A Novel Screening Method for Evaluation of Lettuce Germplasm for Bacterial Leaf Spot Resistance

Abstract

<jats:p>Bacterial leaf spot of lettuce, caused by <jats:italic>Xanthomonas campestris</jats:italic> pv. <jats:italic>vitians</jats:italic>, is a devastating disease of lettuce worldwide. Because there are no chemicals available for effective control of the disease, host-plant resistance is highly desirable to protect lettuce production. A new method for fast screening and accurate identification of bacterial leaf spot (BLS)-resistant lettuce has been developed in our laboratory. A total of 79 lettuce genotypes (69 germplasm lines and 10 adapted cultivars) were evaluated with this technique for response to <jats:italic>X. c. vitians</jats:italic>. Disease incidences ranged from 92% to 100% and disease severities were between 1.6 and 3.6 on the 0 to 4 scale. No highly resistant genotypes were identified. However, 12 genotypes did not significantly differ for disease severities from the moderately resistant ‘Little Gem’ lettuce that was used as a resistant control. Comparison of disease severities of 10 commercial cultivars and three moderately resistant germplasm lines tested at the seedling stage and adult stage showed a high positive correlation (<jats:italic>r</jats:italic> = 0.87, <jats:italic>P</jats:italic> &lt; 0.0001) between tests. The new screening method should be useful in breeding programs, in which great numbers of plants need to be tested during germplasm evaluation, and for single plant selection as well as other studies. The identification of new sources of moderate resistance in this study could facilitate development of cultivars with a higher level of resistance through the gene pyramiding approach.</jats:p>

Diverse Responses of Lettuce Cultivars and Germplasm Lines to Infections of Three Isolates of Xanthomonas campestris pv. vitians

“…However, four resistant sources were identified among the butterhead, leaf, and latin lettuce types. The results were in agreement with the BLS reactions of lettuce types to the Xcv isolate L7 discovered in Pernezny et al (2000), Lu and Raid (2013), and Lu et al (2014). According to Mikel (2007), out of the 328 lettuce cultivars which were registered in the United States from 1970 through 2004, 61.6% were crisphead, 19.5% loose leaf, 13.7% romaine, 4.9% butterhead, and 0.3% latin lettuce types.…”

“…2B). These results were in agreement with those of Lu and Raid (2013). Because NF1 was a weak Xcv isolate, all but five lettuce genotypes ('10178', '50100', 'h1078', 'Okeechobee', and 'Terrapin') exhibited various degrees of resistance (Fig.…”

“…Pathogenicity tests were conducted according to the method reported by Lu and Raid (2013). The tests were repeated twice.…”

“…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.…”

“…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.…”