A Single Gene Confers Resistance to Bacterial Leaf Spot in the Lettuce Cultivar La Brillante

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.