studies (Bull et al., 2007). Bull et al. (2007) identified a resistant cultivar, Little Gem, by comparing 16 cultivars for response to BLS. ‘Little Gem’ has been introduced into several breeding programs for development of adapted cultivars with resistance to BLS. However, there are two concerns with the use of ‘Little Gem’ for resistant cultivar development. The first is that ‘Little Gem’ is a small Latin lettuce and has undesirable agronomic and horticultural traits. This makes it difficult to transfer BLS resistance to adapted cultivars without affecting other traits of the adapted cultivars such as yield. The second concern is that ‘Little Gem’ exhibited BLS in the experiments of Bull et al. (2007) and our preliminary studies. The resistance in ‘Little Gem’ may not provide sufficient protection of lettuce if a BLS epidemic is extensive and severe in the field, as occurred in Florida in 1992–93 and in 2010. Therefore, efforts need to be made to screen additional cultivars and germplasm lines for new sources of resistance.
Two types of experiments (field and greenhouse) have been used to evaluate lettuce for response to BLS. In the field, plants were maintained according to standard commercial practices, sprayed with bacterial inoculum at thinning and again 2 weeks later, and rated for disease incidence and severity before harvest (Bull et al., 2007). In the greenhouse, lettuce was planted in either pots (Pernezny et al., 1995; Robinson et al., 2006) or cell flats (Bull et al., 2007), and up to three applications of inoculum to plants were made at a 7-d interval (Bull et al., 2007). These screening methods are labor-intensive and thus allow only a small number of cultivars to be tested at a time. For example, Carisse et al. (2000) evaluated only nine cultivars and Bull et al. (2007) tested 16 cultivars.
Breeding lettuce for BLS resistance usually requires testing a great number of plants during the initial screening of germplasm for resistant sources and during the plant selection stage to integrate the resistance into adapted cultivars. It is necessary to establish a method for fast and accurate evaluation of a large number of plants in a short period of time. We report development of a new screening method and identification of new sources of BLS resistance.
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