Role of Stomatal Opening and Frequency on Infection ofLycopersiconspp. byXanthomonas campestrispv.vesicatoria

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Abstract
In Lycopersicon esculentum (tomato) and related Lycopersicon spp., [L. hirsutum, L. pimpinellifolium] bacterial spot disease was significantly reduced when leaf stomatal closure was physiologically induced or the stomatal opening was chemically suppressed by abscisic acid and phenylmercuric acetate before inoculation with Xanthomonas campestris pv. vesicatoria. Both disease incidence and severity were related to stomatal opening at the time of, and immediately following, inoculations. Stomatal frequency on adaxial and abaxial surfaces was correlated with the number of spot lesions produced after the infection. Higher stomatal frequency and number of bacterial lesions were found in L. esculentum ''Flora-Dade'' and ''Florida 1B'', whereas L. hirsutum had lower stomatal frequency and fewer lesions. Midrange values in the disease response and stomatal frequency of hybrids (L. esculentum .times. L. hirsutum) suggested that a moderate level of resistance may be heritable and quantitatively based. Inoculations by misting versus infiltration of the mesophyll provided evidence that the lower disease level of the host is mainly due to an external, morphological resistance (i.e., fewer stomata on the surface of leaves) rather than to an internal, intrinsic resistance. An improved level of control may be achieved by combining low stomatal number with stomatal closure by application of antitranspirants.