2013 ASHS Annual Conference

Bacterial leaf spot (BLS) of lettuce caused by Xanthomonas campestris pv. vitians (Xcv) is an important lettuce disease. Host resistance is known in ‘La Brillante’; nothing is known about the inheritance of this resistance. In other crop species a rapid cell death at the infection site known as a hypersensitive response (HR) is a common mechanism of disease resistance. The objective of this research was to 1) determine if a HR confers resistance in ‘La Brillante’ and 2) determine the inheritance of BLS resistance in a F₇ recombinant inbred line (RIL) population from Salinas 88 x La Brillante. Six plants from 90 RILs, ‘Salinas 88’, and ‘La Brillante’ were evaluated for a HR after infiltrating the leaves of four week old lettuce seedlings with a solution of approximately 1x10⁸ cells/mL using Xcv isolate BS347. The same cultivars and RILs were evaluated for diseases severity (DS) on November 28 in a Salinas, CA, field experiment with natural infection and in two greenhouse experiments (GH1 and GH2). The greenhouse experiments were inoculated by spraying four-week-old seedlings until runoff with a mixed solution of Xcv isolates BS339, BS340, and BS347 at approximately 1x10⁸ cells/mL. The field, GH1, and GH2 experiments used a randomized complete-block design with three blocks. Disease severity was measured on individual plants using a 0 (low disease) to 10 (severe disease) scale at harvest maturity in the field and a 0 (no disease) to 5 (severe disease) scale in GH1 and GH2 one week after inoculation. Segregation of the HR was compared to known genetic models using chi-square. The mean DS of the HR and the non HR RILs and parents were calculated and compared using t-tests. 'La Brillante' expressed the HR while 'Salinas 88' did not. Disease severity means of ‘Salinas 88’ (field: 3.8, GH1: 3.3, GH2: 4.5) and ‘La Brillante’ (field: 0.2, GH1: 0.9, GH2: 2.2) were significantly different (P < 0.01) in all experiments. The RILs segregated 47 HR : 43 not HR, a good fit to 1:1 (χ², 1 df = 0.2, P = 0.7), which indicates a single gene controlling the HR. The mean DS of RILs expressing the HR (field: 2.8, GH1: 1.4, GH2: 2.5) was significantly lower than the non-HR RILs (field: 9.8, GH1: 2.9, GH2: 4.0). Bacterial leaf spot resistance in ‘La Brillante’ is a simply inherited trait and should be useful for breeding resistant cultivars.