Relationships between <i>Xanthomonas campestris</i> pv. <i>vitians</i> population sizes, stomatal density and lettuce resistance to bacterial leaf spot

The resistance of 10 lettuce cultivars to bacterial leaf spot caused by Xanthomonas campestris pv. vitians (Xcv) was evaluated to establish the relationship between stomatal density, variations in pathogen population size, and the presence of epiphytic Bacillus sp. On the basis of average disease severity index ranked on a scale from 0 to 6, the cultivars were divided into three groups: tolerant, with 2.09; intermediate, with 3.59; and susceptible, with 4.92. The initial inoculum population of the pathogen was slightly increased, by less than 1 logCFU g−1 after 14 d, on only the tolerant cultivars ‘Batavia Reine des Glaces’ and ‘Little Gem’. In contrast to Xcv, the initial inoculum population of Bacillus sp. decreased on all the cultivars, by 1 to more than 2 logCFU g−1, without any significant relationship with the resistance groups. The tolerant cultivars exhibited averages of 40 and 57 stomata mm−2 on the adaxial and abaxial sides of the leaves, respectively; those averages were significantly lower than those of the susceptible cultivars, with 67 and 111 stomata mm−2, respectively. However, the stomatal area of the cultivars varied from 110 to 263 µm2/stomata and 126 to 232 µm2/stomata on the adaxial and abaxial sides of the leaves, respectively. Therefore, no relationship could be established between stomatal area and lettuce cultivar susceptibility. The density of stomata seems to be a good criterion for predicting the degree of tolerance of cultivars, especially when the adaxial side of the leaves is considered.